3.16.62-rc1 review patch. If anyone has any objections, please let me know.
------------------
From: Tycho Andersen tycho@tycho.ws
commit a5ba1d95e46ecaea638ddd7cd144107c783acb5d upstream.
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: Tycho Andersen tycho@tycho.ws Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org [bwh: Backported to 3.16: Use uport->lock directly rather than through uart_port_{,un}lock()] Signed-off-by: Ben Hutchings ben@decadent.org.uk --- drivers/tty/serial/serial_core.c | 17 ++++++++++++----- 1 file changed, 12 insertions(+), 5 deletions(-)
--- a/drivers/tty/serial/serial_core.c +++ b/drivers/tty/serial/serial_core.c @@ -131,6 +131,7 @@ static int uart_port_startup(struct tty_ struct uart_port *uport = state->uart_port; struct tty_port *port = &state->port; unsigned long page; + unsigned long flags = 0; int retval = 0;
if (uport->type == PORT_UNKNOWN) @@ -145,15 +146,18 @@ static int uart_port_startup(struct tty_ * 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;
+ spin_lock_irqsave(&uport->lock, flags); + if (!state->xmit.buf) { state->xmit.buf = (unsigned char *) page; uart_circ_clear(&state->xmit); + } else { + free_page(page); } + spin_unlock_irqrestore(&uport->lock, flags);
retval = uport->ops->startup(uport); if (retval == 0) { @@ -232,6 +236,7 @@ static void uart_shutdown(struct tty_str { struct uart_port *uport = state->uart_port; struct tty_port *port = &state->port; + unsigned long flags = 0;
/* * Set the TTY IO error marker @@ -262,10 +267,12 @@ static void uart_shutdown(struct tty_str /* * Free the transmit buffer page. */ + spin_lock_irqsave(&uport->lock, flags); if (state->xmit.buf) { free_page((unsigned long)state->xmit.buf); state->xmit.buf = NULL; } + spin_unlock_irqrestore(&uport->lock, flags); }
/**