Re: [PATCH 3/4] mm: synchronize saved_auxv access with arg_lock

[Michal Koutný]

On Mon, Feb 09, 2026 at 07:06:04PM +0000, Andrei Vagin <avagin@xxxxxxxxxx> wrote:
> The mm->saved_auxv array stores the auxiliary vector, which can be
> modified via prctl(PR_SET_MM_AUXV) or prctl(PR_SET_MM_MAP). Previously,
> accesses to saved_auxv were not synchronized. This was a intentional
> trade-off, as the vector was only used to provide information to
> userspace via /proc/PID/auxv or prctl(PR_GET_AUXV), and consistency
> between the auxv values left to userspace.
> 
> With the introduction of hardware capability (HWCAP) inheritance during
> execve, the kernel now relies on the contents of saved_auxv to configure
> the execution environment of new processes.  An unsynchronized read
> during execve could result in a new process inheriting an inconsistent
> set of capabilities if the parent process updates its auxiliary vector
> concurrently.
> 
> While it is still not strictly required to guarantee the consistency of
> auxv values on the kernel side, doing so is relatively straightforward.
> This change implements synchronization using arg_lock.

(For the clarification, I didn't consider the lack of synchronization a
blocker after your previous explanation. Nevertheless)

Thanks for explicit sync.

One little nit is a missing hunk like below.


--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1205,11 +1205,10 @@ struct mm_struct {
                spinlock_t arg_lock; /* protect the below fields */

                unsigned long start_code, end_code, start_data, end_data;
                unsigned long start_brk, brk, start_stack;
                unsigned long arg_start, arg_end, env_start, env_end;
-
                unsigned long saved_auxv[AT_VECTOR_SIZE]; /* for /proc/PID/auxv */

 #ifdef CONFIG_ARCH_HAS_ELF_CORE_EFLAGS
                /* the ABI-related flags from the ELF header. Used for core dump */
                unsigned long saved_e_flags;



> 
> Signed-off-by: Andrei Vagin <avagin@xxxxxxxxxx>
> ---
>  fs/exec.c      |  8 ++++++--
>  fs/proc/base.c | 12 +++++++++---
>  kernel/fork.c  |  7 ++++++-
>  kernel/sys.c   | 29 ++++++++++++++---------------
>  4 files changed, 35 insertions(+), 21 deletions(-)

I can say
Reviewed-by: Michal Koutný <mkoutny@xxxxxxxx>

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