Hi
On Thu, May 23, 2024, at 4:25 AM, Barnabás Pőcze wrote:
> 2024. május 23., csütörtök 1:23 keltezéssel, Andrew Morton
> <akpm@xxxxxxxxxxxxxxxxxxxx> írta:
>> It's a change to a userspace API, yes? Please let's have a detailed
>> description of why this is OK. Why it won't affect any existing users.
>
> Yes, it is a uAPI change. To trigger user visible change, a program has to
>
> - create a memfd
> - with MFD_NOEXEC_SEAL,
> - without MFD_ALLOW_SEALING;
> - try to add seals / check the seals.
>
> This change in essence reverts the kernel's behaviour to that of Linux
> <6.3, where
> only `MFD_ALLOW_SEALING` enabled sealing. If a program works correctly
> on those
> kernels, it will likely work correctly after this change.
>
> I have looked through Debian Code Search and GitHub, searching for
> `MFD_NOEXEC_SEAL`.
> And I could find only a single breakage that this change would case:
> dbus-broker
> has its own memfd_create() wrapper that is aware of this implicit
> `MFD_ALLOW_SEALING`
> behaviour[0], and tries to work around it. This workaround will break.
> Luckily,
> however, as far as I could tell this only affects the test suite of
> dbus-broker,
> not its normal operations, so I believe it should be fine. I have
> prepared a PR
> with a fix[1].
We asked for exactly this fix before, so I very much support this. Our test-suite in `dbus-broker` merely verifies what the current kernel behavior is (just like the kernel selftests). I am certainly ok if the kernel breaks it. I will gladly adapt the test-suite.
Previous discussion was in:
[PATCH] memfd: support MFD_NOEXEC alongside MFD_EXEC
https://lore.kernel.org/lkml/20230714114753.170814-1-david@xxxxxxxxxxxx/
Note that this fix is particularly important in combination with `vm.memfd_noexec=2`, since this breaks existing user-space by enabling sealing on all memfds unconditionally. I also encourage backporting to stable kernels.
Reviewed-by: David Rheinsberg <david@xxxxxxxxxxxx>
Thanks
David
