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Message-ID: <20180711200056.GA13192@cisco.cisco.com>
Date: Wed, 11 Jul 2018 14:00:56 -0600
From: Tycho Andersen <tycho@...ho.ws>
To: "Serge E. Hallyn" <serge@...lyn.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Jiri Slaby <jslaby@...e.com>, linux-serial@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] uart: fix race between uart_put_char() and
uart_shutdown()
On Wed, Jul 11, 2018 at 02:49:08PM -0500, Serge E. Hallyn wrote:
> Quoting Tycho Andersen (tycho@...ho.ws):
> > 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@...ho.ws>
>
> One question: would it be worth doing:
>
> > ---
> > 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);
>
> ignoring the alloc failure here. (leave a comment for worried
> future reviewers) Then,
>
> > + if (!page)
> > + return -ENOMEM;
> >
> > + if (uport)
> > + spin_lock_irqsave(&uport->lock, flags);
> > + if (state->xmit.buf) {
> > + free_page(page);
> > + } else {
>
> here if page is NULL (unlock and) return ENOMEM.
>
> Since it looks like this fn is called on every device open, that
> would seem to minimize getting lots of needless ENOMEMs.
>
> Another alternative is to just wait to do the alloc until we
> get here, and drop the spinlock here if we need to alloc. Then
> retake the lock, re-check state->xmit.buf; If that is now not
> null then free_page(page), or if it is still NULL and page alloc
> failed, then return ENOEMEM). That is uglier code, but is
> probably the best behavior.
>
> Still the original patch fixes the real bug so I'm fine with it
> as well.
Sure, I'm happy to implement whichever of these we think is best.
Greg?
Thanks,
Tycho
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