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From:   Ondrej Mosnacek <omosnace@redhat.com>
Date:   Sat, 23 May 2020 14:16:37 +0200
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Subject: Re: general protection fault in selinux_socket_recvmsg
To:     syzbot <syzbot+c6bfc3db991edc918432@syzkaller.appspotmail.com>
Cc:     andriin@fb.com, anton@enomsg.org, ast@kernel.org,
        bpf@vger.kernel.org, ccross@android.com, daniel@iogearbox.net,
        Eric Paris <eparis@parisplace.org>, john.fastabend@gmail.com,
        kafai@fb.com, Kees Cook <keescook@chromium.org>,
        kpsingh@chromium.org,
        Linux kernel mailing list <linux-kernel@vger.kernel.org>,
        network dev <netdev@vger.kernel.org>,
        Paul Moore <paul@paul-moore.com>,
        SElinux list <selinux@vger.kernel.org>, songliubraving@fb.com,
        Stephen Smalley <stephen.smalley.work@gmail.com>,
        syzkaller-bugs@googlegroups.com, tony.luck@intel.com, yhs@fb.com,
        Mat Martineau <mathew.j.martineau@linux.intel.com>,
        Matthieu Baerts <matthieu.baerts@tessares.net>,
        mptcp@lists.01.org, Paolo Abeni <pabeni@redhat.com>
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On Sat, May 23, 2020 at 9:14 AM syzbot
<syzbot+c6bfc3db991edc918432@syzkaller.appspotmail.com> wrote:
> Hello,
>
> syzbot found the following crash on:
>
> HEAD commit:    051143e1 Merge tag 'apparmor-pr-2020-05-21' of git://git.k..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1313f016100000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=b3368ce0cc5f5ace
> dashboard link: https://syzkaller.appspot.com/bug?extid=c6bfc3db991edc918432
> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=13eeacba100000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=167163e6100000
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+c6bfc3db991edc918432@syzkaller.appspotmail.com
>
> general protection fault, probably for non-canonical address 0xdffffc0000000003: 0000 [#1] PREEMPT SMP KASAN
> KASAN: null-ptr-deref in range [0x0000000000000018-0x000000000000001f]
> CPU: 0 PID: 7370 Comm: syz-executor283 Not tainted 5.7.0-rc6-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> RIP: 0010:selinux_socket_recvmsg+0x1e/0x40 security/selinux/hooks.c:4841

OK, this is obviously not a SELinux bug - the hook just gets sock ==
NULL from the net stack, which is just plain wrong...

> Code: e8 77 f9 1e fe 48 89 ef 5d eb b1 90 53 48 89 fb e8 67 f9 1e fe 48 8d 7b 18 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c 02 00 75 0f 48 8b 7b 18 be 02 00 00 00 5b e9 7d fc ff ff e8
> RSP: 0018:ffffc900019d7a58 EFLAGS: 00010206
> RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000040000000
> RDX: 0000000000000003 RSI: ffffffff83543bb9 RDI: 0000000000000018
> RBP: dffffc0000000000 R08: ffff88809f45a180 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000000 R12: dffffc0000000000
> R13: ffffc900019d7d78 R14: 0000000000001000 R15: 0000000040000000
> FS:  00007f5ffb311700(0000) GS:ffff8880ae600000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f5ffb2efe78 CR3: 00000000a33c1000 CR4: 00000000001406f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  security_socket_recvmsg+0x78/0xc0 security/security.c:2070
>  sock_recvmsg+0x47/0x110 net/socket.c:902
>  mptcp_recvmsg+0xb3b/0xd90 net/mptcp/protocol.c:891

...but this function looks suspicious. If the "unlikely(ssock)" check
at [1] fails, then we continue in the rest of the function with ssock
== NULL. However, when the "unlikely(__mptcp_tcp_fallback(msk))" check
at [2] succeeds, we jump to the "fallback" label, where it is assumed
that ssock != NULL. That seems like an obvious bug to me and I bet
that's what causes the crash.

If I understand the code correctly, the fix should be just to change
this at [2]:
- if (unlikely(__mptcp_tcp_fallback(msk)))
+ if (unlikely(ssock = __mptcp_tcp_fallback(msk)))

@MPTCP folks, can you have a look?

[1] https://elixir.bootlin.com/linux/v5.7-rc6/source/net/mptcp/protocol.c#L886
[2] https://elixir.bootlin.com/linux/v5.7-rc6/source/net/mptcp/protocol.c#L957

>  inet_recvmsg+0x121/0x5d0 net/ipv4/af_inet.c:838
>  sock_recvmsg_nosec net/socket.c:886 [inline]
>  sock_recvmsg net/socket.c:904 [inline]
>  sock_recvmsg+0xca/0x110 net/socket.c:900
>  __sys_recvfrom+0x1c5/0x2f0 net/socket.c:2057
>  __do_sys_recvfrom net/socket.c:2075 [inline]
>  __se_sys_recvfrom net/socket.c:2071 [inline]
>  __x64_sys_recvfrom+0xdd/0x1b0 net/socket.c:2071
>  do_syscall_64+0xf6/0x7d0 arch/x86/entry/common.c:295
>  entry_SYSCALL_64_after_hwframe+0x49/0xb3
> RIP: 0033:0x448ef9
> Code: e8 cc 14 03 00 48 83 c4 18 c3 0f 1f 80 00 00 00 00 48 89 f8 48 89 f7 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 9b 0c fc ff c3 66 2e 0f 1f 84 00 00 00 00
> RSP: 002b:00007f5ffb310da8 EFLAGS: 00000246 ORIG_RAX: 000000000000002d
> RAX: ffffffffffffffda RBX: 00000000006dec28 RCX: 0000000000448ef9
> RDX: 0000000000001000 RSI: 00000000200004c0 RDI: 0000000000000003
> RBP: 00000000006dec20 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000040000000 R11: 0000000000000246 R12: 00000000006dec2c
> R13: 00007ffc3a001ccf R14: 00007f5ffb3119c0 R15: 00000000006dec2c
> Modules linked in:
> ---[ end trace 60e1f3eb5a5b83ce ]---
> RIP: 0010:selinux_socket_recvmsg+0x1e/0x40 security/selinux/hooks.c:4841
> Code: e8 77 f9 1e fe 48 89 ef 5d eb b1 90 53 48 89 fb e8 67 f9 1e fe 48 8d 7b 18 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c 02 00 75 0f 48 8b 7b 18 be 02 00 00 00 5b e9 7d fc ff ff e8
> RSP: 0018:ffffc900019d7a58 EFLAGS: 00010206
> RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000040000000
> RDX: 0000000000000003 RSI: ffffffff83543bb9 RDI: 0000000000000018
> RBP: dffffc0000000000 R08: ffff88809f45a180 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000000 R12: dffffc0000000000
> R13: ffffc900019d7d78 R14: 0000000000001000 R15: 0000000040000000
> FS:  00007f5ffb311700(0000) GS:ffff8880ae600000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f5ffb2efe78 CR3: 00000000a33c1000 CR4: 00000000001406f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>
>
> ---
> 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 syzkaller@googlegroups.com.
>
> syzbot will keep track of this bug report. See:
> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> syzbot can test patches for this bug, for details see:
> https://goo.gl/tpsmEJ#testing-patches
>

-- 
Ondrej Mosnacek <omosnace at redhat dot com>
Software Engineer, Security Technologies
Red Hat, Inc.