Hi On Thu, May 23, 2024, at 6:55 PM, Jeff Xu wrote: > On Thu, May 23, 2024 at 9:20 AM Jeff Xu <jeffxu@xxxxxxxxxx> wrote: >> On Thu, May 23, 2024 at 1:24 AM David Rheinsberg <david@xxxxxxxxxxxx> wrote: >> > 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. >> > > memfd is by default not sealable, and file is by default sealable, > right ? that makes the memfd semantics different from other objects > in linux. > I wonder what is the original reason to have memfd this way? shmem-files are *not* sealable by default. This design was followed for backward compatibility reasons, since shmem-files predate sealing and silently enabling sealing on all shmem-files would have broken existing users (see shmem.c which initializes seals to F_SEAL_SEAL). I am not sure what you mean with "makes [memfd] semantics different from other objects in linux". Can you elaborate? Since `memfd_create` was introduced at the same time as shmem-sealing, it could certainly have enabled sealing by default. Not sure whether this would be preferable, though. > Another solution is to change memfd to be by-default sealable, > although that will be an api change, but what side effect will it be > ? > If we are worried about the memfd being sealed by an attacker, the > malicious code could also overwrite the content since memfd is not > sealed. You cannot change the default-seals retrospectively. There are existing shmem-users that share file-descriptors and *expect* the other party to be able to override data, but do *not* expect the other party to be able to apply seals. Note that these models explicitly *want* shared, writable access to the buffer (e.g., render-client shares a buffer with the display server for scanout), so just because you can *write* to a shmem-file does not mean that sharing is unsafe (e.g., using SIGBUS+mmap can safely deal with page-faults). Thanks David
