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Message-Id: <20180605000127.5495-1-tycho@tycho.ws>
Date: Mon, 4 Jun 2018 18:01:27 -0600
From: Tycho Andersen <tycho@...ho.ws>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Jiri Slaby <jslaby@...e.com>
Cc: linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
"Serge E . Hallyn" <serge@...lyn.com>,
Tycho Andersen <tycho@...ho.ws>
Subject: [PATCH] uart: fix race between uart_put_char() and uart_shutdown()
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, we simply also acquire state->port.mutex.
Unfortunately, I don't have any insightful thoughts about how to test this.
Ideas are appreciated :)
Signed-off-by: Tycho Andersen <tycho@...ho.ws>
---
drivers/tty/serial/serial_core.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)
diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 0466f9f08a91..883a8c15510c 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -532,9 +532,15 @@ static int uart_put_char(struct tty_struct *tty, unsigned char c)
unsigned long flags;
int ret = 0;
+ /*
+ * state->xmit.buf is protected by state->port.mutex, see the note in
+ * uart_port_startup()
+ */
+ mutex_lock(&state->port.mutex);
+
circ = &state->xmit;
if (!circ->buf)
- return 0;
+ goto out;
port = uart_port_lock(state, flags);
if (port && uart_circ_chars_free(circ) != 0) {
@@ -543,6 +549,9 @@ static int uart_put_char(struct tty_struct *tty, unsigned char c)
ret = 1;
}
uart_port_unlock(port, flags);
+
+out:
+ mutex_unlock(&state->port.mutex);
return ret;
}
--
2.17.0
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