Messages in this thread | | | Date | Thu, 2 Mar 2017 20:27:31 +0100 | From | Greg Kroah-Hartman <> | Subject | Re: tty: panic in tty_ldisc_restore |
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On Thu, Mar 02, 2017 at 07:27:35PM +0100, Dmitry Vyukov wrote: > On Tue, Feb 28, 2017 at 7:11 PM, Dmitry Vyukov <dvyukov@google.com> wrote: > > On Fri, Feb 17, 2017 at 10:51 PM, Dmitry Vyukov <dvyukov@google.com> wrote: > >>>>> >> >> Hello, > >>>>> >> >> > >>>>> >> >> Syzkaller fuzzer started crashing kernel with the following panics: > >>>>> >> >> > >>>>> >> >> Kernel panic - not syncing: Couldn't open N_TTY ldisc for ircomm0 --- error -12. > >>>>> >> >> CPU: 0 PID: 5637 Comm: syz-executor3 Not tainted 4.9.0 #6 > >>>>> >> >> Hardware name: Google Google Compute Engine/Google Compute Engine, > >>>>> >> >> BIOS Google 01/01/2011 > >>>>> >> >> ffff8801d4ba7a18 ffffffff8234d0df ffffffff00000000 1ffff1003a974ed6 > >>>>> >> >> ffffed003a974ece 0000000041b58ab3 ffffffff84b38180 ffffffff8234cdf1 > >>>>> >> >> 0000000000000000 0000000000000000 ffff8801d4ba76a8 00000000dabb4fad > >>>>> >> >> Call Trace: > >>>>> >> >> [<ffffffff8234d0df>] __dump_stack lib/dump_stack.c:15 [inline] > >>>>> >> >> [<ffffffff8234d0df>] dump_stack+0x2ee/0x3ef lib/dump_stack.c:51 > >>>>> >> >> [<ffffffff818280d4>] panic+0x1fb/0x412 kernel/panic.c:179 > >>>>> >> >> [<ffffffff826bb0d4>] tty_ldisc_restore drivers/tty/tty_ldisc.c:520 [inline] > >>>>> >> >> [<ffffffff826bb0d4>] tty_set_ldisc+0x704/0x8b0 drivers/tty/tty_ldisc.c:579 > >>>>> >> >> [<ffffffff826a3a93>] tiocsetd drivers/tty/tty_io.c:2667 [inline] > >>>>> >> >> [<ffffffff826a3a93>] tty_ioctl+0xc63/0x2370 drivers/tty/tty_io.c:2924 > >>>>> >> >> [<ffffffff81a7a22f>] vfs_ioctl fs/ioctl.c:43 [inline] > >>>>> >> >> [<ffffffff81a7a22f>] do_vfs_ioctl+0x1bf/0x1630 fs/ioctl.c:679 > >>>>> >> >> [<ffffffff81a7b72f>] SYSC_ioctl fs/ioctl.c:694 [inline] > >>>>> >> >> [<ffffffff81a7b72f>] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:685 > >>>>> >> >> [<ffffffff84377941>] entry_SYSCALL_64_fastpath+0x1f/0xc2 > >>>>> >> >> > >>>>> >> >> Kernel panic - not syncing: Couldn't open N_TTY ldisc for ptm2 --- error -12. > >>>>> >> >> CPU: 0 PID: 7844 Comm: syz-executor0 Not tainted 4.9.0 #6 > >>>>> >> >> Hardware name: Google Google Compute Engine/Google Compute Engine, > >>>>> >> >> BIOS Google 01/01/2011 > >>>>> >> >> ffff8801c3307a18 ffffffff8234d0df ffffffff00000000 1ffff10038660ed6 > >>>>> >> >> ffffed0038660ece 0000000041b58ab3 ffffffff84b38180 ffffffff8234cdf1 > >>>>> >> >> 0000000000000000 0000000000000000 ffff8801c33076a8 00000000dabb4fad > >>>>> >> >> Call Trace: > >>>>> >> >> [<ffffffff8234d0df>] __dump_stack lib/dump_stack.c:15 [inline] > >>>>> >> >> [<ffffffff8234d0df>] dump_stack+0x2ee/0x3ef lib/dump_stack.c:51 > >>>>> >> >> [<ffffffff818280d4>] panic+0x1fb/0x412 kernel/panic.c:179 > >>>>> >> >> [<ffffffff826bb0d4>] tty_ldisc_restore drivers/tty/tty_ldisc.c:520 [inline] > >>>>> >> >> [<ffffffff826bb0d4>] tty_set_ldisc+0x704/0x8b0 drivers/tty/tty_ldisc.c:579 > >>>>> >> >> [<ffffffff826a3a93>] tiocsetd drivers/tty/tty_io.c:2667 [inline] > >>>>> >> >> [<ffffffff826a3a93>] tty_ioctl+0xc63/0x2370 drivers/tty/tty_io.c:2924 > >>>>> >> >> [<ffffffff81a7a22f>] vfs_ioctl fs/ioctl.c:43 [inline] > >>>>> >> >> [<ffffffff81a7a22f>] do_vfs_ioctl+0x1bf/0x1630 fs/ioctl.c:679 > >>>>> >> >> [<ffffffff81a7b72f>] SYSC_ioctl fs/ioctl.c:694 [inline] > >>>>> >> >> [<ffffffff81a7b72f>] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:685 > >>>>> >> >> [<ffffffff84377941>] entry_SYSCALL_64_fastpath+0x1f/0xc2 > >>>>> >> >> > >>>>> >> >> > >>>>> >> >> In all cases there is a vmalloc failure right before that: > >>>>> >> >> > >>>>> >> >> syz-executor4: vmalloc: allocation failure, allocated 0 of 16384 > >>>>> >> >> bytes, mode:0x14000c2(GFP_KERNEL|__GFP_HIGHMEM), nodemask=(null) > >>>>> >> >> syz-executor4 cpuset=/ mems_allowed=0 > >>>>> >> >> CPU: 1 PID: 4852 Comm: syz-executor4 Not tainted 4.9.0 #6 > >>>>> >> >> Hardware name: Google Google Compute Engine/Google Compute Engine, > >>>>> >> >> BIOS Google 01/01/2011 > >>>>> >> >> ffff8801c41df898 ffffffff8234d0df ffffffff00000001 1ffff1003883bea6 > >>>>> >> >> ffffed003883be9e 0000000041b58ab3 ffffffff84b38180 ffffffff8234cdf1 > >>>>> >> >> 0000000000000282 ffffffff84fd53c0 ffff8801dae65b38 ffff8801c41df4d0 > >>>>> >> >> Call Trace: > >>>>> >> >> [< inline >] __dump_stack lib/dump_stack.c:15 > >>>>> >> >> [<ffffffff8234d0df>] dump_stack+0x2ee/0x3ef lib/dump_stack.c:51 > >>>>> >> >> [<ffffffff8186530f>] warn_alloc+0x21f/0x360 > >>>>> >> >> [<ffffffff819792c9>] __vmalloc_node_range+0x4e9/0x770 > >>>>> >> >> [< inline >] __vmalloc_node mm/vmalloc.c:1749 > >>>>> >> >> [< inline >] __vmalloc_node_flags mm/vmalloc.c:1763 > >>>>> >> >> [<ffffffff8197961b>] vmalloc+0x5b/0x70 mm/vmalloc.c:1778 > >>>>> >> >> [<ffffffff826ad77b>] n_tty_open+0x1b/0x470 drivers/tty/n_tty.c:1883 > >>>>> >> >> [<ffffffff826ba973>] tty_ldisc_open.isra.3+0x73/0xd0 > >>>>> >> >> drivers/tty/tty_ldisc.c:463 > >>>>> >> >> [< inline >] tty_ldisc_restore drivers/tty/tty_ldisc.c:510 > >>>>> >> >> [<ffffffff826bafb4>] tty_set_ldisc+0x5e4/0x8b0 drivers/tty/tty_ldisc.c:579 > >>>>> >> >> [< inline >] tiocsetd drivers/tty/tty_io.c:2667 > >>>>> >> >> [<ffffffff826a3a93>] tty_ioctl+0xc63/0x2370 drivers/tty/tty_io.c:2924 > >>>>> >> >> [<ffffffff81a7a22f>] do_vfs_ioctl+0x1bf/0x1630 > >>>>> >> >> [< inline >] SYSC_ioctl fs/ioctl.c:698 > >>>>> >> >> [<ffffffff81a7b72f>] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:689 > >>>>> >> >> [<ffffffff84377941>] entry_SYSCALL_64_fastpath+0x1f/0xc2 > >>>>> >> >> arch/x86/entry/entry_64.S:204 > >>>>> >> >> > >>>>> >> >> > >>>>> >> >> I've found that it's even documented