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

[Adrian Ratiu]

On Wednesday, July 31, 2024 02:08 EEST, Jeff Xu <jeffxu@xxxxxxxxxx> wrote:

> 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.

Thanks Jeff for spotting this! :)

In addition to adding all the 3 entries as you suggested, I think we
can also do the following:

proc_mem_force_override = lookup_constant(proc_mem_force_table,
                                                 buf, proc_mem_force_override);

This will ensure that if something like "invalid" gets passed, the
proc_mem_force_override value remains unchanged. In other words
it remains equal to the default choice set via Kconfig and correctly
matches the doc description.

I'll address this before sending v5 in a few days to give others time
to review.

> 
> 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
> >