Forwarded: [PATCH] ext4: reject system.data xattr with external inode storage

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Subject: [PATCH] ext4: reject system.data xattr with external inode storage
Author: kartikey406@gmail.com

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

Prevent use-after-free in ext4_search_dir by rejecting malformed
inline directory xattr entries during validation.

ext4 uses the system.data xattr to store inline directory entries
within the inode. This xattr must always use inline storage
(e_value_inum == 0). However, a corrupted filesystem can craft a
system.data xattr entry with e_value_inum != 0, bypassing the existing
validation in check_xattrs() which only validates e_value_offs when
e_value_inum == 0.

Later, when ext4_find_inline_entry() is called, ext4_get_inline_xattr_pos()
reads the corrupt e_value_offs and calculates an inline_start pointer
that can point outside the inode buffer, potentially into freed memory.
When ext4_search_dir() attempts to access this invalid pointer, it
results in a KASAN use-after-free.

Fix this by explicitly validating that system.data xattr entries always
have e_value_inum == 0 in check_xattrs(). This catches the corruption
at validation time during inode load, before the corrupt pointer can be
used.

Reported-by: syzbot+3ee481e21fd75e14c397@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=3ee481e21fd75e14c397
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
 fs/ext4/xattr.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
index 5a6fe1513fd2..a9057d0f9e24 100644
--- a/fs/ext4/xattr.c
+++ b/fs/ext4/xattr.c
@@ -251,6 +251,13 @@ check_xattrs(struct inode *inode, struct buffer_head *bh,
 			err_str = "invalid ea_ino";
 			goto errout;
 		}
+		if (entry->e_name_index == EXT4_XATTR_INDEX_SYSTEM &&
+		    entry->e_name_len == sizeof(EXT4_XATTR_SYSTEM_DATA) - 1 &&
+		    !memcmp(entry->e_name, EXT4_XATTR_SYSTEM_DATA, entry->e_name_len) &&
+		    ea_ino != 0) {
+			err_str = "system.data xattr cannot use external inode storage";
+			goto errout;
+		}
 		if (size > EXT4_XATTR_SIZE_MAX) {
 			err_str = "e_value size too large";
 			goto errout;
-- 
2.43.0