KMSAN: kernel-infoleak in copy_page_to_iter (2)

[syzbot]

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+2dcfea...@syzkaller.appspotmail.com

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 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 syzk...@googlegroups.com.

syzbot will keep track of this bug report. See:
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syzbot.
syzbot can test patches for this bug, for details see:
https://goo.gl/tpsmEJ#testing-patches

[Alexander Potapenko]

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?

>
> ---
> 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 syzk...@googlegroups.com.
>
> syzbot will keep track of this bug report. See:
> https://goo.gl/tpsmEJ#bug-status-tracking for how to communicate with
> syzbot.
> syzbot can test patches for this bug, for details see:
> https://goo.gl/tpsmEJ#testing-patches
>

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--
Alexander Potapenko
Software Engineer

Google Germany GmbH
Erika-Mann-Straße, 33
80636 München

Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg

[Alexander Potapenko]

A friendly ping, this bug is still reproducible on syzbot.

[Jens Axboe]

Does this fix it?


diff --git a/drivers/block/null_blk_main.c b/drivers/block/null_blk_main.c
index 62c9654b9ce8..08808c572692 100644
--- a/drivers/block/null_blk_main.c
+++ b/drivers/block/null_blk_main.c
@@ -655,7 +655,7 @@ static struct nullb_page *null_alloc_page(gfp_t gfp_flags)
if (!t_page)
goto out;

- t_page->page = alloc_pages(gfp_flags, 0);
+ t_page->page = alloc_pages(gfp_flags | __GFP_ZERO, 0);
if (!t_page->page)
goto out_freepage;


--
Jens Axboe

[Alexander Potapenko]

There must be something wrong with my tool, as it stops reporting this
bug when I apply your patch.
However when I run `cat /dev/nullb0 | strings` and wait long enough I
start seeing meaningful strings (file names, env dumps etc.)
I suspect this is still unexpected, right?

>
> diff --git a/drivers/block/null_blk_main.c b/drivers/block/null_blk_main.c
> index 62c9654b9ce8..08808c572692 100644
> --- a/drivers/block/null_blk_main.c
> +++ b/drivers/block/null_blk_main.c
> @@ -655,7 +655,7 @@ static struct nullb_page *null_alloc_page(gfp_t gfp_flags)
> if (!t_page)
> goto out;
>
> - t_page->page = alloc_pages(gfp_flags, 0);
> + t_page->page = alloc_pages(gfp_flags | __GFP_ZERO, 0);
> if (!t_page->page)
> goto out_freepage;
>
>
> --
> Jens Axboe
>

[Alexander Potapenko]

From: Alexander Potapenko <gli...@google.com>
Date: Wed, Jan 2, 2019 at 11:09 AM
To: Jens Axboe
Cc: Andrew Morton, <bart.va...@wdc.com>,
<matias....@wdc.com>, Andi Kleen, <ja...@suse.cz>,
<jla...@redhat.com>, LKML, Linux Memory Management List,
<mawi...@microsoft.com>, <mgo...@techsingularity.net>,
<syzkall...@googlegroups.com>

A friendly ping, as we're still seeing similar errors.