[PATCH] udf: fix OOB read for a crafted unallocSpaceEntry

[Baul Lee <baul.lee@xbow.com>]

udf_read_inode() sanity-checks the on-disk allocation-descriptor length
before it is used:

	if (iinfo->i_lenEAttr > bs || iinfo->i_lenAlloc > bs)
		goto out;
	/* Now do exact checks */
	if (udf_file_entry_alloc_offset(inode) + iinfo->i_lenAlloc > bs)
		goto out;

but the TAG_IDENT_USE branch sets i_lenAlloc from the on-disk
lengthAllocDescs and returns before ever reaching them.  A USE inode
therefore keeps a completely unvalidated le32 there, while i_data is
allocated at only bs - sizeof(struct unallocSpaceEntry) bytes.

udf_current_aext() computes, in its in-ICB branch,

	alen = udf_file_entry_alloc_offset(inode) + iinfo->i_lenAlloc;

with no clamp and passes alen to udf_get_fileshortad() as maxoffset.
That helper only checks (*offset + sizeof(struct short_ad)) > maxoffset,
so an inflated i_lenAlloc lets the walk step past the end of i_data and
dereference sa->extLength in adjacent slab memory.

A Type-1 partition whose unallocSpaceTable extent is non-zero loads the
USE table inode at mount and sets UDF_PART_FLAG_UNALLOC_TABLE.  An
unprivileged statfs(2) then reaches udf_count_free_table() ->
udf_next_aext() -> udf_current_aext(), and KASAN reports a
slab-out-of-bounds read just past the 472-byte i_data allocation.

Apply the same exact check on the USE path, before i_data is allocated
and walked.  ret is still -EIO there, so the offending inode fails to
load.

Discovered by XBOW, triaged by Baul Lee <baul.lee@xbow.com>

Fixes: 23b133bdc452 ("udf: Check length of extended attributes and allocation descriptors")
Reported-by: Federico Kirschbaum <federico.kirschbaum@xbow.com>
Reported-by: Baul Lee <baul.lee@xbow.com>
Cc: stable@vger.kernel.org
Signed-off-by: Baul Lee <baul.lee@xbow.com>
---
 fs/udf/inode.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/udf/inode.c b/fs/udf/inode.c
index c97914aa8d8b..8a0c9ea31a11 100644
--- a/fs/udf/inode.c
+++ b/fs/udf/inode.c
@@ -1475,6 +1475,8 @@ static int udf_read_inode(struct inode *inode, bool hidden_inode)
 		iinfo->i_lenAlloc = le32_to_cpu(
 				((struct unallocSpaceEntry *)bh->b_data)->
 				 lengthAllocDescs);
+		if (udf_file_entry_alloc_offset(inode) + iinfo->i_lenAlloc > bs)
+			goto out;
 		ret = udf_alloc_i_data(inode, bs -
 					sizeof(struct unallocSpaceEntry));
 		if (ret)
-- 
2.50.1 (Apple Git-155)