Subject: Re: [syzbot] [ext4?] KASAN: slab-out-of-bounds Read in
 ext4_group_desc_csum
To:     Jan Kara <jack@suse.cz>, Tudor Ambarus <tudor.ambarus@linaro.org>
References: <000000000000ef6cf905f496e40b@google.com>
 <7e4a0f15-4d82-6026-c14b-59852ffab08e@linaro.org>
 <20230307103958.lo6ynoypgwreqmnq@quack3>
 <60788e5d-5c7c-1142-e554-c21d709acfd9@linaro.org>
 <976a7f24-0446-182f-c99e-98f3b98aef49@linaro.org>
 <20230313115728.2wxy2qj4mqpwgrx7@quack3>
Cc:     syzbot <syzbot+8785e41224a3afd04321@syzkaller.appspotmail.com>,
        adilger.kernel@dilger.ca, linux-ext4@vger.kernel.org,
        linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
        nathan@kernel.org, ndesaulniers@google.com,
        syzkaller-bugs@googlegroups.com, trix@redhat.com, tytso@mit.edu,
        Lee Jones <joneslee@google.com>
From:   yebin <yebin@huaweicloud.com>
Message-ID: <640F16B6.10100@huaweicloud.com>
Date:   Mon, 13 Mar 2023 20:27:34 +0800
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On 2023/3/13 19:57, Jan Kara wrote:
> Hi Tudor!
>
> On Mon 13-03-23 11:11:18, Tudor Ambarus wrote:
>> On 3/7/23 11:02, Tudor Ambarus wrote:
>>> On 3/7/23 10:39, Jan Kara wrote:
>>>> On Wed 01-03-23 12:13:51, Tudor Ambarus wrote:
>>>>> 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.
>>>> If there's something corrupting the superblock while the filesystem is
>>>> mounted, we need to find what is corrupting the SB and fix *that*. Not
>>>> try
>>>> to paper over the problem by not using the on-disk data... Maybe journal
>>>> replay is corrupting the value or something like that?
>>>>
>>>>                                  Honza
>>>>
>>> Ok, I agree. First thing would be to understand the reproducer and to
>>> simplify it if possible. I haven't yet decoded what the syz repro is
>>> doing at
>>> https://syzkaller.appspot.com/text?tag=ReproSyz&x=16ce3de4c80000
>>> Will reply to this email thread once I understand what's happening. If
>>> you or someone else can decode the syz repro faster than me, shoot.
>>>
>> I can now explain how the contents of the super block of the buffer get
>> corrupted. After the ext4 fs is mounted to the target ("./bus"), the
>> reproducer maps 6MB of data starting at offset 0 in the target's file
>> ("./bus"), then it starts overriding the data with something else, by
>> using memcpy, memset, individual byte inits. Does that mean that we
>> shouldn't rely on the contents of the super block in the buffer after we
>> mount the file system? If so, then my patch stands. I'll be happy to
>> extend it if needed. Below one may find a step by step interpretation of
>> the reproducer.
>>
>> We have a strace log for the same bug, but on Android 5.15:
>> https://syzkaller.appspot.com/text?tag=CrashLog&x=14ecec8cc80000
>>
>> Look for pid 328. You notice that the bpf() syscalls return error, so I
>> commented them out in the c repro to confirm that they are not the
>> cause. The bug reproduced without the bpf() calls. One can find the c
>> repro at:
>> https://syzkaller.appspot.com/text?tag=ReproC&x=17c5fc50c80000
>>
>> Let's look at these calls, just before the bug was hit:
>> [pid   328] open("./bus",
>> O_RDWR|O_CREAT|O_TRUNC|O_NONBLOCK|O_SYNC|O_DIRECT|O_LARGEFILE|O_NOATIME,
>> 000) = 4
>> [pid   328] mount("/dev/loop0", "./bus", NULL, MS_BIND, NULL) = 0
>> [pid   328] open("./bus", O_RDWR|O_SYNC|O_NOATIME|0x3c) = 5
>> [pid   328] mmap(0x20000000, 6291456,
>> PROT_READ|PROT_WRITE|PROT_EXEC|PROT_SEM|0x47ffff0, MAP_SHARED|MAP_FIXED,
>> 5, 0) = 0x20000000
> Yeah, looking at the reproducer, before this the reproducer also mounts
> /dev/loop0 as ext4 filesystem.
>
>> - ./bus is created (if it does not exist), fd 4 is returned.
>> - /dev/loop0 is mounted to ./bus
>> - then it creates a new file descriptor (5) for the same ./bus
>> - then it creates a mapping for ./bus starting at offset zero. The
>> mapped area is at 0x20000000 and is of 0x600000ul length.
> So the result is that the reproducer modified the block device while it is
> mounted by the filesystem. We know cases like this can crash the kernel and
> it is inherently difficult to fix. We have to trust the buffer cache
> contents as otherwise the performance will be unacceptable. For historical
> reasons we also have to allow modifications of buffer cache while ext4 is
> mounted because tune2fs uses this to e.g. update the label of a mounted
> filesystem.
>
> Long-term we are moving ext4 in a direction where we can disallow block
> device modifications while the fs is mounted but we are not there yet. I've
> discussed some shorter-term solution to avoid such known problems with syzbot
> developers and what seems plausible would be a kconfig option to disallow
> writing to a block device when it is exclusively open by someone else.
> But so far I didn't get to trying whether this would reasonably work. Would
> you be interested in having a look into this?
I am interested in this job. The file system is often damaged by writing 
block devices,
which is a headache. I have always wanted to eradicate this kind of problem.
A few months ago, I tried to add a mount parameter to prohibit 
modification after the
block device is mounted.But I encountered several problems that led to 
the termination
of my attempt. First of all, the 32-bit super block flags have been used 
up and need to
be extended. Secondly, I don't know how to handle read-only flag in the 
case of multiple
mount points.
  "disallow writing to a block device when it is exclusively open by 
someone else. "
-> Perhaps we can add a new IOCTL command to control whether write 
operations are
allowed after the block device has been exclusively opened. I don't know 
if this is feasible?
Do you have any good suggestions?
> 								Honza