Date:   Wed, 19 Dec 2018 06:23:32 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Alexander Potapenko <glider@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        bart.vanassche@....com, matias.bjorling@....com
Cc:     Andi Kleen <ak@...ux.intel.com>, jack@...e.cz, jlayton@...hat.com,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        mawilcox@...rosoft.com, mgorman@...hsingularity.net,
        syzkaller-bugs@...glegroups.com
Subject: Re: KMSAN: kernel-infoleak in copy_page_to_iter (2)

On 12/19/18 3:23 AM, Alexander Potapenko wrote:
> On Thu, Sep 13, 2018 at 11:23 AM Alexander Potapenko <glider@...gle.com> wrote:
>>
>> On Thu, Sep 13, 2018 at 11:18 AM syzbot
>> <syzbot+2dcfeaf8cb49b05e8f1a@...kaller.appspotmail.com> 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@...kaller.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 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?
> A friendly ping, this bug is still reproducible on syzbot.

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

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