Re: [PATCH v2 3/5] memfd: improve userspace warnings for missing exec-related flags

[Damian Tometzki <dtometzki@fedoraproject.org>]

On Mon, 14. Aug 18:40, Aleksa Sarai wrote:
> In order to incentivise userspace to switch to passing MFD_EXEC and
> MFD_NOEXEC_SEAL, we need to provide a warning on each attempt to call
> memfd_create() without the new flags. pr_warn_once() is not useful
> because on most systems the one warning is burned up during the boot
> process (on my system, systemd does this within the first second of
> boot) and thus userspace will in practice never see the warnings to push
> them to switch to the new flags.
> 
> The original patchset[1] used pr_warn_ratelimited(), however there were
> concerns about the degree of spam in the kernel log[2,3]. The resulting
> inability to detect every case was flagged as an issue at the time[4].
> 
> While we could come up with an alternative rate-limiting scheme such as
> only outputting the message if vm.memfd_noexec has been modified, or
> only outputting the message once for a given task, these alternatives
> have downsides that don't make sense given how low-stakes a single
> kernel warning message is. Switching to pr_info_ratelimited() instead
> should be fine -- it's possible some monitoring tool will be unhappy
> with a stream of warning-level messages but there's already plenty of
> info-level message spam in dmesg.
> 
> [1]: https://lore.kernel.org/20221215001205.51969-4-jeffxu@google.com/
> [2]: https://lore.kernel.org/202212161233.85C9783FB@keescook/
> [3]: https://lore.kernel.org/Y5yS8wCnuYGLHMj4@x1n/
> [4]: https://lore.kernel.org/f185bb42-b29c-977e-312e-3349eea15383@linuxfoundation.org/
> 
> Cc: stable@vger.kernel.org # v6.3+
> Fixes: 105ff5339f49 ("mm/memfd: add MFD_NOEXEC_SEAL and MFD_EXEC")
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> ---
>  mm/memfd.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/memfd.c b/mm/memfd.c
> index d65485c762de..aa46521057ab 100644
> --- a/mm/memfd.c
> +++ b/mm/memfd.c
> @@ -315,7 +315,7 @@ SYSCALL_DEFINE2(memfd_create,
>  		return -EINVAL;
>  
>  	if (!(flags & (MFD_EXEC | MFD_NOEXEC_SEAL))) {
> -		pr_warn_once(
> +		pr_info_ratelimited(
>  			"%s[%d]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set\n",
>  			current->comm, task_pid_nr(current));
>  	}
> 
> -- 
> 2.41.0
>
Hello Sarai,

i got a lot of messages in dmesg with this. DMESG is unuseable with
this. 
[ 1390.349462] __do_sys_memfd_create: 5 callbacks suppressed
[ 1390.349468] pipewire-pulse[2930]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1390.350106] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1390.350366] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1390.359390] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1390.359453] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1390.848813] pipewire-pulse[2930]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1390.849425] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1390.849673] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1390.857629] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1390.857674] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1404.819637] __do_sys_memfd_create: 105 callbacks suppressed
[ 1404.819641] pipewire-pulse[2930]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1404.819950] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1404.820054] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1404.824240] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1404.824279] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1430.373186] pipewire-pulse[2930]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1430.373906] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1430.374131] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1430.382397] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1430.382485] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1430.499581] pipewire-pulse[2930]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1430.500077] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1430.500265] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1430.512772] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1430.512840] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1444.388519] __do_sys_memfd_create: 60 callbacks suppressed
[ 1444.388525] pipewire-pulse[2930]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1444.389061] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1444.389335] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1444.397909] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1444.397965] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1444.503514] pipewire-pulse[2930]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1444.503658] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1444.503726] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1444.507841] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1444.507870] pipewire[2712]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set
[ 1449.707966] __do_sys_memfd_create: 25 callbacks suppressed

Best regards
Damian