• Tycho Andersen's avatar
    uart: fix race between uart_put_char() and uart_shutdown() · a5ba1d95
    Tycho Andersen authored
    We have reports of the following crash:
    
        PID: 7 TASK: ffff88085c6d61c0 CPU: 1 COMMAND: "kworker/u25:0"
        #0 [ffff88085c6db710] machine_kexec at ffffffff81046239
        #1 [ffff88085c6db760] crash_kexec at ffffffff810fc248
        #2 [ffff88085c6db830] oops_end at ffffffff81008ae7
        #3 [ffff88085c6db860] no_context at ffffffff81050b8f
        #4 [ffff88085c6db8b0] __bad_area_nosemaphore at ffffffff81050d75
        #5 [ffff88085c6db900] bad_area_nosemaphore at ffffffff81050e83
        #6 [ffff88085c6db910] __do_page_fault at ffffffff8105132e
        #7 [ffff88085c6db9b0] do_page_fault at ffffffff8105152c
        #8 [ffff88085c6db9c0] page_fault at ffffffff81a3f122
        [exception RIP: uart_put_char+149]
        RIP: ffffffff814b67b5 RSP: ffff88085c6dba78 RFLAGS: 00010006
        RAX: 0000000000000292 RBX: ffffffff827c5120 RCX: 0000000000000081
        RDX: 0000000000000000 RSI: 000000000000005f RDI: ffffffff827c5120
        RBP: ffff88085c6dba98 R8: 000000000000012c R9: ffffffff822ea320
        R10: ffff88085fe4db04 R11: 0000000000000001 R12: ffff881059f9c000
        R13: 0000000000000001 R14: 000000000000005f R15: 0000000000000fba
        ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018
        #9 [ffff88085c6dbaa0] tty_put_char at ffffffff81497544
        #10 [ffff88085c6dbac0] do_output_char at ffffffff8149c91c
        #11 [ffff88085c6dbae0] __process_echoes at ffffffff8149cb8b
        #12 [ffff88085c6dbb30] commit_echoes at ffffffff8149cdc2
        #13 [ffff88085c6dbb60] n_tty_receive_buf_fast at ffffffff8149e49b
        #14 [ffff88085c6dbbc0] __receive_buf at ffffffff8149ef5a
        #15 [ffff88085c6dbc20] n_tty_receive_buf_common at ffffffff8149f016
        #16 [ffff88085c6dbca0] n_tty_receive_buf2 at ffffffff8149f194
        #17 [ffff88085c6dbcb0] flush_to_ldisc at ffffffff814a238a
        #18 [ffff88085c6dbd50] process_one_work at ffffffff81090be2
        #19 [ffff88085c6dbe20] worker_thread at ffffffff81091b4d
        #20 [ffff88085c6dbeb0] kthread at ffffffff81096384
        #21 [ffff88085c6dbf50] ret_from_fork at ffffffff81a3d69f​
    
    after slogging through some dissasembly:
    
