Re: [PATCH net-next v6 06/11] seccomp,landlock: Handle Landlock events per process hierarchy

From: MickaÃl SalaÃn
Date: Tue Apr 18 2017 - 19:25:54 EST

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On 19/04/2017 00:53, Kees Cook wrote:
> On Tue, Mar 28, 2017 at 4:46 PM, MickaÃl SalaÃn <mic@xxxxxxxxxxx> wrote:
>> The seccomp(2) syscall can be used by a task to apply a Landlock rule to
>> itself. As a seccomp filter, a Landlock rule is enforced for the current
>> task and all its future children. A rule is immutable and a task can
>> only add new restricting rules to itself, forming a chain of rules.
>>
>> A Landlock rule is tied to a Landlock event. If the use of a kernel
>> object is allowed by the other Linux security mechanisms (e.g. DAC,
>> capabilities, other LSM), then a Landlock event related to this kind of
>> object is triggered. The chain of rules for this event is then
>> evaluated. Each rule return a 32-bit value which can deny the use of a
>> kernel object with a non-zero value. If every rules of the chain return
>> zero, then the use of the object is allowed.
>>
>> Changes since v5:
>> * remove struct landlock_node and use a similar inheritance mechanisme
>> as seccomp-bpf (requested by Andy Lutomirski)
>> * rename SECCOMP_ADD_LANDLOCK_RULE to SECCOMP_APPEND_LANDLOCK_RULE
>> * rename file manager.c to providers.c
>> * add comments
>> * typo and cosmetic fixes
>>
>> Changes since v4:
>> * merge manager and seccomp patches
>> * return -EFAULT in seccomp(2) when user_bpf_fd is null to easely check
>> if Landlock is supported
>> * only allow a process with the global CAP_SYS_ADMIN to use Landlock
>> (will be lifted in the future)
>> * add an early check to exit as soon as possible if the current process
>> does not have Landlock rules
>>
>> Changes since v3:
>> * remove the hard link with seccomp (suggested by Andy Lutomirski and
>> Kees Cook):
>> * remove the cookie which could imply multiple evaluation of Landlock
>> rules
>> * remove the origin field in struct landlock_data
>> * remove documentation fix (merged upstream)
>> * rename the new seccomp command to SECCOMP_ADD_LANDLOCK_RULE
>> * internal renaming
>> * split commit
>> * new design to be able to inherit on the fly the parent rules
>>
>> Changes since v2:
>> * Landlock programs can now be run without seccomp filter but for any
>> syscall (from the process) or interruption
>> * move Landlock related functions and structs into security/landlock/*
>> (to manage cgroups as well)
>> * fix seccomp filter handling: run Landlock programs for each of their
>> legitimate seccomp filter
>> * properly clean up all seccomp results
>> * cosmetic changes to ease the understanding
>> * fix some ifdef
>>
>> Signed-off-by: MickaÃl SalaÃn <mic@xxxxxxxxxxx>
>> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
>> Cc: James Morris <james.l.morris@xxxxxxxxxx>
>> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
>> Cc: Serge E. Hallyn <serge@xxxxxxxxxx>
>> Cc: Will Drewry <wad@xxxxxxxxxxxx>
>> Link: https://lkml.kernel.org/r/c10a503d-5e35-7785-2f3d-25ed8dd63fab@xxxxxxxxxxx
>> ---
>> include/linux/landlock.h | 36 +++++++
>> include/linux/seccomp.h | 8 ++
>> include/uapi/linux/seccomp.h | 1 +
>> kernel/fork.c | 14 ++-
>> kernel/seccomp.c | 8 ++
>> security/landlock/Makefile | 2 +-
>> security/landlock/hooks.c | 37 +++++++
>> security/landlock/hooks.h | 5 +
>> security/landlock/init.c | 3 +-
>> security/landlock/providers.c | 232 ++++++++++++++++++++++++++++++++++++++++++
>> 10 files changed, 342 insertions(+), 4 deletions(-)
>> create mode 100644 security/landlock/providers.c
>>
>> diff --git a/include/linux/landlock.h b/include/linux/landlock.h
>> index 53013dc374fe..c40ee78e86e0 100644
>> --- a/include/linux/landlock.h
>> +++ b/include/linux/landlock.h
>> @@ -12,6 +12,9 @@
>> #define _LINUX_LANDLOCK_H
>> #ifdef CONFIG_SECURITY_LANDLOCK
>>
>> +#include <linux/bpf.h> /* _LANDLOCK_SUBTYPE_EVENT_LAST */
>> +#include <linux/types.h> /* atomic_t */
>> +
>> /*
>> * This is not intended for the UAPI headers. Each userland software should use
>> * a static minimal version for the required features as explained in the
>> @@ -19,5 +22,38 @@
>> */
>> #define LANDLOCK_VERSION 1
>>
>> +struct landlock_rule {
>> + atomic_t usage;
>
> This should be refcount_t. (And I should convert seccomp to use
> refcount_t too!) :)

