Re: [PATCH v4] proc: add config & param to block forcing mem writes

[Jeff Xu]

On Tue, Jul 30, 2024 at 6:25 AM Adrian Ratiu <adrian.ratiu@xxxxxxxxxxxxx> wrote:
>
> This adds a Kconfig option and boot param to allow removing
> the FOLL_FORCE flag from /proc/pid/mem write calls because
> it can be abused.
>
> The traditional forcing behavior is kept as default because
> it can break GDB and some other use cases.
>
> Previously we tried a more sophisticated approach allowing
> distributions to fine-tune /proc/pid/mem behavior, however
> that got NAK-ed by Linus [1], who prefers this simpler
> approach with semantics also easier to understand for users.
>
> Link: https://lore.kernel.org/lkml/CAHk-=wiGWLChxYmUA5HrT5aopZrB7_2VTa0NLZcxORgkUe5tEQ@xxxxxxxxxxxxxx/ [1]
> Cc: Doug Anderson <dianders@xxxxxxxxxxxx>
> Cc: Jeff Xu <jeffxu@xxxxxxxxxx>
> Cc: Jann Horn <jannh@xxxxxxxxxx>
> Cc: Kees Cook <kees@xxxxxxxxxx>
> Cc: Ard Biesheuvel <ardb@xxxxxxxxxx>
> Cc: Christian Brauner <brauner@xxxxxxxxxx>
> Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Adrian Ratiu <adrian.ratiu@xxxxxxxxxxxxx>
> ---
> Changes in v4:
> * Fixed doc punctuation, used passive tense, improved
>   wording consistency, fixed default value wording
> * Made struct constant_table a static const __initconst
> * Reworked proc_mem_foll_force() indentation and var
>   declarations to make code clearer
> * Reworked enum + struct definition so lookup_constant()
>   defaults to 'always'.
>
> Changes in v3:
> * Simplified code to use shorthand ifs and a
>   lookup_constant() table
>
> Changes in v2:
> * Added bootparam on top of Linus' patch
> * Slightly reworded commit msg
> ---
>  .../admin-guide/kernel-parameters.txt         | 10 ++++
>  fs/proc/base.c                                | 54 ++++++++++++++++++-
>  security/Kconfig                              | 32 +++++++++++
>  3 files changed, 95 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index f1384c7b59c9..8396e015aab3 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -4788,6 +4788,16 @@
>         printk.time=    Show timing data prefixed to each printk message line
>                         Format: <bool>  (1/Y/y=enable, 0/N/n=disable)
>
> +       proc_mem.force_override= [KNL]
> +                       Format: {always | ptrace | never}
> +                       Traditionally /proc/pid/mem allows memory permissions to be
> +                       overridden without restrictions. This option may be set to
> +                       restrict that. Can be one of:
> +                       - 'always': traditional behavior always allows mem overrides.
> +                       - 'ptrace': only allow mem overrides for active ptracers.
> +                       - 'never':  never allow mem overrides.
> +                       If not specified, default is the CONFIG_PROC_MEM_* choice.
> +
>         processor.max_cstate=   [HW,ACPI]
>                         Limit processor to maximum C-state
>                         max_cstate=9 overrides any DMI blacklist limit.
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 72a1acd03675..daacb8070042 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -85,6 +85,7 @@
>  #include <linux/elf.h>
>  #include <linux/pid_namespace.h>
>  #include <linux/user_namespace.h>
> +#include <linux/fs_parser.h>
>  #include <linux/fs_struct.h>
>  #include <linux/slab.h>
>  #include <linux/sched/autogroup.h>
> @@ -117,6 +118,35 @@
>  static u8 nlink_tid __ro_after_init;
>  static u8 nlink_tgid __ro_after_init;
>
> +enum proc_mem_force {
> +       PROC_MEM_FORCE_ALWAYS,
> +       PROC_MEM_FORCE_PTRACE,
> +       PROC_MEM_FORCE_NEVER
> +};
> +
> +static enum proc_mem_force proc_mem_force_override __ro_after_init =
> +       IS_ENABLED(CONFIG_PROC_MEM_NO_FORCE) ? PROC_MEM_FORCE_NEVER :
> +       IS_ENABLED(CONFIG_PROC_MEM_FORCE_PTRACE) ? PROC_MEM_FORCE_PTRACE :
> +       PROC_MEM_FORCE_ALWAYS;
> +
> +static const struct constant_table proc_mem_force_table[] __initconst = {
> +       { "never", PROC_MEM_FORCE_NEVER },
> +       { "ptrace", PROC_MEM_FORCE_PTRACE },
> +       { }
> +};
> +
> +static int __init early_proc_mem_force_override(char *buf)
> +{
> +       if (!buf)
> +               return -EINVAL;
> +
> +       proc_mem_force_override = lookup_constant(proc_mem_force_table,
> +                                                 buf, PROC_MEM_FORCE_ALWAYS);
proc_mem_force_table has two entries, this means:
if kernel cmdline has proc_mem.force_override="invalid",
    PROC_MEM_FORCE_ALWAYS will be used.