    ffffffff814b6720 <uart_put_char>:
    ffffffff814b6720:	55                   	push   %rbp
    ffffffff814b6721:	48 89 e5             	mov    %rsp,%rbp
    ffffffff814b6724:	48 83 ec 20          	sub    $0x20,%rsp
    ffffffff814b6728:	48 89 1c 24          	mov    %rbx,(%rsp)
    ffffffff814b672c:	4c 89 64 24 08       	mov    %r12,0x8(%rsp)
    ffffffff814b6731:	4c 89 6c 24 10       	mov    %r13,0x10(%rsp)
    ffffffff814b6736:	4c 89 74 24 18       	mov    %r14,0x18(%rsp)
    ffffffff814b673b:	e8 b0 8e 58 00       	callq  ffffffff81a3f5f0 <mcount>
    ffffffff814b6740:	4c 8b a7 88 02 00 00 	mov    0x288(%rdi),%r12
    ffffffff814b6747:	45 31 ed             	xor    %r13d,%r13d
    ffffffff814b674a:	41 89 f6             	mov    %esi,%r14d
    ffffffff814b674d:	49 83 bc 24 70 01 00 	cmpq   $0x0,0x170(%r12)
    ffffffff814b6754:	00 00
    ffffffff814b6756:	49 8b 9c 24 80 01 00 	mov    0x180(%r12),%rbx
    ffffffff814b675d:	00
    ffffffff814b675e:	74 2f                	je     ffffffff814b678f <uart_put_char+0x6f>
    ffffffff814b6760:	48 89 df             	mov    %rbx,%rdi
    ffffffff814b6763:	e8 a8 67 58 00       	callq  ffffffff81a3cf10 <_raw_spin_lock_irqsave>
    ffffffff814b6768:	41 8b 8c 24 78 01 00 	mov    0x178(%r12),%ecx
    ffffffff814b676f:	00
    ffffffff814b6770:	89 ca                	mov    %ecx,%edx
    ffffffff814b6772:	f7 d2                	not    %edx
    ffffffff814b6774:	41 03 94 24 7c 01 00 	add    0x17c(%r12),%edx
    ffffffff814b677b:	00
    ffffffff814b677c:	81 e2 ff 0f 00 00    	and    $0xfff,%edx
    ffffffff814b6782:	75 23                	jne    ffffffff814b67a7 <uart_put_char+0x87>
    ffffffff814b6784:	48 89 c6             	mov    %rax,%rsi
    ffffffff814b6787:	48 89 df             	mov    %rbx,%rdi
    ffffffff814b678a:	e8 e1 64 58 00       	callq  ffffffff81a3cc70 <_raw_spin_unlock_irqrestore>
    ffffffff814b678f:	44 89 e8             	mov    %r13d,%eax
    ffffffff814b6792:	48 8b 1c 24          	mov    (%rsp),%rbx
    ffffffff814b6796:	4c 8b 64 24 08       	mov    0x8(%rsp),%r12
    ffffffff814b679b:	4c 8b 6c 24 10       	mov    0x10(%rsp),%r13
    ffffffff814b67a0:	4c 8b 74 24 18       	mov    0x18(%rsp),%r14
    ffffffff814b67a5:	c9                   	leaveq
    ffffffff814b67a6:	c3                   	retq
    ffffffff814b67a7:	49 8b 94 24 70 01 00 	mov    0x170(%r12),%rdx
    ffffffff814b67ae:	00
    ffffffff814b67af:	48 63 c9             	movslq %ecx,%rcx
    ffffffff814b67b2:	41 b5 01             	mov    $0x1,%r13b
    ffffffff814b67b5:	44 88 34 0a          	mov    %r14b,(%rdx,%rcx,1)
    ffffffff814b67b9:	41 8b 94 24 78 01 00 	mov    0x178(%r12),%edx
    ffffffff814b67c0:	00
    ffffffff814b67c1:	83 c2 01             	add    $0x1,%edx
    ffffffff814b67c4:	81 e2 ff 0f 00 00    	and    $0xfff,%edx
    ffffffff814b67ca:	41 89 94 24 78 01 00 	mov    %edx,0x178(%r12)
    ffffffff814b67d1:	00
    ffffffff814b67d2:	eb b0                	jmp    ffffffff814b6784 <uart_put_char+0x64>
    ffffffff814b67d4:	66 66 66 2e 0f 1f 84 	data32 data32 nopw %cs:0x0(%rax,%rax,1)
    ffffffff814b67db:	00 00 00 00 00
    
    for our build, this is crashing at:
    
        circ->buf[circ->head] = c;
    
    Looking in uart_port_startup(), it seems that circ->buf (state->xmit.buf)
    protected by the "per-port mutex", which based on uart_port_check() is
    state->port.mutex. Indeed, the lock acquired in uart_put_char() is
    uport->lock, i.e. not the same lock.
    
    Anyway, since the lock is not acquired, if uart_shutdown() is called, the
    last chunk of that function may release state->xmit.buf before its assigned
    to null, and cause the race above.
    
    To fix it, let's lock uport->lock when allocating/deallocating
    state->xmit.buf in addition to the per-port mutex.
    
    v2: switch to locking uport->lock on allocation/deallocation instead of
        locking the per-port mutex in uart_put_char. Note that since
        uport->lock is a spin lock, we have to switch the allocation to
        GFP_ATOMIC.
    v3: move the allocation outside the lock, so we can switch back to
        GFP_KERNEL
    Signed-off-by: default avatarTycho Andersen <tycho@tycho.ws>
    Cc: stable <stable@vger.kernel.org>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    a5ba1d95
serial_core.c 75 KB