On Tue, Feb 7, 2017 at 11:51 AM, Dmitry Vyukov <dvyu...@google.com> wrote: > On Tue, Feb 7, 2017 at 11:43 AM, Greg Kroah-Hartman > <gre...@linuxfoundation.org> wrote: >> On Tue, Feb 07, 2017 at 11:24:13AM +0100, Dmitry Vyukov wrote: >>> On Thu, Feb 2, 2017 at 7:23 PM, Greg Kroah-Hartman >>> <gre...@linuxfoundation.org> wrote: >>> > On Thu, Feb 02, 2017 at 07:03:41PM +0100, Dmitry Vyukov wrote: >>> >> On Thu, Feb 2, 2017 at 6:55 PM, Greg Kroah-Hartman >>> >> <gre...@linuxfoundation.org> wrote: >>> >> > On Thu, Feb 02, 2017 at 06:48:48PM +0100, Dmitry Vyukov 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.