tty: panic in tty_ldisc_restore

[Dmitry Vyukov]

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));
        if (!ldata)
                goto err;


On commit 510948533b059f4f5033464f9f4a0c32d4ab0c08 of mmotm tree.