Quoting Tycho Andersen (tycho@xxxxxxxx):
> 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
> v4: * switch to positive condition of state->xmit.buf in
> uart_port_startup()
> * declare `flags` on its own line
> * use the result of uart_port_lock() in uart_shutdown() to avoid
> uninitialized warning
> * don't use the uart_port_lock/unlock macros in uart_port_startup,
> instead test against uport directly; the compiler can't seem to "see"
> through the macros/ref/unref calls to not warn about uninitialized
> flags. We don't need to do a ref here since we hold the per-port
> mutex anyway.
>
> Signed-off-by: Tycho Andersen <tycho@xxxxxxxx>
Reviewed-by: Serge Hallyn <serge@xxxxxxxxxx>
Thanks, Tycho.
> ---
> drivers/tty/serial/serial_core.c | 19 ++++++++++++++-----
> 1 file changed, 14 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index 9c14a453f73c..406e8382d3de 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -182,6 +182,7 @@ static int uart_port_startup(struct tty_struct *tty, struct uart_state *state,
> {
> struct uart_port *uport = uart_port_check(state);
> unsigned long page;
> + unsigned long flags;
> int retval = 0;
>
> if (uport->type == PORT_UNKNOWN)
> @@ -196,15 +197,20 @@ static int uart_port_startup(struct tty_struct *tty, struct uart_state *state,
> * Initialise and allocate the transmit and temporary
> * buffer.
> */
> - if (!state->xmit.buf) {
> - /* This is protected by the per port mutex */
> - page = get_zeroed_page(GFP_KERNEL);
> - if (!page)
> - return -ENOMEM;
> + page = get_zeroed_page(GFP_KERNEL);
> + if (!page)
> + return -ENOMEM;
>
> + if (uport)
> + spin_lock_irqsave(&uport->lock, flags);
> + if (state->xmit.buf) {
> + free_page(page);
> + } else {
> state->xmit.buf = (unsigned char *) page;
> uart_circ_clear(&state->xmit);
> }
> + if (uport)
> + spin_unlock_irqrestore(&uport->lock, flags);
>
> retval = uport->ops->startup(uport);
> if (retval == 0) {
> @@ -263,6 +269,7 @@ static void uart_shutdown(struct tty_struct *tty, struct uart_state *state)
> {
> struct uart_port *uport = uart_port_check(state);
> struct tty_port *port = &state->port;
> + unsigned long flags;
>
> /*
> * Set the TTY IO error marker
> @@ -295,10 +302,12 @@ static void uart_shutdown(struct tty_struct *tty, struct uart_state *state)
> /*
> * Free the transmit buffer page.
> */
> + uport = uart_port_lock(state, flags);
> if (state->xmit.buf) {
> free_page((unsigned long)state->xmit.buf);
> state->xmit.buf = NULL;
> }
> + uart_port_unlock(uport, flags);
> }
>
> /**
> --
> 2.17.1
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