OK

>
>> + struct landlock_rule *prev;
>> + struct bpf_prog *prog;
>> +};
>> +
>> +/**
>> + * struct landlock_events - Landlock event rules enforced on a thread
>> + *
>> + * This is used for low performance impact when forking a process. Instead of
>> + * copying the full array and incrementing the usage of each entries, only
>> + * create a pointer to &struct landlock_events and increments its usage. When
>> + * appending a new rule, if &struct landlock_events is shared with other tasks,
>> + * then duplicate it and append the rule to this new &struct landlock_events.
>> + *
>> + * @usage: reference count to manage the object lifetime. When a thread need to
>> + * add Landlock rules and if @usage is greater than 1, then the thread
>> + * must duplicate &struct landlock_events to not change the children's
>> + * rules as well.
>> + * @rules: array of non-NULL &struct landlock_rule pointers
>> + */
>> +struct landlock_events {
>> + atomic_t usage;
>> + struct landlock_rule *rules[_LANDLOCK_SUBTYPE_EVENT_LAST];
>> +};
>> +
>> +void put_landlock_events(struct landlock_events *events);
>> +
>> +#ifdef CONFIG_SECCOMP_FILTER
>
> Isn't CONFIG_SECCOMP_FILTER already required for landlock?

Yes it is, but Landlock could only/also be used through cgroups in the
future. :)

>
>> +int landlock_seccomp_append_prog(unsigned int flags,
>> + const char __user *user_bpf_fd);
>> +#endif /* CONFIG_SECCOMP_FILTER */
>> +
>> #endif /* CONFIG_SECURITY_LANDLOCK */
>> #endif /* _LINUX_LANDLOCK_H */
>> diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
>> index e25aee2cdfc0..9a38de3c0e72 100644
>> --- a/include/linux/seccomp.h
>> +++ b/include/linux/seccomp.h
>> @@ -10,6 +10,10 @@
>> #include <linux/thread_info.h>
>> #include <asm/seccomp.h>
>>
>> +#if defined(CONFIG_SECCOMP_FILTER) && defined(CONFIG_SECURITY_LANDLOCK)
>> +struct landlock_events;
>> +#endif /* CONFIG_SECCOMP_FILTER && CONFIG_SECURITY_LANDLOCK */
>
> Testing LANDLOCK should be sufficient, since it requires ..._FILTER.
>
>> +
>> struct seccomp_filter;
>> /**
>> * struct seccomp - the state of a seccomp'ed process
>> @@ -18,6 +22,7 @@ struct seccomp_filter;
>> * system calls available to a process.
>> * @filter: must always point to a valid seccomp-filter or NULL as it is
>> * accessed without locking during system call entry.
>> + * @landlock_events: contains an array of Landlock rules.
>> *
>> * @filter must only be accessed from the context of current as there
>> * is no read locking.
>> @@ -25,6 +30,9 @@ struct seccomp_filter;
>> struct seccomp {
>> int mode;
>> struct seccomp_filter *filter;
>> +#if defined(CONFIG_SECCOMP_FILTER) && defined(CONFIG_SECURITY_LANDLOCK)
>> + struct landlock_events *landlock_events;
>> +#endif /* CONFIG_SECCOMP_FILTER && CONFIG_SECURITY_LANDLOCK */
>
> Same.
>
>> };
>>
>> #ifdef CONFIG_HAVE_ARCH_SECCOMP_FILTER
>> diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
>> index 0f238a43ff1e..74891cf60ca6 100644
>> --- a/include/uapi/linux/seccomp.h
>> +++ b/include/uapi/linux/seccomp.h
>> @@ -13,6 +13,7 @@
>> /* Valid operations for seccomp syscall. */
>> #define SECCOMP_SET_MODE_STRICT 0
>> #define SECCOMP_SET_MODE_FILTER 1
>> +#define SECCOMP_APPEND_LANDLOCK_RULE 2
>>
>> /* Valid flags for SECCOMP_SET_MODE_FILTER */
>> #define SECCOMP_FILTER_FLAG_TSYNC 1
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index a27d8e67ce33..14c09486c565 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -47,6 +47,7 @@
>> #include <linux/security.h>
>> #include <linux/hugetlb.h>
>> #include <linux/seccomp.h>
>> +#include <linux/landlock.h>
>> #include <linux/swap.h>
>> #include <linux/syscalls.h>
>> #include <linux/jiffies.h>
>> @@ -528,7 +529,10 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
>> * the usage counts on the error path calling free_task.
>> */
>> tsk->seccomp.filter = NULL;
>> -#endif
>> +#ifdef CONFIG_SECURITY_LANDLOCK
>> + tsk->seccomp.landlock_events = NULL;
>> +#endif /* CONFIG_SECURITY_LANDLOCK */
>> +#endif /* CONFIG_SECCOMP */
>>
>> setup_thread_stack(tsk, orig);
>> clear_user_return_notifier(tsk);
>> @@ -1405,7 +1409,13 @@ static void copy_seccomp(struct task_struct *p)
>>
>> /* Ref-count the new filter user, and assign it. */
>> get_seccomp_filter(current);
>> - p->seccomp = current->seccomp;
>> + p->seccomp.mode = current->seccomp.mode;
>> + p->seccomp.filter = current->seccomp.filter;
>> +#if defined(CONFIG_SECCOMP_FILTER) && defined(CONFIG_SECURITY_LANDLOCK)
>> + p->seccomp.landlock_events = current->seccomp.landlock_events;
>> + if (p->seccomp.landlock_events)
>> + atomic_inc(&p->seccomp.landlock_events->usage);
>> +#endif /* CONFIG_SECCOMP_FILTER && CONFIG_SECURITY_LANDLOCK */
>
> Hrm. So, this needs config cleanup as above. Also, I'm going to need
> some help understanding the usage tracking on landlock_events (which
> should use a get/put rather than open-coding the _inc). I don't see
> why individual assignments are needed here. The only thing that
> matters is the usage bump. I would have expected no changes at all in
> this code, actually. The filter and the events share the same usage
> don't they?

Right, I can move the struct landlock_event into the struct
seccomp_filter. This should make the code cleaner.

>
>> /*
>> * Explicitly enable no_new_privs here in case it got set
>> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
>> index 326f79e32127..d122829e6da1 100644
>> --- a/kernel/seccomp.c
>> +++ b/kernel/seccomp.c
>> @@ -34,6 +34,7 @@
>> #include <linux/security.h>
>> #include <linux/tracehook.h>
>> #include <linux/uaccess.h>
>> +#include <linux/landlock.h>
>>
>> /**
>> * struct seccomp_filter - container for seccomp BPF programs
>> @@ -494,6 +495,9 @@ static void put_seccomp_filter(struct seccomp_filter *filter)
>> void put_seccomp(struct task_struct *tsk)
>> {
>> put_seccomp_filter(tsk->seccomp.filter);
>> +#ifdef CONFIG_SECURITY_LANDLOCK
>> + put_landlock_events(tsk->seccomp.landlock_events);
>> +#endif /* CONFIG_SECURITY_LANDLOCK */
>
> put_landlock_events() should be defined in a header file to be a
> static inline no-op if ..._LANDLOCK isn't defined. That way we can
> avoid all the #ifdef stuff here. Similarly, I think all of these
> should take just task_struct so that there isn't an exposed structure
> name which lets you avoid the #ifdef too.

Right

>
>> }
>>
>> static void seccomp_init_siginfo(siginfo_t *info, int syscall, int reason)
>> @@ -813,6 +817,10 @@ static long do_seccomp(unsigned int op, unsigned int flags,
>> return seccomp_set_mode_strict();
>> case SECCOMP_SET_MODE_FILTER:
>> return seccomp_set_mode_filter(flags, uargs);
>> +#if defined(CONFIG_SECCOMP_FILTER) && defined(CONFIG_SECURITY_LANDLOCK)
>> + case SECCOMP_APPEND_LANDLOCK_RULE:
>> + return landlock_seccomp_append_prog(flags, uargs);
>> +#endif /* CONFIG_SECCOMP_FILTER && CONFIG_SECURITY_LANDLOCK */
>> default:
>> return -EINVAL;
>> }
>> diff --git a/security/landlock/Makefile b/security/landlock/Makefile
>> index c0db504a6335..da8ba8b5183e 100644
>> --- a/security/landlock/Makefile
>> +++ b/security/landlock/Makefile
>> @@ -2,4 +2,4 @@ ccflags-$(CONFIG_SECURITY_LANDLOCK) += -Werror=unused-function
>>
>> obj-$(CONFIG_SECURITY_LANDLOCK) := landlock.o
>>
>> -landlock-y := init.o hooks.o hooks_fs.o
>> +landlock-y := init.o providers.o hooks.o hooks_fs.o
>> diff --git a/security/landlock/hooks.c b/security/landlock/hooks.c
>> index eaee8162ff70..4fa7d0b38d41 100644
>> --- a/security/landlock/hooks.c
>> +++ b/security/landlock/hooks.c
>> @@ -95,6 +95,38 @@ bool landlock_is_valid_access(int off, int size, enum bpf_access_type type,
>> return true;
>> }
>>
>> +/**
>> + * landlock_event_deny - run Landlock rules tied to an event
>> + *
>> + * @event_idx: event index in the rules array
>> + * @ctx: non-NULL eBPF context
>> + * @events: Landlock events pointer
>> + *
>> + * Return true if at least one rule deny the event.
>> + */
>> +static bool landlock_event_deny(u32 event_idx, const struct landlock_context *ctx,
>> + struct landlock_events *events)
>> +{
>> + struct landlock_rule *rule;
>> +
>> + if (!events)
>> + return false;
>> +
>> + for (rule = events->rules[event_idx]; rule; rule = rule->prev) {
>> + u32 ret;
>> +
>> + if (WARN_ON(!rule->prog))
>> + continue;
>> + rcu_read_lock();
>> + ret = BPF_PROG_RUN(rule->prog, (void *)ctx);
>> + rcu_read_unlock();
>> + /* deny access if a program returns a value different than 0 */
>> + if (ret)
>> + return true;
>> + }
>> + return false;
>> +}
>> +
>> int landlock_decide(enum landlock_subtype_event event,
>> __u64 ctx_values[CTX_ARG_NB], u32 cmd, const char *hook)
>> {
>> @@ -111,5 +143,10 @@ int landlock_decide(enum landlock_subtype_event event,
>> .arg2 = ctx_values[1],
>> };
>>
>> +#ifdef CONFIG_SECCOMP_FILTER
>> + deny = landlock_event_deny(event_idx, &ctx,
>> + current->seccomp.landlock_events);
>> +#endif /* CONFIG_SECCOMP_FILTER */
>
> Isn't ..._FILTER required?

Same as above, this is required for now but could change in the near
future. :)

>
>> +
>> return deny ? -EPERM : 0;
>> }
>> diff --git a/security/landlock/hooks.h b/security/landlock/hooks.h
>> index 2e180f6ed86b..dd0486a4c284 100644
>> --- a/security/landlock/hooks.h
>> +++ b/security/landlock/hooks.h
>> @@ -12,6 +12,7 @@
>> #include <linux/bpf.h> /* enum bpf_access_type */
>> #include <linux/lsm_hooks.h>
>> #include <linux/sched.h> /* struct task_struct */
>> +#include <linux/seccomp.h>
>>
>> /* separators */
>> #define SEP_COMMA() ,
>> @@ -163,7 +164,11 @@ WRAP_TYPE_RAW_C;
>>
>> static inline bool landlocked(const struct task_struct *task)
>> {
>> +#ifdef CONFIG_SECCOMP_FILTER
>> + return !!(task->seccomp.landlock_events);
>> +#else
>> return false;
>> +#endif /* CONFIG_SECCOMP_FILTER */
>> }
>>
>> __init void landlock_register_hooks(struct security_hook_list *hooks, int count);
>> diff --git a/security/landlock/init.c b/security/landlock/init.c
>> index 1c2750e12dfa..ef8a3da69860 100644
>> --- a/security/landlock/init.c
>> +++ b/security/landlock/init.c
>> @@ -135,7 +135,8 @@ static struct bpf_prog_type_list bpf_landlock_type __ro_after_init = {
>>
>> void __init landlock_add_hooks(void)
>> {
>> - pr_info("landlock: Version %u", LANDLOCK_VERSION);
>> + pr_info("landlock: Version %u, ready to sandbox with %s\n",
>> + LANDLOCK_VERSION, "seccomp");
>> landlock_add_hooks_fs();
>> security_add_hooks(NULL, 0, "landlock");
>> bpf_register_prog_type(&bpf_landlock_type);
>> diff --git a/security/landlock/providers.c b/security/landlock/providers.c
>> new file mode 100644
>> index 000000000000..6d867a39c947
>> --- /dev/null
>> +++ b/security/landlock/providers.c
>> @@ -0,0 +1,232 @@
>> +/*
>> + * Landlock LSM - seccomp provider
>> + *
>> + * Copyright Â 2017 MickaÃl SalaÃn <mic@xxxxxxxxxxx>
>> + *
>> + * This program is free software; you can redistribute it and/or modify
>> + * it under the terms of the GNU General Public License version 2, as
>> + * published by the Free Software Foundation.
>> + */
>> +
>> +#include <asm/page.h> /* PAGE_SIZE */
>> +#include <linux/atomic.h> /* atomic_*(), smp_store_release() */
>> +#include <linux/bpf.h> /* bpf_prog_put() */
>> +#include <linux/filter.h> /* struct bpf_prog */
>> +#include <linux/kernel.h> /* round_up() */
>> +#include <linux/landlock.h>
>> +#include <linux/sched.h> /* current_cred(), task_no_new_privs() */
>> +#include <linux/security.h> /* security_capable_noaudit() */
>> +#include <linux/slab.h> /* alloc(), kfree() */
>> +#include <linux/types.h> /* atomic_t */
>> +#include <linux/uaccess.h> /* copy_from_user() */
>> +
>> +#include "common.h"
>> +
>> +static void put_landlock_rule(struct landlock_rule *rule)
>> +{
>> + struct landlock_rule *orig = rule;
>> +
>> + /* clean up single-reference branches iteratively */
>> + while (orig && atomic_dec_and_test(&orig->usage)) {
>> + struct landlock_rule *freeme = orig;
>> +
>> + bpf_prog_put(orig->prog);
>> + orig = orig->prev;
>> + kfree(freeme);
>> + }
>> +}
>> +
>> +void put_landlock_events(struct landlock_events *events)
>> +{
>> + if (events && atomic_dec_and_test(&events->usage)) {
>> + size_t i;
>> +
>> + for (i = 0; i < ARRAY_SIZE(events->rules); i++)
