ext4: fix use-after-free in ext4_search_dir

We got issue as follows:
EXT4-fs (loop0): mounted filesystem without journal. Opts: ,errors=continue
==================================================================
BUG: KASAN: use-after-free in ext4_search_dir fs/ext4/namei.c:1394 [inline]
BUG: KASAN: use-after-free in search_dirblock fs/ext4/namei.c:1199 [inline]
BUG: KASAN: use-after-free in __ext4_find_entry+0xdca/0x1210 fs/ext4/namei.c:1553
Read of size 1 at addr ffff8881317c3005 by task syz-executor117/2331

CPU: 1 PID: 2331 Comm: syz-executor117 Not tainted 5.10.0+ #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
Call Trace:
 __dump_stack lib/dump_stack.c:83 [inline]
 dump_stack+0x144/0x187 lib/dump_stack.c:124
 print_address_description+0x7d/0x630 mm/kasan/report.c:387
 __kasan_report+0x132/0x190 mm/kasan/report.c:547
 kasan_report+0x47/0x60 mm/kasan/report.c:564
 ext4_search_dir fs/ext4/namei.c:1394 [inline]
 search_dirblock fs/ext4/namei.c:1199 [inline]
 __ext4_find_entry+0xdca/0x1210 fs/ext4/namei.c:1553
 ext4_lookup_entry fs/ext4/namei.c:1622 [inline]
 ext4_lookup+0xb8/0x3a0 fs/ext4/namei.c:1690
 __lookup_hash+0xc5/0x190 fs/namei.c:1451
 do_rmdir+0x19e/0x310 fs/namei.c:3760
 do_syscall_64+0x33/0x40 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x445e59
Code: 4d c7 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 1b c7 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fff2277fac8 EFLAGS: 00000246 ORIG_RAX: 0000000000000054
RAX: ffffffffffffffda RBX: 0000000000400280 RCX: 0000000000445e59
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 00000000200000c0
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000002
R10: 00007fff2277f990 R11: 0000000000000246 R12: 0000000000000000
R13: 431bde82d7b634db R14: 0000000000000000 R15: 0000000000000000

The buggy address belongs to the page:
page:0000000048cd3304 refcount:0 mapcount:0 mapping:0000000000000000 index:0x1 pfn:0x1317c3
flags: 0x200000000000000()
raw: 0200000000000000 ffffea0004526588 ffffea0004528088 0000000000000000
raw: 0000000000000001 0000000000000000 00000000ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8881317c2f00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff8881317c2f80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffff8881317c3000: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                   ^
 ffff8881317c3080: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff8881317c3100: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
==================================================================

ext4_search_dir:
  ...
  de = (struct ext4_dir_entry_2 *)search_buf;
  dlimit = search_buf + buf_size;
  while ((char *) de < dlimit) {
  ...
    if ((char *) de + de->name_len <= dlimit &&
	 ext4_match(dir, fname, de)) {
	    ...
    }
  ...
    de_len = ext4_rec_len_from_disk(de->rec_len, dir->i_sb->s_blocksize);
    if (de_len <= 0)
      return -1;
    offset += de_len;
    de = (struct ext4_dir_entry_2 *) ((char *) de + de_len);
  }

Assume:
de=0xffff8881317c2fff
dlimit=0x0xffff8881317c3000

If read 'de->name_len' which address is 0xffff8881317c3005, obviously is
out of range, then will trigger use-after-free.
To solve this issue, 'dlimit' must reserve 8 bytes, as we will read
'de->name_len' to judge if '(char *) de + de->name_len' out of range.
Signed-off-by: Ye Bin <yebin10@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220324064816.1209985-1-yebin10@huawei.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org

fs/ext4/ext4.h

@@ -2273,6 +2273,10 @@ static inline int ext4_forced_shutdown(struct ext4_sb_info *sbi)
  * Structure of a directory entry
  */
 #define EXT4_NAME_LEN 255
+/*
+ * Base length of the ext4 directory entry excluding the name length
+ */
+#define EXT4_BASE_DIR_LEN (sizeof(struct ext4_dir_entry_2) - EXT4_NAME_LEN)
 
 struct ext4_dir_entry {
 	__le32	inode;			/* Inode number */

fs/ext4/namei.c

@@ -1466,10 +1466,10 @@ int ext4_search_dir(struct buffer_head *bh, char *search_buf, int buf_size,
 
 	de = (struct ext4_dir_entry_2 *)search_buf;
 	dlimit = search_buf + buf_size;
-	while ((char *) de < dlimit) {
+	while ((char *) de < dlimit - EXT4_BASE_DIR_LEN) {
 		/* this code is executed quadratically often */
 		/* do minimal checking `by hand' */
-		if ((char *) de + de->name_len <= dlimit &&
+		if (de->name + de->name_len <= dlimit &&
 		    ext4_match(dir, fname, de)) {
 			/* found a match - just to be sure, do
 			 * a full check */