Re: [syzbot] [ext4?] KASAN: slab-out-of-bounds Read in ext4_group_desc_csum

[Tudor Ambarus <tudor.ambarus@linaro.org>]

Hi!

On 2/13/23 15:56, syzbot wrote:
> syzbot has found a reproducer for the following issue on:
> 
> HEAD commit:    ceaa837f96ad Linux 6.2-rc8
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=11727cc7480000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=42ba4da8e1e6af9f
> dashboard link: https://syzkaller.appspot.com/bug?extid=8785e41224a3afd04321
> compiler:       Debian clang version 15.0.7, GNU ld (GNU Binutils for Debian) 2.35.2
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=14392a4f480000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/88042f9b5fc8/disk-ceaa837f.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/9945b57ec9ee/vmlinux-ceaa837f.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/72ff118ed96b/bzImage-ceaa837f.xz
> mounted in repro: https://storage.googleapis.com/syzbot-assets/dabec17b2679/mount_0.gz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+8785e41224a3afd04321@syzkaller.appspotmail.com
> 
> ==================================================================
> BUG: KASAN: use-after-free in crc16+0x1fb/0x280 lib/crc16.c:58
> Read of size 1 at addr ffff88807de00000 by task syz-executor.1/5339
> 
> CPU: 1 PID: 5339 Comm: syz-executor.1 Not tainted 6.2.0-rc8-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/21/2023
> Call Trace:
>   <TASK>
>   __dump_stack lib/dump_stack.c:88 [inline]
>   dump_stack_lvl+0x1e7/0x2d0 lib/dump_stack.c:106
>   print_address_description mm/kasan/report.c:306 [inline]
>   print_report+0x163/0x4f0 mm/kasan/report.c:417
>   kasan_report+0x13a/0x170 mm/kasan/report.c:517
>   crc16+0x1fb/0x280 lib/crc16.c:58
>   ext4_group_desc_csum+0x90f/0xc50 fs/ext4/super.c:3187
>   ext4_group_desc_csum_set+0x19b/0x240 fs/ext4/super.c:3210
>   ext4_mb_clear_bb fs/ext4/mballoc.c:6027 [inline]
>   ext4_free_blocks+0x1c57/0x3010 fs/ext4/mballoc.c:6173
>   ext4_remove_blocks fs/ext4/extents.c:2527 [inline]
>   ext4_ext_rm_leaf fs/ext4/extents.c:2710 [inline]
>   ext4_ext_remove_space+0x289e/0x5270 fs/ext4/extents.c:2958
>   ext4_ext_truncate+0x176/0x210 fs/ext4/extents.c:4416
>   ext4_truncate+0xafa/0x1450 fs/ext4/inode.c:4342
>   ext4_evict_inode+0xc40/0x1230 fs/ext4/inode.c:286
>   evict+0x2a4/0x620 fs/inode.c:664
>   do_unlinkat+0x4f1/0x930 fs/namei.c:4327
>   __do_sys_unlink fs/namei.c:4368 [inline]
>   __se_sys_unlink fs/namei.c:4366 [inline]
>   __x64_sys_unlink+0x49/0x50 fs/namei.c:4366
>   do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>   do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
>   entry_SYSCALL_64_after_hwframe+0x63/0xcd
> RIP: 0033:0x7fbc85a8c0f9
> Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 f1 19 00 00 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 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007fbc86838168 EFLAGS: 00000246 ORIG_RAX: 0000000000000057
> RAX: ffffffffffffffda RBX: 00007fbc85babf80 RCX: 00007fbc85a8c0f9
> RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000020000000
> RBP: 00007fbc85ae7ae9 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> R13: 00007ffd5743beaf R14: 00007fbc86838300 R15: 0000000000022000
>   </TASK>
> 
> The buggy address belongs to the physical page:
> page:ffffea0001f78000 refcount:0 mapcount:-128 mapping:0000000000000000 index:0x0 pfn:0x7de00
> flags: 0xfff00000000000(node=0|zone=1|lastcpupid=0x7ff)
> raw: 00fff00000000000 ffffea0001f86008 ffffea0001db2a08 0000000000000000
> raw: 0000000000000000 0000000000000001 00000000ffffff7f 0000000000000000
> page dumped because: kasan: bad access detected
> page_owner tracks the page as freed
> page last allocated via order 1, migratetype Unmovable, gfp_mask 0xd20c0(__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 4855, tgid 4855 (sshd), ts 43553490210, free_ts 58249059760
>   prep_new_page mm/page_alloc.c:2531 [inline]
>   get_page_from_freelist+0x3449/0x35c0 mm/page_alloc.c:4283
>   __alloc_pages+0x291/0x7e0 mm/page_alloc.c:5549
>   alloc_slab_page+0x6a/0x160 mm/slub.c:1851
>   allocate_slab mm/slub.c:1998 [inline]
>   new_slab+0x84/0x2f0 mm/slub.c:2051
>   ___slab_alloc+0xa85/0x10a0 mm/slub.c:3193
>   __kmem_cache_alloc_bulk mm/slub.c:3951 [inline]
>   kmem_cache_alloc_bulk+0x160/0x430 mm/slub.c:4026
>   mt_alloc_bulk lib/maple_tree.c:157 [inline]
>   mas_alloc_nodes+0x381/0x640 lib/maple_tree.c:1257
>   mas_node_count_gfp lib/maple_tree.c:1316 [inline]
>   mas_preallocate+0x131/0x350 lib/maple_tree.c:5724
>   vma_expand+0x277/0x850 mm/mmap.c:541
>   mmap_region+0xc43/0x1fb0 mm/mmap.c:2592
>   do_mmap+0x8c9/0xf70 mm/mmap.c:1411
>   vm_mmap_pgoff+0x1ce/0x2e0 mm/util.c:520
>   do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>   do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
>   entry_SYSCALL_64_after_hwframe+0x63/0xcd
> page last free stack trace:
>   reset_page_owner include/linux/page_owner.h:24 [inline]
>   free_pages_prepare mm/page_alloc.c:1446 [inline]
>   free_pcp_prepare mm/page_alloc.c:1496 [inline]
>   free_unref_page_prepare+0xf3a/0x1040 mm/page_alloc.c:3369
>   free_unref_page+0x37/0x3f0 mm/page_alloc.c:3464
>   qlist_free_all+0x22/0x60 mm/kasan/quarantine.c:187
>   kasan_quarantine_reduce+0x15a/0x170 mm/kasan/quarantine.c:294
>   __kasan_slab_alloc+0x23/0x80 mm/kasan/common.c:302
>   kasan_slab_alloc include/linux/kasan.h:201 [inline]
>   slab_post_alloc_hook+0x68/0x390 mm/slab.h:761
>   slab_alloc_node mm/slub.c:3452 [inline]
>   kmem_cache_alloc_node+0x158/0x2c0 mm/slub.c:3497
>   __alloc_skb+0xd6/0x2d0 net/core/skbuff.c:552
>   alloc_skb include/linux/skbuff.h:1270 [inline]
>   alloc_skb_with_frags+0xa8/0x750 net/core/skbuff.c:6194
>   sock_alloc_send_pskb+0x919/0xa50 net/core/sock.c:2743
>   unix_dgram_sendmsg+0x5b5/0x2050 net/unix/af_unix.c:1943
>   sock_sendmsg_nosec net/socket.c:714 [inline]
>   sock_sendmsg net/socket.c:734 [inline]
>   __sys_sendto+0x475/0x5f0 net/socket.c:2117
>   __do_sys_sendto net/socket.c:2129 [inline]
>   __se_sys_sendto net/socket.c:2125 [inline]
>   __x64_sys_sendto+0xde/0xf0 net/socket.c:2125
>   do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>   do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
>   entry_SYSCALL_64_after_hwframe+0x63/0xcd
> 
> Memory state around the buggy address:
>   ffff88807ddfff00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>   ffff88807ddfff80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>> ffff88807de00000: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>                     ^
>   ffff88807de00080: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>   ffff88807de00100: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
> ==================================================================
> 