in the source code, but it does > >>>>> >> >> not look like a good failure mode for allocation failure: > >>>>> >> >> > >>>>> >> >> static int n_tty_open(struct tty_struct *tty) > >>>>> >> >> { > >>>>> >> >> struct n_tty_data *ldata; > >>>>> >> >> > >>>>> >> >> /* Currently a malloc failure here can panic */ > >>>>> >> >> ldata = vmalloc(sizeof(*ldata)); > >>>>> >> > > >>>>> >> > How are you running out of vmalloc() memory? > >>>>> >> > >>>>> >> > >>>>> >> I don't know exactly. But it does not seem to represent a problem for > >>>>> >> the fuzzer. > >>>>> >> Is it meant to be very hard to do? > >>>>> > > >>>>> > Yes, do you know of any normal way to cause it to fail? > >>>>> > >>>>> > >>>>> I don't. But I means approximately nothing. > >>>>> Do you mean that it is not possible to trigger? > >>>>> Doesn't simply creating lots of kernel resources (files, sockets, > >>>>> pipe) will do the trick? Or just paging in lots of memory? Even if the > >>>>> process itself will be chosen as OOM kill target, it will still take > >>>>> the machine down with itself due to the panic while returning from the > >>>>> syscall, no? > >>>> > >>>> I'm not saying that it's impossible, just an "almost" impossible thing > >>>> to hit. Obviously you have hit it, so it can happen :) > >>>> > >>>> But, how to fix it? I really don't know. Unwinding a failure at this > >>>> point in time is very tough, as that comment shows. Any suggestions of > >>>> how it could be resolved are greatly appreciated. > >>> > >>> Is it possible to not shutdown the old discipline tty_set_ldisc before > >>> we prepare everything for the new one: > >>> > >>> /* Shutdown the old discipline. */ > >>> tty_ldisc_close(tty, old_ldisc); > >>> > >>> Currently it does: > >>> > >>> close(old) > >>> if (open(new)) > >>> open(old) // assume never fails > >>> > >>> it looks inherently problematic. > >>> Couldn't we do: > >>> > >>> if (open(new)) > >>> return -ESOMETHING > >>> close(old) > >>> > >>> ? > >> > >> > >> Or can we just kill the task? Still better than kernel panic. > > > > I guess we can't get away with killing the task as tty will be left in > > inconsistent state and it is accessible to other tasks. > > But what creating new ldisk first and then, if that succeeds, > > destroying the old one? > > > This is hurting us badly.
Really? How? Are you hitting this a lot? Why now and never before? Are you really out of memory?
> Opening new disk before closing the old one turned out to be hard (too > much state saved in tty). > How about this one? It reuses the existing tty_ldisc_reinit helper. If > opening the old disk and N_TTY fails, it leaves ldisk == NULL. But > it's already possible in tty_ldisc_hangup, and the code seems to be > prepared for this.
<snip>
I'll look at this after -rc1 is out, thanks.
greg k-h
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