From: Dmitry Vyukov <dvyukov@google.com>
To: Josef Bacik <josef@toxicpanda.com>
Cc: syzbot <syzbot+8aa9678d1cda7a0432b7@syzkaller.appspotmail.com>,
clm@fb.com, dsterba@suse.com, glider@google.com,
linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org,
syzkaller-bugs@googlegroups.com
Subject: Re: [syzbot] KMSAN: uninit-value in generic_bin_search (2)
Date: Tue, 15 Jun 2021 10:32:53 +0200 [thread overview]
Message-ID: <CACT4Y+anyMbe-GQ040BN1dRO=stayqGDPLUpV=c89BUs6qzTCQ@mail.gmail.com> (raw)
In-Reply-To: <8ba465e6-f933-6224-ef74-c5aa898a1022@toxicpanda.com>
_data_ On Mon, Jun 14, 2021 at 8:53 PM Josef Bacik <josef@toxicpanda.com> wrote:
>
> On 6/14/21 1:41 AM, syzbot wrote:
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit: 6099c9da x86: entry: speculatively unpoison pt_regs in do_..
> > git tree: https://github.com/google/kmsan.git master
> > console output: https://syzkaller.appspot.com/x/log.txt?x=12c7a057d00000
> > kernel config: https://syzkaller.appspot.com/x/.config?x=4e6842a91012889c
> > dashboard link: https://syzkaller.appspot.com/bug?extid=8aa9678d1cda7a0432b7
> > compiler: Debian clang version 11.0.1-2
> > userspace arch: i386
> >
> > Unfortunately, I don't have any reproducer for this issue yet.
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+8aa9678d1cda7a0432b7@syzkaller.appspotmail.com
> >
> > tracehook_notify_resume include/linux/tracehook.h:189 [inline]
> > exit_to_user_mode_loop kernel/entry/common.c:174 [inline]
> > exit_to_user_mode_prepare+0x486/0x560 kernel/entry/common.c:208
> > __syscall_exit_to_user_mode_work kernel/entry/common.c:290 [inline]
> > syscall_exit_to_user_mode+0x35/0x60 kernel/entry/common.c:301
> > __do_fast_syscall_32+0x14f/0x180 arch/x86/entry/common.c:145
> > do_fast_syscall_32+0x77/0xd0 arch/x86/entry/common.c:168
> > do_SYSENTER_32+0x73/0x90 arch/x86/entry/common.c:211
> > entry_SYSENTER_compat_after_hwframe+0x4d/0x5c
> > =====================================================
> > =====================================================
> > BUG: KMSAN: uninit-value in btrfs_comp_cpu_keys fs/btrfs/ctree.c:1556 [inline]
> > BUG: KMSAN: uninit-value in comp_keys fs/btrfs/ctree.c:1528 [inline]
> > BUG: KMSAN: uninit-value in generic_bin_search+0x799/0xbc0 fs/btrfs/ctree.c:1702
> > CPU: 1 PID: 9364 Comm: syz-executor.2 Tainted: G B 5.12.0-rc6-syzkaller #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > Call Trace:
> > __dump_stack lib/dump_stack.c:79 [inline]
> > dump_stack+0x24c/0x2e0 lib/dump_stack.c:120
> > kmsan_report+0xfb/0x1e0 mm/kmsan/kmsan_report.c:118
> > __msan_warning+0x5c/0xa0 mm/kmsan/kmsan_instr.c:197
> > btrfs_comp_cpu_keys fs/btrfs/ctree.c:1556 [inline]
> > comp_keys fs/btrfs/ctree.c:1528 [inline]
> > generic_bin_search+0x799/0xbc0 fs/btrfs/ctree.c:1702
> > btrfs_bin_search fs/btrfs/ctree.c:1724 [inline]
>
> This appears to be a bug in KMSAN, the code is doing the correct thing and it
> appears to be complaining about tmp, which is initialized either in the if or
> else part. The else part may be what's confusing KMSAM here as it's essentially
>
> struct btrfs_disk_key tmp;
> struct btrfs_disk_key unaligned;
>
> else {
> memcpy(&unaligned, ptr, len); // read_extent_buffer is basically memcpy
> tmp = &unaligned;
> }
Hi Josef,
Thanks for looking into this.
The mere fact of assigning something to a variable does not make it initialized.
Namely,
int y;
int x = y;
if (x) ...
is not much different from:
int x;
if (x) ...
Are you sure the data used in the code is in fact initialized?
KMSAN says the data originated in btrfs_alloc_tree_block. Was it
initialized? Where?
prev parent reply other threads:[~2021-06-15 8:33 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-14 5:41 [syzbot] KMSAN: uninit-value in generic_bin_search (2) syzbot
2021-06-14 18:52 ` Josef Bacik
2021-06-15 8:32 ` Dmitry Vyukov [this message]
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