[PATCH v4] erofs-utils: lib: harden h_shared_count in erofs_init_inode_xattrs()

[Utkal Singh]

`u8 h_shared_count` indicates the shared xattr count of an inode. It is
read from the on-disk xattr ibody header, which should be corrupted if
the size of the shared xattr array exceeds the space available in
`xattr_isize`.

It does not cause harmful consequence (e.g. crashes), since the image is
already considered corrupted, it indeed results in the silent processing
of garbage metadata.

Let's harden it to report -EFSCORRUPTED earlier.

Reproducer:
  mkdir testdir && echo hello > testdir/a.txt
  setfattr -n user.test -v val testdir/a.txt
  mkfs.erofs test.img testdir
  # corrupt h_shared_count (offset = nid*32 + inode_size + 4) to 0xFF
  # then: fsck.erofs --extract=/tmp/out --xattrs test_corrupted.img
  # Without patch: silently processes invalid shared xattr IDs
  # With patch: returns -EFSCORRUPTED

Signed-off-by: Utkal Singh <singhutkal015 at gmail.com>
---
 lib/xattr.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/lib/xattr.c b/lib/xattr.c
index 565070a..9d52a18 100644
--- a/lib/xattr.c
+++ b/lib/xattr.c
@@ -1182,6 +1182,13 @@ static int erofs_init_inode_xattrs(struct erofs_inode *vi)
 
 	ih = it.kaddr;
 	vi->xattr_shared_count = ih->h_shared_count;
+	if (vi->xattr_shared_count * sizeof(__le32) >
+	    vi->xattr_isize - sizeof(struct erofs_xattr_ibody_header)) {
+		erofs_err("invalid h_shared_count %u in nid %llu",
+			  vi->xattr_shared_count, vi->nid | 0ULL);
+		erofs_put_metabuf(&it.buf);
+		return -EFSCORRUPTED;
+	}
 	vi->xattr_shared_xattrs = malloc(vi->xattr_shared_count * sizeof(uint));
 	if (!vi->xattr_shared_xattrs) {
 		erofs_put_metabuf(&it.buf);
-- 
2.43.0