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

[Christian Brauner <brauner@kernel.org>]

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/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>
> ---

Reviewed-by: Christian Brauner <brauner@kernel.org>