2023-08-14 09:27:41

by Aleksa Sarai

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

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/[email protected]/
[2]: https://lore.kernel.org/202212161233.85C9783FB@keescook/
[3]: https://lore.kernel.org/Y5yS8wCnuYGLHMj4@x1n/
[4]: https://lore.kernel.org/[email protected]/

Cc: [email protected] # v6.3+
Fixes: 105ff5339f49 ("mm/memfd: add MFD_NOEXEC_SEAL and MFD_EXEC")
Signed-off-by: Aleksa Sarai <[email protected]>
---
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

2023-08-22 09:52:12

by Christian Brauner

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

On Mon, Aug 14, 2023 at 06:40:59PM +1000, 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/[email protected]/
> [2]: https://lore.kernel.org/202212161233.85C9783FB@keescook/
> [3]: https://lore.kernel.org/Y5yS8wCnuYGLHMj4@x1n/
> [4]: https://lore.kernel.org/[email protected]/
>
> Cc: [email protected] # v6.3+
> Fixes: 105ff5339f49 ("mm/memfd: add MFD_NOEXEC_SEAL and MFD_EXEC")
> Signed-off-by: Aleksa Sarai <[email protected]>
> ---

Reviewed-by: Christian Brauner <[email protected]>

2023-09-01 16:14:35

by Damian Tometzki

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

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/[email protected]/
> [2]: https://lore.kernel.org/202212161233.85C9783FB@keescook/
> [3]: https://lore.kernel.org/Y5yS8wCnuYGLHMj4@x1n/
> [4]: https://lore.kernel.org/[email protected]/
>
> Cc: [email protected] # v6.3+
> Fixes: 105ff5339f49 ("mm/memfd: add MFD_NOEXEC_SEAL and MFD_EXEC")
> Signed-off-by: Aleksa Sarai <[email protected]>
> ---
> 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

2023-09-05 16:06:34

by Andrew Morton

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

On Fri, 1 Sep 2023 07:13:45 +0200 Damian Tometzki <[email protected]> wrote:

> > 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

OK, thanks, I'll revert this. Spamming everyone even harder isn't a
good way to get developers to fix their stuff.