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Date:   Thu, 19 Jul 2018 17:07:18 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     syzbot <syzbot+b173e77096a8ba815511@syzkaller.appspotmail.com>
Cc:     jack@suse.cz, jlayton@redhat.com, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, mgorman@techsingularity.net,
        syzkaller-bugs@googlegroups.com, willy@infradead.org,
        v9fs-developer@lists.sourceforge.net
Subject: Re: KASAN: use-after-free Read in generic_perform_write
Message-Id: <20180719170718.8d4e7344fe79b2ad411dde98@linux-foundation.org>
In-Reply-To: <00000000000047116205715df655@google.com>
References: <00000000000047116205715df655@google.com>
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On Thu, 19 Jul 2018 11:01:01 -0700 syzbot <syzbot+b173e77096a8ba815511@syzkaller.appspotmail.com> wrote:

> Hello,
> 
> syzbot found the following crash on:
> 
> HEAD commit:    1c34981993da Add linux-next specific files for 20180719
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=16e6ac44400000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=7002497517b09aec
> dashboard link: https://syzkaller.appspot.com/bug?extid=b173e77096a8ba815511
> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
> 
> Unfortunately, I don't have any reproducer for this crash yet.

Thanks.  I cc'ed v9fs-developer, optimistically.  That list manager is
weird :(

I'm suspecting v9fs.  Does that fs attempt to write to the fs from a
kmalloced buffer?

Full report below...

> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+b173e77096a8ba815511@syzkaller.appspotmail.com
> 
> IPVS: length: 141 != 24
> XFS (loop5): Invalid superblock magic number
> ==================================================================
> BUG: KASAN: use-after-free in memcpy include/linux/string.h:345 [inline]
> BUG: KASAN: use-after-free in iov_iter_copy_from_user_atomic+0xb8d/0xfa0  
> lib/iov_iter.c:916
> Read of size 21 at addr ffff880190103660 by task kworker/0:3/4927
> 
> CPU: 0 PID: 4927 Comm: kworker/0:3 Not tainted 4.18.0-rc5-next-20180719+ #11
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
> Google 01/01/2011
> Workqueue: events p9_write_work
> Call Trace:
>   __dump_stack lib/dump_stack.c:77 [inline]
>   dump_stack+0x1c9/0x2b4 lib/dump_stack.c:113
>   print_address_description+0x6c/0x20b mm/kasan/report.c:256
>   kasan_report_error mm/kasan/report.c:354 [inline]
>   kasan_report.cold.7+0x242/0x30d mm/kasan/report.c:412
>   check_memory_region_inline mm/kasan/kasan.c:260 [inline]
>   check_memory_region+0x13e/0x1b0 mm/kasan/kasan.c:267
>   memcpy+0x23/0x50 mm/kasan/kasan.c:302
>   memcpy include/linux/string.h:345 [inline]
>   iov_iter_copy_from_user_atomic+0xb8d/0xfa0 lib/iov_iter.c:916
>   generic_perform_write+0x469/0x6c0 mm/filemap.c:3058
>   __generic_file_write_iter+0x26e/0x630 mm/filemap.c:3175
>   ext4_file_write_iter+0x390/0x1450 fs/ext4/file.c:266
>   call_write_iter include/linux/fs.h:1826 [inline]
>   new_sync_write fs/read_write.c:474 [inline]
>   __vfs_write+0x6af/0x9d0 fs/read_write.c:487
>   vfs_write+0x1fc/0x560 fs/read_write.c:549
>   kernel_write+0xab/0x120 fs/read_write.c:526
>   p9_fd_write net/9p/trans_fd.c:427 [inline]
>   p9_write_work+0x6f1/0xd50 net/9p/trans_fd.c:476
>   process_one_work+0xc73/0x1ba0 kernel/workqueue.c:2153
>   worker_thread+0x189/0x13c0 kernel/workqueue.c:2296
>   kthread+0x345/0x410 kernel/kthread.c:246
>   ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:415
> 
> Allocated by task 13072:
>   save_stack+0x43/0xd0 mm/kasan/kasan.c:448
>   set_track mm/kasan/kasan.c:460 [inline]
>   kasan_kmalloc+0xc4/0xe0 mm/kasan/kasan.c:553
>   __do_kmalloc mm/slab.c:3718 [inline]
>   __kmalloc+0x14e/0x760 mm/slab.c:3727
>   kmalloc include/linux/slab.h:518 [inline]
>   p9_fcall_alloc+0x1e/0x90 net/9p/client.c:237
>   p9_tag_alloc net/9p/client.c:266 [inline]
>   p9_client_prepare_req.part.8+0x107/0xa00 net/9p/client.c:640
>   p9_client_prepare_req net/9p/client.c:675 [inline]
>   p9_client_rpc+0x242/0x1330 net/9p/client.c:675
>   p9_client_version net/9p/client.c:890 [inline]
>   p9_client_create+0xca4/0x1537 net/9p/client.c:974
>   v9fs_session_init+0x21a/0x1a80 fs/9p/v9fs.c:400
>   v9fs_mount+0x7c/0x900 fs/9p/vfs_super.c:135
>   legacy_get_tree+0x131/0x460 fs/fs_context.c:674
>   vfs_get_tree+0x1cb/0x5c0 fs/super.c:1743
>   do_new_mount fs/namespace.c:2603 [inline]
>   do_mount+0x6f2/0x1e20 fs/namespace.c:2927
>   ksys_mount+0x12d/0x140 fs/namespace.c:3143
>   __do_sys_mount fs/namespace.c:3157 [inline]
>   __se_sys_mount fs/namespace.c:3154 [inline]
>   __x64_sys_mount+0xbe/0x150 fs/namespace.c:3154
>   do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
>   entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
> Freed by task 13072:
>   save_stack+0x43/0xd0 mm/kasan/kasan.c:448
>   set_track mm/kasan/kasan.c:460 [inline]
>   __kasan_slab_free+0x11a/0x170 mm/kasan/kasan.c:521
>   kasan_slab_free+0xe/0x10 mm/kasan/kasan.c:528
>   __cache_free mm/slab.c:3498 [inline]
>   kfree+0xd9/0x260 mm/slab.c:3813
>   p9_free_req+0xb5/0x120 net/9p/client.c:338
>   p9_client_rpc+0xa8e/0x1330 net/9p/client.c:739
>   p9_client_version net/9p/client.c:890 [inline]
>   p9_client_create+0xca4/0x1537 net/9p/client.c:974
>   v9fs_session_init+0x21a/0x1a80 fs/9p/v9fs.c:400
>   v9fs_mount+0x7c/0x900 fs/9p/vfs_super.c:135
>   legacy_get_tree+0x131/0x460 fs/fs_context.c:674
>   vfs_get_tree+0x1cb/0x5c0 fs/super.c:1743
>   do_new_mount fs/namespace.c:2603 [inline]
>   do_mount+0x6f2/0x1e20 fs/namespace.c:2927
>   ksys_mount+0x12d/0x140 fs/namespace.c:3143
>   __do_sys_mount fs/namespace.c:3157 [inline]
>   __se_sys_mount fs/namespace.c:3154 [inline]
>   __x64_sys_mount+0xbe/0x150 fs/namespace.c:3154
>   do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
>   entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
> The buggy address belongs to the object at ffff880190103640
>   which belongs to the cache kmalloc-16384 of size 16384
> The buggy address is located 32 bytes inside of
>   16384-byte region [ffff880190103640, ffff880190107640)
> The buggy address belongs to the page:
> page:ffffea0006404000 count:1 mapcount:0 mapping:ffff8801da802200 index:0x0  
> compound_mapcount: 0
> flags: 0x2fffc0000010200(slab|head)
> raw: 02fffc0000010200 ffffea000643b808 ffffea0006452c08 ffff8801da802200
> raw: 0000000000000000 ffff880190103640 0000000100000001 0000000000000000
> page dumped because: kasan: bad access detected
> 
> Memory state around the buggy address:
>   ffff880190103500: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>   ffff880190103580: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> > ffff880190103600: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
>                                                         ^
>   ffff880190103680: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>   ffff880190103700: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ==================================================================
> 
> 
> ---
> This bug is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
> syzbot engineers can be reached at syzkaller@googlegroups.com.
> 
> syzbot will keep track of this bug report. See:
> https://goo.gl/tpsmEJ#bug-status-tracking for how to communicate with  
> syzbot.