[PATCH v2 1/3] ntfs: fix use-after-free in ntfs_attr_find()

[Hawkins Jiawei <yin31149@gmail.com>]

Syzkaller reported use-after-free read as follows:
==================================================================
BUG: KASAN: use-after-free in ntfs_attr_find+0xc02/0xce0 fs/ntfs/attrib.c:597
Read of size 2 at addr ffff88807e352009 by task syz-executor153/3607

[...]
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
 print_address_description mm/kasan/report.c:317 [inline]
 print_report.cold+0x2ba/0x719 mm/kasan/report.c:433
 kasan_report+0xb1/0x1e0 mm/kasan/report.c:495
 ntfs_attr_find+0xc02/0xce0 fs/ntfs/attrib.c:597
 ntfs_attr_lookup+0x1056/0x2070 fs/ntfs/attrib.c:1193
 ntfs_read_inode_mount+0x89a/0x2580 fs/ntfs/inode.c:1845
 ntfs_fill_super+0x1799/0x9320 fs/ntfs/super.c:2854
 mount_bdev+0x34d/0x410 fs/super.c:1400
 legacy_get_tree+0x105/0x220 fs/fs_context.c:610
 vfs_get_tree+0x89/0x2f0 fs/super.c:1530
 do_new_mount fs/namespace.c:3040 [inline]
 path_mount+0x1326/0x1e20 fs/namespace.c:3370
 do_mount fs/namespace.c:3383 [inline]
 __do_sys_mount fs/namespace.c:3591 [inline]
 __se_sys_mount fs/namespace.c:3568 [inline]
 __x64_sys_mount+0x27f/0x300 fs/namespace.c:3568
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd
 [...]
 </TASK>

The buggy address belongs to the physical page:
page:ffffea0001f8d400 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x7e350
head:ffffea0001f8d400 order:3 compound_mapcount:0 compound_pincount:0
flags: 0xfff00000010200(slab|head|node=0|zone=1|lastcpupid=0x7ff)
raw: 00fff00000010200 0000000000000000 dead000000000122 ffff888011842140
raw: 0000000000000000 0000000000040004 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected
Memory state around the buggy address:
 ffff88807e351f00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff88807e351f80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff88807e352000: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                      ^
 ffff88807e352080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff88807e352100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================

Kernel will loads $MFT/$DATA's first mft record in
ntfs_read_inode_mount().

Yet the problem is that after loading, kernel doesn't check whether
attrs_offset field is a valid value.

To be more specific, if attrs_offset field is larger
than bytes_allocated field, then it may trigger the out-of-bounds read
bug(reported as use-after-free bug) in ntfs_attr_find(), when kernel
tries to access the corresponding mft record's attribute.

This patch solves it by adding the sanity check between attrs_offset
field and bytes_allocated field, after loading the first mft record.

Signed-off-by: Hawkins Jiawei <yin31149@gmail.com>
---
v1->v2:
 * No change.

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

diff --git a/fs/ntfs/inode.c b/fs/ntfs/inode.c
index db0f1995aedd..08c659332e26 100644
--- a/fs/ntfs/inode.c
+++ b/fs/ntfs/inode.c
@@ -1829,6 +1829,13 @@ int ntfs_read_inode_mount(struct inode *vi)
 		goto err_out;
 	}
 
+	/* Sanity check offset to the first attribute */
+	if (le16_to_cpu(m->attrs_offset) >= le32_to_cpu(m->bytes_allocated)) {
+		ntfs_error(sb, "Incorrect mft offset to the first attribute %u in superblock.",
+			       le16_to_cpu(m->attrs_offset));
+		goto err_out;
+	}
+
 	/* Need this to sanity check attribute list references to $MFT. */
 	vi->i_generation = ni->seq_no = le16_to_cpu(m->sequence_number);
 
-- 
2.25.1