I think the patch from below should fix it.

I printed le16_to_cpu(sbi->s_es->s_desc_size) and it was greater than
EXT4_MAX_DESC_SIZE. What I think it happens is that the contents of the
super block in the buffer get corrupted sometime after the .get_tree
(which eventually calls __ext4_fill_super()) is called. So instead of
relying on the contents of the buffer, we should instead rely on the
s_desc_size initialized at the __ext4_fill_super() time.

If someone finds this good (or bad), or has a more in depth explanation,
please let me know, it will help me better understand the subsystem. In
the meantime I'll continue to investigate this and prepare a patch for
it.

Cheers,
ta

index 260c1b3e3ef2..91d41e84da32 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -3182,11 +3182,9 @@ static __le16 ext4_group_desc_csum(struct 
super_block *sb, __u32 block_group,
         crc = crc16(crc, (__u8 *)gdp, offset);
         offset += sizeof(gdp->bg_checksum); /* skip checksum */
         /* for checksum of struct ext4_group_desc do the rest...*/
-       if (ext4_has_feature_64bit(sb) &&
-           offset < le16_to_cpu(sbi->s_es->s_desc_size))
+       if (ext4_has_feature_64bit(sb) && offset < sbi->s_desc_size)
                 crc = crc16(crc, (__u8 *)gdp + offset,
-                           le16_to_cpu(sbi->s_es->s_desc_size) -
-                               offset);
+                           sbi->s_desc_size - offset);

  out:
         return cpu_to_le16(crc);