Another option is to have 3 entries in proc_mem_force_table: adding
{"aways", PROC_MEM_FORCE_ALWAYS}

and let lookup_constant return -1 when not found, and not override
proc_mem_force_override.

This enforces the kernel cmd line must be set to one of three choices
"always|ptrace|never" to be effective.

If you choose this path: please modify kernel-parameters.txt to
"If not specified or invalid, default is the CONFIG_PROC_MEM_* choice."

or else please clarify in the kernel-parameters.text:
If not specified, default is the CONFIG_PROC_MEM_* choice
If invalid str or empty string, PROC_MEM_FORCE_ALWAYS will be used
regardless CONFIG_PROC_MEM_* choice

> +
> +       return 0;
> +}
> +early_param("proc_mem.force_override", early_proc_mem_force_override);
> +
>  struct pid_entry {
>         const char *name;
>         unsigned int len;
> @@ -835,6 +865,26 @@ static int mem_open(struct inode *inode, struct file *file)
>         return ret;
>  }
>
> +static bool proc_mem_foll_force(struct file *file, struct mm_struct *mm)
> +{
> +       struct task_struct *task;
> +       bool ptrace_active = false;
> +
> +       switch (proc_mem_force_override) {
> +       case PROC_MEM_FORCE_NEVER:
> +               return false;
> +       case PROC_MEM_FORCE_PTRACE:
> +               task = get_proc_task(file_inode(file));
> +               if (task) {
> +                       ptrace_active = task->ptrace && task->mm == mm && task->parent == current;
Do we need to call "read_lock(&tasklist_lock);" ?
see comments in ptrace_check_attach() of  kernel/ptrace.c



> +                       put_task_struct(task);
> +               }
> +               return ptrace_active;
> +       default:
> +               return true;
> +       }
> +}
> +
>  static ssize_t mem_rw(struct file *file, char __user *buf,
>                         size_t count, loff_t *ppos, int write)
>  {
> @@ -855,7 +905,9 @@ static ssize_t mem_rw(struct file *file, char __user *buf,
>         if (!mmget_not_zero(mm))
>                 goto free;
>
> -       flags = FOLL_FORCE | (write ? FOLL_WRITE : 0);
> +       flags = write ? FOLL_WRITE : 0;
> +       if (proc_mem_foll_force(file, mm))
> +               flags |= FOLL_FORCE;
>
>         while (count > 0) {
>                 size_t this_len = min_t(size_t, count, PAGE_SIZE);
> diff --git a/security/Kconfig b/security/Kconfig
> index 412e76f1575d..a93c1a9b7c28 100644
> --- a/security/Kconfig
> +++ b/security/Kconfig
> @@ -19,6 +19,38 @@ config SECURITY_DMESG_RESTRICT
>
>           If you are unsure how to answer this question, answer N.
>
> +choice
> +       prompt "Allow /proc/pid/mem access override"
> +       default PROC_MEM_ALWAYS_FORCE
> +       help
> +         Traditionally /proc/pid/mem allows users to override memory
> +         permissions for users like ptrace, assuming they have ptrace
> +         capability.
> +
> +         This allows people to limit that - either never override, or
> +         require actual active ptrace attachment.
> +
> +         Defaults to the traditional behavior (for now)
> +
> +config PROC_MEM_ALWAYS_FORCE
> +       bool "Traditional /proc/pid/mem behavior"
> +       help
> +         This allows /proc/pid/mem accesses to override memory mapping
> +         permissions if you have ptrace access rights.
> +
> +config PROC_MEM_FORCE_PTRACE
> +       bool "Require active ptrace() use for access override"
> +       help
> +         This allows /proc/pid/mem accesses to override memory mapping
> +         permissions for active ptracers like gdb.
> +
> +config PROC_MEM_NO_FORCE
> +       bool "Never"
> +       help
> +         Never override memory mapping permissions
> +
> +endchoice
> +
>  config SECURITY
>         bool "Enable different security models"
>         depends on SYSFS
> --
> 2.44.2
>