[PATCH] udf: validate free block extents against the partition length

[Michael Bommarito <michael.bommarito@gmail.com>]

udf_free_blocks() checks the logical block number and count against the
partition length, but drops the extent offset from that final bound.  A
crafted extent can pass the guard while logicalBlockNum + offset + count
points past the partition, which later indexes past the space bitmap
array.

A single ftruncate(2) on a file backed by such an extent reliably
panics the kernel.  This is a local availability issue.  On desktop
systems where UDisks/polkit allows the active user to mount removable
UDF media without CAP_SYS_ADMIN, an unprivileged local user can supply
the crafted filesystem and trigger the panic by truncating a writable
file on it.  Systems that require root or CAP_SYS_ADMIN to mount the
image have a higher prerequisite.

No confidentiality or integrity impact is claimed: the reproduced
primitive is an out-of-bounds read of a bitmap pointer slot followed by
a kernel panic.

Use the already computed logicalBlockNum + offset + count value for the
partition length check.  Also make load_block_bitmap() reject an
out-of-range block group before indexing s_block_bitmap[], so corrupted
callers cannot walk past the flexible array.

Fixes: 56e69e59751d ("udf: prevent integer overflow in udf_bitmap_free_blocks()")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
 fs/udf/balloc.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/udf/balloc.c b/fs/udf/balloc.c
index 807c493ed0cd..cc6dc6e1d84d 100644
--- a/fs/udf/balloc.c
+++ b/fs/udf/balloc.c
@@ -82,8 +82,9 @@ static int load_block_bitmap(struct super_block *sb,
 	int nr_groups = bitmap->s_nr_groups;
 
 	if (block_group >= nr_groups) {
-		udf_debug("block_group (%u) > nr_groups (%d)\n",
+		udf_debug("block_group (%u) >= nr_groups (%d)\n",
 			  block_group, nr_groups);
+		return -EFSCORRUPTED;
 	}
 
 	if (bitmap->s_block_bitmap[block_group]) {
@@ -662,7 +663,7 @@ void udf_free_blocks(struct super_block *sb, struct inode *inode,
 
 	if (check_add_overflow(bloc->logicalBlockNum, offset, &blk) ||
 	    check_add_overflow(blk, count, &blk) ||
-	    bloc->logicalBlockNum + count > map->s_partition_len) {
+	    blk > map->s_partition_len) {
 		udf_debug("Invalid request to free blocks: (%d, %u), off %u, "
 			  "len %u, partition len %u\n",
 			  partition, bloc->logicalBlockNum, offset, count,
-- 
2.53.0