Re: [PATCH] selinux: optimize major part with a kernel config in selinux_mmap_addr()

[Paul Moore]

On Mon, Jul 10, 2023 at 4:25 AM Leesoo Ahn <lsahn@xxxxxxxxxx> wrote:
>
> The major part, the conditional branch in selinux_mmap_addr() is always to be
> false so long as CONFIG_LSM_MMAP_MIN_ADDR is set to zero at compile time.
>
> This usually happens in some linux distros, for instance Ubuntu, which
> the config is set to zero in release version. Therefore it could be a bit
> optimized with '#if <expr>' at compile time.
>
> Signed-off-by: Leesoo Ahn <lsahn@xxxxxxxxxxxxxx>
> ---
>  security/selinux/hooks.c | 2 ++
>  1 file changed, 2 insertions(+)

First, I agree with Stephen's comments that you should ask your distro
(you mentioned Debian) to move MIN_ADDR higher.  Beyond that, I have
one request, see below ...

> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index d06e350fedee..a049aab6524b 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -3723,11 +3723,13 @@ static int selinux_mmap_addr(unsigned long addr)
>  {
>         int rc = 0;
>
> +#if CONFIG_LSM_MMAP_MIN_ADDR > 0
>         if (addr < CONFIG_LSM_MMAP_MIN_ADDR) {
>                 u32 sid = current_sid();
>                 rc = avc_has_perm(sid, sid, SECCLASS_MEMPROTECT,
>                                   MEMPROTECT__MMAP_ZERO, NULL);
>         }
> +#endif
>
>         return rc;
>  }

Pre-processor conditionals inside a function are generally something
we don't recommend.  In this case I would suggest doing something like
this:

#if (MMAP_MIN_ADDR > 0)
static int selinux_mmap_addr(...)
{
  /* current func definition */
}
#else /* MMAP_MIN_ADDR > 0 */
static int selinux_mmap_addr(...)
{
  return 0;
}
#endif /* MMAP_MIN_ADDR > 0 */

-- 
paul-moore.com