On Wed, May 22, 2024 at 4:23 PM Andrew Morton akpm@linux-foundation.org wrote:
On Wed, 15 May 2024 23:11:12 -0700 Jeff Xu jeffxu@google.com wrote:
On Mon, May 13, 2024 at 12:15 PM Barnabás Pőcze pobrn@protonmail.com wrote:
`MFD_NOEXEC_SEAL` should remove the executable bits and set `F_SEAL_EXEC` to prevent further modifications to the executable bits as per the comment in the uapi header file:
not executable and sealed to prevent changing to executable
However, currently, it also unsets `F_SEAL_SEAL`, essentially acting as a superset of `MFD_ALLOW_SEALING`. Nothing implies that it should be so, and indeed up until the second version of the of the patchset[0] that introduced `MFD_EXEC` and `MFD_NOEXEC_SEAL`, `F_SEAL_SEAL` was not removed, however it was changed in the third revision of the patchset[1] without a clear explanation.
This behaviour is suprising for application developers, there is no documentation that would reveal that `MFD_NOEXEC_SEAL` has the additional effect of `MFD_ALLOW_SEALING`.
Ya, I agree that there should be documentation, such as a man page. I will work on that.
So do not remove `F_SEAL_SEAL` when `MFD_NOEXEC_SEAL` is requested. This is technically an ABI break, but it seems very unlikely that an application would depend on this behaviour (unless by accident).
...
Reviewed-by: Jeff Xu jeffxu@google.com
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.
Unfortunately, this is a breaking change that might break a application if they do below: memfd_create("", MFD_NOEXEC_SEAL) fcntl(fd, F_ADD_SEALS, F_SEAL_WRITE); <-- this will fail in new semantics, due to mfd not being sealable.
However, I still think the new semantics is a better, the reason is due to the sysctl: memfd_noexec_scope Currently, when the sysctl is set to MEMFD_NOEXEC_SCOPE_NOEXEC_SEAL kernel adds MFD_NOEXEC_SEAL to memfd_create, and the memfd becomes sealable. E.g. When the sysctl is set to MEMFD_NOEXEC_SCOPE_NOEXEC_SEAL The app calls memfd_create("",0) application will get sealable memfd, which might be a surprise to application.
If the app doesn't want this behavior, they will need one of two below in current implementation. 1> set the sysctl: memfd_noexec_scope to 0. So the kernel doesn't overwrite the mdmfd_create
2> modify their code to get non-sealable NOEXEC memfd. memfd_create("", MEMFD_NOEXEC_SCOPE_NOEXEC) fcntl(fd, F_ADD_SEALS, F_SEAL_SEAL)
The new semantics works better with the sysctl.
Since memfd noexec is new, maybe there is no application using the MFD_NOEXEC_SEAL to create sealable memfd. They mostly likely use memfd(MFD_NOEXEC_SEAL|MFD_ALLOW_SEALING) instead. I think it might benefit in the long term with the new semantics.
If breaking change is not recommended, the alternative is to introduce a new flag. MFD_NOEXEC_SEAL_SEAL. (I can't find a better name...)
What do you think ?
Also, please let's give consideration to a -stable backport so that all kernel versions will eventually behave in the same manner.
Yes. If the new semantics is acceptable, backport is needed as bugfix to all kernel versions. I can do that if someone helps me with the process.
And sorry about this bug that I created.