[PATCH 2/2] squashfs: make the fragment index table bounds check overflow-safe

[Karl Mehltretter <kmehltretter@gmail.com>]

squashfs_read_fragment_index_table() checks that the table fits before
the next one with:

	if (fragment_table_start + length > next_table)
		return ERR_PTR(-EINVAL);

fragment_table_start comes from the superblock and is not validated
before this point. A start of 2^64 - length wraps the sum to zero, so
the check passes regardless of next_table and fails to reject the
invalid table ordering.

length then reaches kmalloc() through squashfs_read_table(). A
fragment count of 0xffffffff asks for 64MB, order 14. GFP_KERNEL does
not include __GFP_NOWARN, so the page allocator warns before the mount
fails with -ENOMEM. With panic_on_warn, the warning panics the kernel.

Compare the operands instead of adding them. id.c and export.c avoid
the same wrap with an exact-size check. Keep the inequality here because
a gap before the next table is still allowed.

Fixes: 1cac63cc9b2f ("Squashfs: add sanity checks to fragment reading at mount time")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-5
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 fs/squashfs/fragment.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/squashfs/fragment.c b/fs/squashfs/fragment.c
index 49602b9a42e19..b46673d1077a0 100644
--- a/fs/squashfs/fragment.c
+++ b/fs/squashfs/fragment.c
@@ -69,9 +69,11 @@ __le64 *squashfs_read_fragment_index_table(struct super_block *sb,
 	/*
 	 * Sanity check, length bytes should not extend into the next table -
 	 * this check also traps instances where fragment_table_start is
-	 * incorrectly larger than the next table start
+	 * incorrectly larger than the next table start.  Both values are read
+	 * from the filesystem image, so compare without adding them.
 	 */
-	if (fragment_table_start + length > next_table)
+	if (fragment_table_start > next_table ||
+	    length > next_table - fragment_table_start)
 		return ERR_PTR(-EINVAL);
 
 	table = squashfs_read_table(sb, fragment_table_start, length);
-- 
2.53.0