[PATCH] udf: reject VAT indexes equal to the entry count

[David Lee]

UDF 1.50 virtual partition mapping uses the VAT as an array of physical
block mappings. s_num_entries stores the number of entries in that array,
not the highest valid index. The valid VAT indexes are therefore below
s_num_entries.

udf_get_pblock_virt15() currently rejects only indexes greater than
s_num_entries. A crafted image can request index s_num_entries, pass the
bounds check, and make the kernel read one entry past the allocated VAT table.

Change the check to reject block >= s_num_entries, so the count is handled as
an exclusive upper bound.

A crafted UDF image reproduced this on origin/master commit
0e35b9b6ec0ffcc5e23cbdec09f5c622ad532b53 with a KASAN slab-out-of-bounds
report in udf_get_pblock_virt15().

Trail of Bits has a reproducer that triggers kernel panic demonstrating the bug, and can share it if needed.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: David Lee <david.lee@xxxxxxxxxxxxxxx>
Assisted-by: Codex:gpt-5.5
---
fs/udf/partition.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/udf/partition.c b/fs/udf/partition.c
index 2b85c95..ad8dced 100644
--- a/fs/udf/partition.c
+++ b/fs/udf/partition.c
@@ -55,7 +55,7 @@ uint32_t udf_get_pblock_virt15(struct super_block *sb, uint32_t block,
 	map = &sbi->s_partmaps[partition];
 	vdata = &map->s_type_specific.s_virtual;
 
-	if (block > vdata->s_num_entries) {
+	if (block >= vdata->s_num_entries) {
 		udf_debug("Trying to access block beyond end of VAT (%u max %u)\n",
 			  block, vdata->s_num_entries);
 		return 0xFFFFFFFF;