fsverity: Be more careful with return value of ->get_verity_descriptor()

If fsverity_operations::get_verity_descriptor() returns that the verity
descriptor has 0 size, then a zero-size buffer is allocated to hold it,
and it's called again with bufsize=0.

But bufsize=0 is a special case that says to return the actual size.  If
in the meantime the filesystem has decided that the size is some
actually valid size, it will return that size, and then
validate_fsverity_descriptor() will read from ZERO_SIZE_PTR and crash.

Of course, for the filesystem to misbehave in that strange way, it
generally would require direct writes to the block device.

Regardless, fsverity_get_descriptor() might as well guard against it.
Make it do so.  Specifically, validate that the first call to
->get_verity_descriptor() returns at least the minimum descriptor size,
and validate that the second call returns the same size as the first.

Fixes: fd2d1acfcadf ("fs-verity: add the hook for file ->open()")
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20261025203831.120095-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

0 files changed, 0 insertions, 0 deletions