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

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Message-Id: <20230814-memfd-vm-noexec-uapi-fixes-v2-3-7ff9e3e10ba6@cyphar.com>
Date:   Mon, 14 Aug 2023 18:40:59 +1000
From:   Aleksa Sarai <cyphar@...har.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>, Jeff Xu <jeffxu@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Daniel Verkamp <dverkamp@...omium.org>
Cc:     Christian Brauner <brauner@...nel.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        stable@...r.kernel.org, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org, Aleksa Sarai <cyphar@...har.com>
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/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@...r.kernel.org # v6.3+
Fixes: 105ff5339f49 ("mm/memfd: add MFD_NOEXEC_SEAL and MFD_EXEC")
Signed-off-by: Aleksa Sarai <cyphar@...har.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