>> + /* XXX: Do we need to use lockless_dereference() here? */
>> + put_landlock_rule(events->rules[i]);
>> + kfree(events);
>> + }
>> +}
>> +
>> +static struct landlock_events *new_landlock_events(void)
>> +{
>> + struct landlock_events *ret;
>> +
>> + /* array filled with NULL values */
>> + ret = kzalloc(sizeof(*ret), GFP_KERNEL);
>> + if (!ret)
>> + return ERR_PTR(-ENOMEM);
>> + atomic_set(&ret->usage, 1);
>> + return ret;
>> +}
>> +
>> +static void add_landlock_rule(struct landlock_events *events,
>> + struct landlock_rule *rule)
>> +{
>> + /* subtype.landlock_rule.event > 0 for loaded programs */
>> + u32 event_idx = get_index(rule->prog->subtype.landlock_rule.event);
>> +
>> + rule->prev = events->rules[event_idx];
>> + WARN_ON(atomic_read(&rule->usage));
>> + atomic_set(&rule->usage, 1);
>> + /* do not increment the previous rule usage */
>> + smp_store_release(&events->rules[event_idx], rule);
>> +}
>> +
>> +/* limit Landlock events to 256KB */
>> +#define LANDLOCK_EVENTS_MAX_PAGES (1 << 6)
>> +
>> +/**
>> + * landlock_append_prog - attach a Landlock rule to @current_events
>> + *
>> + * @current_events: landlock_events pointer, must be locked (if needed) to
>> + * prevent a concurrent put/free. This pointer must not be
>> + * freed after the call.
>> + * @prog: non-NULL Landlock rule to append to @current_events. @prog will be
>> + * owned by landlock_append_prog() and freed if an error happened.
>> + *
>> + * Return @current_events or a new pointer when OK. Return a pointer error
>> + * otherwise.
>> + */
>> +static struct landlock_events *landlock_append_prog(
>> + struct landlock_events *current_events, struct bpf_prog *prog)
>> +{
>> + struct landlock_events *new_events = current_events;
>> + unsigned long pages;
>> + struct landlock_rule *rule;
>> + u32 event_idx;
>> +
>> + if (prog->type != BPF_PROG_TYPE_LANDLOCK) {
>> + new_events = ERR_PTR(-EINVAL);
>> + goto put_prog;
>> + }
>> +
>> + /* validate memory size allocation */
>> + pages = prog->pages;
>> + if (current_events) {
>> + size_t i;
>> +
>> + for (i = 0; i < ARRAY_SIZE(current_events->rules); i++) {
>> + struct landlock_rule *walker_r;
>> +
>> + for (walker_r = current_events->rules[i]; walker_r;
>> + walker_r = walker_r->prev)
>> + pages += walker_r->prog->pages;
>> + }
>> + /* count a struct landlock_events if we need to allocate one */
>> + if (atomic_read(&current_events->usage) != 1)
>> + pages += round_up(sizeof(*current_events), PAGE_SIZE) /
>> + PAGE_SIZE;
>> + }
>> + if (pages > LANDLOCK_EVENTS_MAX_PAGES) {
>> + new_events = ERR_PTR(-E2BIG);
>> + goto put_prog;
>> + }
>> +
>> + rule = kzalloc(sizeof(*rule), GFP_KERNEL);
>> + if (!rule) {
>> + new_events = ERR_PTR(-ENOMEM);
>> + goto put_prog;
>> + }
>> + rule->prog = prog;
>> +
>> + /* subtype.landlock_rule.event > 0 for loaded programs */
>> + event_idx = get_index(rule->prog->subtype.landlock_rule.event);
>> +
>> + if (!new_events) {
>> + /*
>> + * If there is no Landlock events used by the current task,
>> + * then create a new one.
>> + */
>> + new_events = new_landlock_events();
>> + if (IS_ERR(new_events))
>> + goto put_rule;
>
> Shouldn't bpf_prog_put() get called in the face of a rule failure too?
> Why separate exit paths?

