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From:   Alexander Potapenko <glider@google.com>
Date:   Thu, 12 Jul 2018 15:36:21 +0200
Message-ID: <CAG_fn=XDS-HTNQnzLixYHVFmg0EZkiFvZosOCALjoCMeNxYwWQ@mail.gmail.com>
Subject: Re: KMSAN: uninit-value in p9_client_rpc
To:     syzbot+4de40388f584432bf004@syzkaller.appspotmail.com
Cc:     David Miller <davem@davemloft.net>, ericvh@gmail.com,
        LKML <linux-kernel@vger.kernel.org>, lucho@ionkov.net,
        Networking <netdev@vger.kernel.org>, rminnich@sandia.gov,
        syzkaller-bugs@googlegroups.com,
        v9fs-developer@lists.sourceforge.net
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On Thu, Jul 12, 2018 at 11:24 AM syzbot
<syzbot+4de40388f584432bf004@syzkaller.appspotmail.com> wrote:
>
> syzbot has found a reproducer for the following crash on:
>
> HEAD commit:    b64f7ec04e12 kmsan: implement kmsan_memmove_shadow() and =
k..
> git tree:       https://github.com/google/kmsan.git/master
> console output: https://syzkaller.appspot.com/x/log.txt?x=3D12f6791c40000=
0
> kernel config:  https://syzkaller.appspot.com/x/.config?x=3D93d57043084ee=
e38
> dashboard link: https://syzkaller.appspot.com/bug?extid=3D4de40388f584432=
bf004
> compiler:       clang version 7.0.0 (trunk 334104)
> syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=3D147c36dc400=
000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=3D14e8704440000=
0
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit=
:
> Reported-by: syzbot+4de40388f584432bf004@syzkaller.appspotmail.com
>
> FS-Cache: O-key=3D[10] '34323934373135343132'
> FS-Cache: N-cookie c=3D(____ptrval____) [p=3D(____ptrval____) fl=3D2 nc=
=3D0 na=3D1]
> FS-Cache: N-cookie d=3D(____ptrval____) n=3D(____ptrval____)
> FS-Cache: N-key=3D[10] '34323934373135343132'
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> BUG: KMSAN: uninit-value in p9_client_rpc+0x194c/0x1dc0 net/9p/client.c:8=
18
> CPU: 1 PID: 4620 Comm: syz-executor262 Not tainted 4.18.0-rc4+ #24
> 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+0x185/0x1e0 lib/dump_stack.c:113
>   kmsan_report+0x195/0x2c0 mm/kmsan/kmsan.c:1092
>   __msan_warning_32+0x7d/0xe0 mm/kmsan/kmsan_instr.c:640
>   p9_client_rpc+0x194c/0x1dc0 net/9p/client.c:818
>   p9_client_attach+0x35b/0xc30 net/9p/client.c:1147
>   v9fs_session_init+0x24b9/0x2970 fs/9p/v9fs.c:449
>   v9fs_mount+0x107/0x11b0 fs/9p/vfs_super.c:135
>   mount_fs+0x29b/0x780 fs/super.c:1277
>   vfs_kern_mount+0x222/0x990 fs/namespace.c:1037
>   do_new_mount fs/namespace.c:2518 [inline]
>   do_mount+0xd30/0x5310 fs/namespace.c:2848
>   ksys_mount+0x32e/0x3d0 fs/namespace.c:3064
>   __do_sys_mount fs/namespace.c:3078 [inline]
>   __se_sys_mount fs/namespace.c:3075 [inline]
>   __x64_sys_mount+0x157/0x1c0 fs/namespace.c:3075
>   do_syscall_64+0x15b/0x230 arch/x86/entry/common.c:290
>   entry_SYSCALL_64_after_hwframe+0x63/0xe7
> RIP: 0033:0x445f79
> Code: e8 cc e6 ff ff 48 83 c4 18 c3 0f 1f 80 00 00 00 00 48 89 f8 48 89 f=
7
> 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff
> ff 0f 83 3b 0d fc ff c3 66 2e 0f 1f 84 00 00 00 00
> RSP: 002b:00007ffb83104da8 EFLAGS: 00000202 ORIG_RAX: 00000000000000a5
> RAX: ffffffffffffffda RBX: 00000000006dbc3c RCX: 0000000000445f79
> RDX: 0000000020000180 RSI: 00000000200000c0 RDI: 0000000000000000
> RBP: 00000000006dbc38 R08: 00000000200001c0 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000202 R12: 0030656c69662f2e
> R13: 64663d736e617274 R14: 7974697275636573 R15: 0000000000000001
>
> Local variable description: ----ecode.i@p9_client_rpc
> Variable was created at:
>   p9_client_rpc+0x183/0x1dc0 net/9p/client.c:750
>   p9_client_attach+0x35b/0xc30 net/9p/client.c:1147
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
FWIW the bug occurs here:
https://elixir.bootlin.com/linux/latest/source/net/9p/client.c#L560

  } else {
    err =3D p9pdu_readf(req->rc, c->proto_version, "d", &ecode);
    err =3D -ecode;

    p9_debug(P9_DEBUG_9P, "<<< RLERROR (%d)\n", -ecode);
  }

When p9pdu_readf() fails with -EFAULT, |ecode| may remain uninitialized.
We need to check the value of |err| before assigning |-ecode| to it.

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--=20
Alexander Potapenko
Software Engineer

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