Hi
On Thu, May 23, 2024, at 4:25 AM, Barnabás Pőcze wrote:
- május 23., csütörtök 1:23 keltezéssel, Andrew Morton
akpm@linux-foundation.org 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@readahead.eu/
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@readahead.eu
Thanks David