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

Alexander Mikhalitsyn
2026-02-10T20:36:58Z

Am Mo., 9. Feb. 2026 um 20:06 Uhr schrieb Andrei Vagin <avagin@google.com>:
>
> 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.
>
> Signed-off-by: Andrei Vagin <avagin@google.com>

Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@futurfusion.io>

> ---
> fs/exec.c | 8 ++++++--
> fs/proc/base.c | 12 +++++++++---
> kernel/fork.c | 7 ++++++-
> kernel/sys.c | 29 ++++++++++++++---------------
> 4 files changed, 35 insertions(+), 21 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index 7401efbe4ba0..d7e3ad8c8051 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1793,6 +1793,7 @@ static int bprm_execve(struct linux_binprm *bprm)
>
> static void inherit_hwcap(struct linux_binprm *bprm)
> {
> + struct mm_struct *mm = current->mm;
> int i, n;
>
> #ifdef ELF_HWCAP4
> @@ -1805,10 +1806,12 @@ static void inherit_hwcap(struct linux_binprm *bprm)
> n = 1;
> #endif
>
> + spin_lock(&mm->arg_lock);
> for (i = 0; n && i < AT_VECTOR_SIZE; i += 2) {
> - long val = current->mm->saved_auxv[i + 1];
> + unsigned long type = mm->saved_auxv[i];
> + unsigned long val = mm->saved_auxv[i + 1];
>
> - switch (current->mm->saved_auxv[i]) {
> + switch (type) {
> case AT_NULL:
> goto done;
> case AT_HWCAP:
> @@ -1835,6 +1838,7 @@ static void inherit_hwcap(struct linux_binprm *bprm)
> n--;
> }
> done:
> + spin_unlock(&mm->arg_lock);
> mm_flags_set(MMF_USER_HWCAP, bprm->mm);
> }
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 4eec684baca9..09d887741268 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -1083,14 +1083,20 @@ static ssize_t auxv_read(struct file *file, char __user *buf,
> {
> struct mm_struct *mm = file->private_data;
> unsigned int nwords = 0;
> + unsigned long saved_auxv[AT_VECTOR_SIZE];
>
> if (!mm)
> return 0;
> +
> + spin_lock(&mm->arg_lock);
> + memcpy(saved_auxv, mm->saved_auxv, sizeof(saved_auxv));
> + spin_unlock(&mm->arg_lock);
> +
> do {
> nwords += 2;
> - } while (mm->saved_auxv[nwords - 2] != 0); /* AT_NULL */
> - return simple_read_from_buffer(buf, count, ppos, mm->saved_auxv,
> - nwords * sizeof(mm->saved_auxv[0]));
> + } while (saved_auxv[nwords - 2] != 0); /* AT_NULL */
> + return simple_read_from_buffer(buf, count, ppos, saved_auxv,
> + nwords * sizeof(saved_auxv[0]));
> }
>
> static const struct file_operations proc_auxv_operations = {
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 0091315643de..c0a3dd94df22 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1104,8 +1104,13 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
> __mm_flags_overwrite_word(mm, mmf_init_legacy_flags(flags));
> mm->def_flags = current->mm->def_flags & VM_INIT_DEF_MASK;
>
> - if (mm_flags_test(MMF_USER_HWCAP, current->mm))
> + if (mm_flags_test(MMF_USER_HWCAP, current->mm)) {
> + spin_lock(&current->mm->arg_lock);
> mm_flags_set(MMF_USER_HWCAP, mm);
> + memcpy(mm->saved_auxv, current->mm->saved_auxv,
> + sizeof(mm->saved_auxv));

nit: I was looking for this memcpy(mm->saved_auxv,
current->mm->saved_auxv, sizeof(mm->saved_auxv)) while reviewing
a previous patch. Shouldn't it be there?

LGTM.

> + spin_unlock(&current->mm->arg_lock);
> + }
> } else {
> __mm_flags_overwrite_word(mm, default_dump_filter);
> mm->def_flags = 0;
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 6fbd7be21a5f..eafb5f75cb5c 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2147,20 +2147,11 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
> mm->arg_end = prctl_map.arg_end;
> mm->env_start = prctl_map.env_start;
> mm->env_end = prctl_map.env_end;
> - spin_unlock(&mm->arg_lock);
> -
> - /*
> - * Note this update of @saved_auxv is lockless thus
> - * if someone reads this member in procfs while we're
> - * updating -- it may get partly updated results. It's
> - * known and acceptable trade off: we leave it as is to
> - * not introduce additional locks here making the kernel
> - * more complex.
> - */
> if (prctl_map.auxv_size) {
> - memcpy(mm->saved_auxv, user_auxv, sizeof(user_auxv));
> mm_flags_set(MMF_USER_HWCAP, current->mm);
> + memcpy(mm->saved_auxv, user_auxv, sizeof(user_auxv));
> }
> + spin_unlock(&mm->arg_lock);
>
> mmap_read_unlock(mm);
> return 0;
> @@ -2190,10 +2181,10 @@ static int prctl_set_auxv(struct mm_struct *mm, unsigned long addr,
>
> BUILD_BUG_ON(sizeof(user_auxv) != sizeof(mm->saved_auxv));
>
> - task_lock(current);
> - memcpy(mm->saved_auxv, user_auxv, len);
> + spin_lock(&mm->arg_lock);
> mm_flags_set(MMF_USER_HWCAP, current->mm);
> - task_unlock(current);
> + memcpy(mm->saved_auxv, user_auxv, len);
> + spin_unlock(&mm->arg_lock);
>
> return 0;
> }
> @@ -2466,9 +2457,17 @@ static inline int prctl_get_mdwe(unsigned long arg2, unsigned long arg3,
> static int prctl_get_auxv(void __user *addr, unsigned long len)
> {
> struct mm_struct *mm = current->mm;
> + unsigned long auxv[AT_VECTOR_SIZE];
> unsigned long size = min_t(unsigned long, sizeof(mm->saved_auxv), len);
>
> - if (size && copy_to_user(addr, mm->saved_auxv, size))
> + if (!size)
> + return sizeof(mm->saved_auxv);
> +
> + spin_lock(&mm->arg_lock);
> + memcpy(auxv, mm->saved_auxv, size);
> + spin_unlock(&mm->arg_lock);
> +
> + if (copy_to_user(addr, auxv, size))
> return -EFAULT;
> return sizeof(mm->saved_auxv);
> }
> --
> 2.53.0.239.g8d8fc8a987-goog
>