[PATCH] udf: bound lengthAllocDescs from unallocated space entry

[Jay Vadayath <jay@artiphishell.com>]

udf_read_inode() copies the on-disk lengthAllocDescs field of a USE
(unallocSpaceEntry) inode into iinfo->i_lenAlloc without checking that
it fits in the i_data buffer that is subsequently allocated for the
inode. udf_count_free_table(), called from udf_statfs(), then walks the
allocation descriptor array up to i_lenAlloc bytes, so a crafted UDF
image with lengthAllocDescs larger than (blocksize - sizeof(struct
unallocSpaceEntry)) causes udf_get_fileshortad() to read past the end
of the kmalloc'd i_data buffer.

KASAN report from mounting a crafted UDF image and calling statfs()
from an unprivileged process:

  BUG: KASAN: slab-out-of-bounds in udf_get_fileshortad+0x126/0x130
  Read of size 4 at addr ffff8880042137d8 by task poc/65
  Call Trace:
   dump_stack_lvl+0x53/0x70
   print_report+0xce/0x610
   kasan_report+0xce/0x100
   udf_get_fileshortad+0x126/0x130
   udf_current_aext+0x3c4/0xa10
   udf_next_aext+0x241/0x440
   udf_statfs+0xb7d/0x11c0
   statfs_by_dentry+0x117/0x1e0
   user_statfs+0xac/0x130
   __do_sys_statfs+0x80/0xe0
   do_syscall_64+0x102/0x5a0
   entry_SYSCALL_64_after_hwframe+0x77/0x7f

Reject USE inodes whose lengthAllocDescs would place descriptors past
the end of the i_data buffer, mirroring the checks the rest of the UDF
code performs on descriptor lengths.

This bug was discovered by Artiphishell's vTriage pipeline, which
generated a userspace reproducer that reliably triggers the KASAN
report on an unpatched kernel. The fix below was drafted with the
Claude coding assistant; a userspace reproducer (and the crafted UDF
image) is available on request.

Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Jay Vadayath <jay@artiphishell.com>

---
 fs/udf/inode.c | 7 +++++++
 1 file changed, 7 insertions(+)

--- a/fs/udf/inode.c
+++ b/fs/udf/inode.c
@@ -1475,6 +1475,13 @@
 		iinfo->i_lenAlloc = le32_to_cpu(
 				((struct unallocSpaceEntry *)bh->b_data)->
 				 lengthAllocDescs);
+		/*
+		 * Sanity check the length of allocation descriptors so we do
+		 * not read past the end of the allocated i_data buffer when
+		 * walking them later (e.g. from udf_count_free_table()).
+		 */
+		if (iinfo->i_lenAlloc > bs - sizeof(struct unallocSpaceEntry))
+			goto out;
 		ret = udf_alloc_i_data(inode, bs -
 					sizeof(struct unallocSpaceEntry));
 		if (ret)