Squashfs: check block offset is not negative

If a negative offset is read off disk (for example the offset into the
decompressed fragment block), this will cause squashfs_copy_data() to
perform an out of bounds access.

Fix by checking if offset is negative, and returning 0.  This matches
existing behaviour where an offset beyond the block returns 0 bytes
copied.

To trigger this out of bounds access requires a crafted Squashfs
filesystem and CAP_SYS_ADMIN to mount it.  Unprivileged users will not be
able to mount such a filesystem, but once mounted, an unprivileged user
can trigger the out of bounds access by reading the crafted file with the
negative offset.

Link: https://lore.kernel.org/20260807162951.672510-1-phillip@squashfs.org.uk
Fixes: f400e12656ab ("Squashfs: cache operations")
Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
Reported-by: Yuejie Shi <syjcnss@gmail.com>
Closes: https://lore.kernel.org/all/20260803032735.81785-1-syjcnss@gmail.com/
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

0 files changed, 0 insertions, 0 deletions