You're right but put_landlock_rule() call bpf_prog_put() by itself.

>
>> + } else if (atomic_read(&current_events->usage) > 1) {
>> + /*
>> + * If the current task is not the sole user of its Landlock
>> + * events, then duplicate them.
>> + */
>> + size_t i;
>> +
>> + new_events = new_landlock_events();
>> + if (IS_ERR(new_events))
>> + goto put_rule;
>> + for (i = 0; i < ARRAY_SIZE(new_events->rules); i++) {
>> + new_events->rules[i] =
>> + lockless_dereference(current_events->rules[i]);
>> + if (new_events->rules[i])
>> + atomic_inc(&new_events->rules[i]->usage);
>
> I was going to ask: isn't the top-level usage counter sufficient to
> track rule lifetime? But I think I see how things are tracked now:
> each task_struct points to an array of rule list pointers. These
> tables are duplicated when additions are made (which means each table
> needs to be refcounted for the processes using it), and when a new
> table is created, all the refcounters on the rules are bumped (to
> track each table that references the rule), and when a new rule is
> added, it's just prepended to the list for the new table to point at.

That's right.

>
> Does this mean that rules are processed in reverse?

Yes, the rules are processed from the newest to the oldest, as
seccomp-bpf does with filters.

>
>> + }
>> +
>> + /*
>> + * Landlock events from the current task will not be freed here
>> + * because the usage is strictly greater than 1. It is only
>> + * prevented to be freed by another subject thanks to the
>> + * caller of landlock_append_prog() which should be locked if
>> + * needed.
>> + */
>> + put_landlock_events(current_events);
>> + }
>> + add_landlock_rule(new_events, rule);
>> + return new_events;
>> +
>> +put_prog:
>> + bpf_prog_put(prog);
>> + return new_events;
>> +
>> +put_rule:
>> + put_landlock_rule(rule);
>> + return new_events;
>> +}
>> +
>> +/**
>> + * landlock_seccomp_append_prog - attach a Landlock rule to the current process
>> + *
>> + * current->seccomp.landlock_events is lazily allocated. When a process fork,
>> + * only a pointer is copied. When a new event is added by a process, if there
>> + * is other references to this process' landlock_events, then a new allocation
>> + * is made to contain an array pointing to Landlock rule lists. This design
>> + * enable low-performance impact and is memory efficient while keeping the
>> + * property of append-only rules.
>> + *
>> + * @flags: not used for now, but could be used for TSYNC
>> + * @user_bpf_fd: file descriptor pointing to a loaded Landlock rule
>> + */
>> +#ifdef CONFIG_SECCOMP_FILTER
>> +int landlock_seccomp_append_prog(unsigned int flags,
>> + const char __user *user_bpf_fd)
>> +{
>> + struct landlock_events *new_events;
>> + struct bpf_prog *prog;
>> + int bpf_fd;
>> +
>> + /* force no_new_privs to limit privilege escalation */
>> + if (!task_no_new_privs(current))
>> + return -EPERM;
>> + /* will be removed in the future to allow unprivileged tasks */
>> + if (!capable(CAP_SYS_ADMIN))
>> + return -EPERM;
>> + /* enable to check if Landlock is supported with early EFAULT */
>> + if (!user_bpf_fd)
>> + return -EFAULT;
>> + if (flags)
>> + return -EINVAL;
>> + if (copy_from_user(&bpf_fd, user_bpf_fd, sizeof(bpf_fd)))
>> + return -EFAULT;
>
> I think this can just be get_user()?

Yes, I didn't know about that.

>
>> + prog = bpf_prog_get(bpf_fd);
>> + if (IS_ERR(prog))
>> + return PTR_ERR(prog);
>> +
>> + /*
>> + * We don't need to lock anything for the current process hierarchy,
>> + * everything is guarded by the atomic counters.
>> + */
>> + new_events = landlock_append_prog(current->seccomp.landlock_events,
>> + prog);
>> + /* @prog is managed/freed by landlock_append_prog() */
>
> Does kmemcheck notice this "leak"? (i.e. is further annotation needed?)

I didn't enable kmemcheck, I will take a look at it.

>
>> + if (IS_ERR(new_events))
>> + return PTR_ERR(new_events);
>> + current->seccomp.landlock_events = new_events;
>> + return 0;
>> +}
>> +#endif /* CONFIG_SECCOMP_FILTER */
>> --
>> 2.11.0
>>
>
>
>

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