On Sat, May 23, 2020 at 9:14 AM syzbot <syzbot+c6bfc3db991edc918432@xxxxxxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxx > > 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@xxxxxxxxxxxxxxxx. > > 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.
