Re: [PATCH -next] ext4: fix use-after-free in ext4_search_dir

[Jan Kara]

On Wed 23-03-22 11:43:04, Ye Bin wrote:
> 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@xxxxxxxxxx>

Oh, good catch.

> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 3f87cca49f0c..276683f7ab77 100644
> --- a/fs/ext4/ext4.h
> +++ b/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 ext4_dir_entry_2 and ext4_dir_entry exclude name
> + */
> +#define EXT4_BASE_DIR_LEN 8

I'd rather use (sizeof(struct ext4_dir_entry_2) - EXT4_NAME_LEN) here...

>  struct ext4_dir_entry {
>  	__le32	inode;			/* Inode number */
> diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
> index e37da8d5cd0c..4739a5aa13aa 100644
> --- a/fs/ext4/namei.c
> +++ b/fs/ext4/namei.c
> @@ -1465,7 +1465,7 @@ int ext4_search_dir(struct buffer_head *bh, char *search_buf, int buf_size,
>  	int de_len;
>  
>  	de = (struct ext4_dir_entry_2 *)search_buf;
> -	dlimit = search_buf + buf_size;
> +	dlimit = search_buf + buf_size - EXT4_BASE_DIR_LEN;
>  	while ((char *) de < dlimit) {
>  		/* this code is executed quadratically often */
>  		/* do minimal checking `by hand' */

This looks wrong because a bit later we use dlimit to verify
de+de->name_len and that can certainly go upto bufsize. You need to modify
only the condition in the while loop like:

  	while ((char *) de < dlimit - EXT4_BASE_DIR_LEN) {

									Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR