+ memfd-improve-userspace-warnings-for-missing-exec-related-flags.patch added to mm-unstable branch

+ memfd-improve-userspace-warnings-for-missing-exec-related-flags.patch added to mm-unstable branch

[Andrew Morton]

The patch titled
     Subject: memfd: improve userspace warnings for missing exec-related flags
has been added to the -mm mm-unstable branch.  Its filename is
     memfd-improve-userspace-warnings-for-missing-exec-related-flags.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/memfd-improve-userspace-warnings-for-missing-exec-related-flags.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Aleksa Sarai <cyphar@cyphar.com>
Subject: memfd: improve userspace warnings for missing exec-related flags
Date: Mon, 14 Aug 2023 18:40:59 +1000

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/

Link: https://lkml.kernel.org/r/20230814-memfd-vm-noexec-uapi-fixes-v2-3-7ff9e3e10ba6@cyphar.com
Fixes: 105ff5339f49 ("mm/memfd: add MFD_NOEXEC_SEAL and MFD_EXEC")
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Daniel Verkamp <dverkamp@chromium.org>
Cc: Dominique Martinet <asmadeus@codewreck.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/memfd.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/memfd.c~memfd-improve-userspace-warnings-for-missing-exec-related-flags
+++ a/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));
 	}
_

Patches currently in -mm which might be from cyphar@cyphar.com are

selftests-memfd-error-out-test-process-when-child-test-fails.patch
memfd-do-not-eacces-old-memfd_create-users-with-vmmemfd_noexec=2.patch
memfd-improve-userspace-warnings-for-missing-exec-related-flags.patch
memfd-replace-ratcheting-feature-from-vmmemfd_noexec-with-hierarchy.patch
selftests-improve-vmmemfd_noexec-sysctl-tests.patch