From e300eb5002925b29be803d2661af07266cfa267e Mon Sep 17 00:00:00 2001 From: Phillip Lougher Date: Fri, 7 Aug 2026 17:29:51 +0100 Subject: [PATCH] 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 Reported-by: Yuejie Shi Closes: https://lore.kernel.org/all/20260803032735.81785-1-syjcnss@gmail.com/ Signed-off-by: Andrew Morton --- fs/squashfs/cache.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/fs/squashfs/cache.c b/fs/squashfs/cache.c index 67abd4dff222..2807b80d46b7 100644 --- a/fs/squashfs/cache.c +++ b/fs/squashfs/cache.c @@ -299,7 +299,7 @@ int squashfs_copy_data(void *buffer, struct squashfs_cache_entry *entry, { int remaining = length; - if (length == 0) + if (length == 0 || offset < 0) return 0; else if (buffer == NULL) return min(length, entry->length - offset);