Re: [PATCH v3 1/5] libselinux: avc: use pthread mutexes by default when no lock callbacks set

On Mon, Jul 6, 2026 at 10:32 AM Stephen Smalley
<[email protected]> wrote:
>
> The AVC has a set of locking calls but defaults to no locking, and
> only the deprecated avc_init() allowed applications to set their own
> locking callbacks. Since selinux_check_access() is now commonly used
> by applications implementing userspace SELinux permission checks and
> it uses avc_open() internally, such calls are currently not
> thread-safe.  Provide fallbacks to using pthread mutexes by
> default. Single-threaded programs that do not link -lpthread will be
> unaffected due to the existing __pthread_mutex() helpers. Applications
> that explicitly specify locking callbacks via the deprecated
> avc_init() will also be unaffected.
>
> This is the first step toward a thread-safe selinux_check_access();
> the class-string cache and status-page transition handling are
> addressed in follow-up changes.
>
> Fixes: #287 #335 #336
> Link: https://lore.kernel.org/selinux/CAJsHiNx1E7x1jaBkS0i4L1nBWXp8YXLHRWcCaDaH4LOn=zm+Zw@mail.gmail.com/
> Link: https://lore.kernel.org/selinux/[email protected]/
> Reported-by: Seth Moore <[email protected]>
> Reported-by: Purushottam Choudhary <[email protected]>
>
> Signed-off-by: Stephen Smalley <[email protected]>

For these five patches:
Acked-by: James Carter <[email protected]>


> ---
>  libselinux/src/avc_internal.h | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/libselinux/src/avc_internal.h b/libselinux/src/avc_internal.h
> index 719be934..3c21ffef 100644
> --- a/libselinux/src/avc_internal.h
> +++ b/libselinux/src/avc_internal.h
> @@ -14,6 +14,7 @@
>  #include <string.h>
>  #include <selinux/avc.h>
>  #include "callbacks.h"
> +#include "selinux_internal.h"
>
>  /* callback pointers */
>  extern void *(*avc_func_malloc)(size_t);
> @@ -115,25 +116,41 @@ static inline void avc_stop_thread(void *thread)
>
>  static inline void *avc_alloc_lock(void)
>  {
> -       return avc_func_alloc_lock ? avc_func_alloc_lock() : NULL;
> +       pthread_mutex_t *m;
> +
> +       if (avc_func_alloc_lock)
> +               return avc_func_alloc_lock();
> +
> +       m = avc_malloc(sizeof(*m));
> +       if (m)
> +               __pthread_mutex_init(m, NULL);
> +       return m;
>  }
>
>  static inline void avc_get_lock(void *lock)
>  {
>         if (avc_func_get_lock)
>                 avc_func_get_lock(lock);
> +       else if (lock)
> +               __pthread_mutex_lock((pthread_mutex_t *)lock);
>  }
>
>  static inline void avc_release_lock(void *lock)
>  {
>         if (avc_func_release_lock)
>                 avc_func_release_lock(lock);
> +       else if (lock)
> +               __pthread_mutex_unlock((pthread_mutex_t *)lock);
>  }
>
>  static inline void avc_free_lock(void *lock)
>  {
>         if (avc_func_free_lock)
>                 avc_func_free_lock(lock);
> +       else if (lock) {
> +               __pthread_mutex_destroy((pthread_mutex_t *)lock);
> +               avc_free(lock);
> +       }
>  }
>
>  /* statistics helper routines */
> --
> 2.54.0
>