Re: KMSAN: kernel-infoleak in copy_page_to_iter (2)

[Alexander Potapenko]

On Thu, Sep 13, 2018 at 11:18 AM syzbot
<syzbot+2dcfeaf8cb49b05e8f1a@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> Hello,
>
> syzbot found the following crash on:
>
> HEAD commit:    123906095e30 kmsan: introduce kmsan_interrupt_enter()/kmsa..
> git tree:       https://github.com/google/kmsan.git/master
> console output: https://syzkaller.appspot.com/x/log.txt?x=1249fcb8400000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=848e40757852af3e
> dashboard link: https://syzkaller.appspot.com/bug?extid=2dcfeaf8cb49b05e8f1a
> compiler:       clang version 7.0.0 (trunk 334104)
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=116ef050400000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=122870ff800000
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+2dcfeaf8cb49b05e8f1a@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> random: sshd: uninitialized urandom read (32 bytes read)
> ==================================================================
> BUG: KMSAN: kernel-infoleak in copyout lib/iov_iter.c:140 [inline]
> BUG: KMSAN: kernel-infoleak in copy_page_to_iter_iovec lib/iov_iter.c:212
> [inline]
> BUG: KMSAN: kernel-infoleak in copy_page_to_iter+0x754/0x1b70
> lib/iov_iter.c:716
> CPU: 0 PID: 4516 Comm: blkid Not tainted 4.17.0+ #9
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> Google 01/01/2011
> Call Trace:
>   __dump_stack lib/dump_stack.c:77 [inline]
>   dump_stack+0x185/0x1d0 lib/dump_stack.c:113
>   kmsan_report+0x188/0x2a0 mm/kmsan/kmsan.c:1125
>   kmsan_internal_check_memory+0x17e/0x1f0 mm/kmsan/kmsan.c:1238
>   kmsan_copy_to_user+0x7a/0x160 mm/kmsan/kmsan.c:1261
>   copyout lib/iov_iter.c:140 [inline]
>   copy_page_to_iter_iovec lib/iov_iter.c:212 [inline]
>   copy_page_to_iter+0x754/0x1b70 lib/iov_iter.c:716
>   generic_file_buffered_read mm/filemap.c:2185 [inline]
>   generic_file_read_iter+0x2ef8/0x44d0 mm/filemap.c:2362
>   blkdev_read_iter+0x20d/0x280 fs/block_dev.c:1930
>   call_read_iter include/linux/fs.h:1778 [inline]
>   new_sync_read fs/read_write.c:406 [inline]
>   __vfs_read+0x775/0x9d0 fs/read_write.c:418
>   vfs_read+0x36c/0x6b0 fs/read_write.c:452
>   ksys_read fs/read_write.c:578 [inline]
>   __do_sys_read fs/read_write.c:588 [inline]
>   __se_sys_read fs/read_write.c:586 [inline]
>   __x64_sys_read+0x1bf/0x3e0 fs/read_write.c:586
>   do_syscall_64+0x15b/0x230 arch/x86/entry/common.c:287
>   entry_SYSCALL_64_after_hwframe+0x44/0xa9
> RIP: 0033:0x7fdeff68f310
> RSP: 002b:00007ffe999660b8 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
> RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fdeff68f310
> RDX: 0000000000000100 RSI: 0000000001e78df8 RDI: 0000000000000003
> RBP: 0000000001e78dd0 R08: 0000000000000028 R09: 0000000001680000
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000001e78030
> R13: 0000000000000100 R14: 0000000001e78080 R15: 0000000001e78de8
>
> Uninit was created at:
>   kmsan_save_stack_with_flags mm/kmsan/kmsan.c:282 [inline]
>   kmsan_alloc_meta_for_pages+0x161/0x3a0 mm/kmsan/kmsan.c:819
>   kmsan_alloc_page+0x82/0xe0 mm/kmsan/kmsan.c:889
>   __alloc_pages_nodemask+0xf7b/0x5cc0 mm/page_alloc.c:4402
>   alloc_pages_current+0x6b1/0x970 mm/mempolicy.c:2093
>   alloc_pages include/linux/gfp.h:494 [inline]
>   __page_cache_alloc+0x95/0x320 mm/filemap.c:946
>   pagecache_get_page+0x52b/0x1450 mm/filemap.c:1577
>   grab_cache_page_write_begin+0x10d/0x190 mm/filemap.c:3089
>   block_write_begin+0xf9/0x3a0 fs/buffer.c:2068
>   blkdev_write_begin+0xf5/0x110 fs/block_dev.c:584
>   generic_perform_write+0x438/0x9d0 mm/filemap.c:3139
>   __generic_file_write_iter+0x43b/0xa10 mm/filemap.c:3264
>   blkdev_write_iter+0x3a8/0x5f0 fs/block_dev.c:1910
>   do_iter_readv_writev+0x81c/0xa20 include/linux/fs.h:1778
>   do_iter_write+0x30d/0xd50 fs/read_write.c:959
>   vfs_writev fs/read_write.c:1004 [inline]
>   do_writev+0x3be/0x820 fs/read_write.c:1039
>   __do_sys_writev fs/read_write.c:1112 [inline]
>   __se_sys_writev fs/read_write.c:1109 [inline]
>   __x64_sys_writev+0xe1/0x120 fs/read_write.c:1109
>   do_syscall_64+0x15b/0x230 arch/x86/entry/common.c:287
>   entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> Bytes 4-255 of 256 are uninitialized
> Memory access starts at ffff8801b9903000
> ==================================================================
This particular report was caused by the repro program writing a byte
to /dev/nullb0 and /sbin/blkid reading from that device in the
background.
But it turns out that simply running `cat /dev/nullb0` already prints
uninitialized kernel memory.
Is this the intended behavior of the null block driver?
>
> ---
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>
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-- 
Alexander Potapenko
Software Engineer

Google Germany GmbH
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