[PATCH 3/3] selinux: require EXECMEM or PTRACE for FOLL_FORCE introspection

[Stephen Smalley]

On Tue, Aug 18, 2026 at 3:51 PM Jann Horn <jannh at google.com> wrote:
>
> On systems configured with PROC_MEM_FORCE_ALWAYS, ensure that a process can
> only create anonymous executable memory via /proc/self/mem if it has one
> of:
>
>  - EXECMEM (like for other methods of creating anonymous executable pages)
>  - PTRACE (like when using /proc/$pid/mem of another process)

Allowing it if _either_ permission is allowed is unusual in SELinux
and seems prone to errors.
Is there a reason to not just require PTRACE always?

>
> This closes a hole in EXECMEM enforcement that Project Zero has used in a
> remote Android exploit chain:
> It was possible to use a memory corruption bug in a service without EXECMEM
> permission to overwrite executable code via /proc/self/mem, which made it
> possible to load and run shellcode with a kernel exploit.
>
> Signed-off-by: Jann Horn <jannh at google.com>
> ---
>  security/selinux/hooks.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
>
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 18dd28b2bb13..905137c47321 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -2157,6 +2157,32 @@ static int selinux_ptrace_traceme(struct task_struct *parent)
>                             SECCLASS_PROCESS, PROCESS__PTRACE, NULL);
>  }
>
> +/*
> + * Decide whether it should be possible to read non-readable VMAs and write
> + * non-writable VMAs via /proc/self/mem.
> + * This only applies to systems configured with PROC_MEM_FORCE_ALWAYS, and only
> + * triggers on accesses that are not visible to selinux_ptrace_access_check().
> + *
> + * This allows a process to overwrite read-only code in its own address space.
> + *
> + * Creating an audit record on denial doesn't make sense here, since we can't
> + * tell whether FOLL_FORCE matters for the accessed VMAs.
> + */
> +static int selinux_introspect_mem_foll_force(const struct cred *subject)
> +{
> +       struct av_decision avd;
> +       int rc;
> +       u32 sid = cred_sid(subject);
> +
> +       /* Allow if the process is generally allowed to have executable anonymous memory. */
> +       rc = avc_has_perm_noaudit(sid, sid, SECCLASS_PROCESS, PROCESS__EXECMEM, 0, &avd);
> +
> +       /* Also allow if selinux_ptrace_access_check() would allow it. */
> +       if (rc)
> +               rc = avc_has_perm_noaudit(sid, sid, SECCLASS_PROCESS, PROCESS__PTRACE, 0, &avd);
> +       return rc;
> +}
> +
>  static int selinux_capget(const struct task_struct *target, kernel_cap_t *effective,
>                           kernel_cap_t *inheritable, kernel_cap_t *permitted)
>  {
> @@ -7558,6 +7584,7 @@ static struct security_hook_list selinux_hooks[] __ro_after_init = {
>
>         LSM_HOOK_INIT(ptrace_access_check, selinux_ptrace_access_check),
>         LSM_HOOK_INIT(ptrace_traceme, selinux_ptrace_traceme),
> +       LSM_HOOK_INIT(introspect_mem_foll_force, selinux_introspect_mem_foll_force),
>         LSM_HOOK_INIT(capget, selinux_capget),
>         LSM_HOOK_INIT(capset, selinux_capset),
>         LSM_HOOK_INIT(capable, selinux_capable),
>
> --
> 2.55.0.737.g08866a6d13-goog
>