Re: tty/vt/keyboard: out of bounds access in do_compute_shiftstate

[Pavel Machek]

Hi!

(Vojtech added to cc list. This is history.)

> I've hit the following while fuzzing with syzkaller inside a KVM tools guest
> running the latest -next kernel:

What does syzkaller do? Random syscalls?



> [ 2662.777566] BUG: KASAN: global-out-of-bounds in do_compute_shiftstate+0x161/0x370 at addr ffffffffb2e686a0
> 
> [ 2662.777592] Read of size 2 by task syz-executor/30576
> 
> [ 2662.777676] Address belongs to variable plain_map+0x200/0x3540
>

/*
 * Called after returning from RAW mode or when changing consoles - recompute
 * shift_down[] and shift_state from key_down[] maybe called when keymap is
 * undefined, so that shiftkey release is seen. The caller must hold the
 * kbd_event_lock.
 */

static void do_compute_shiftstate(void)
{
	unsigned int i, j, k, sym, val;

	shift_state = 0;
	memset(shift_down, 0, sizeof(shift_down));

	for (i = 0; i < ARRAY_SIZE(key_down); i++) {

		if (!key_down[i])
			continue;

		k = i * BITS_PER_LONG;

		for (j = 0; j < BITS_PER_LONG; j++, k++) {

			if (!test_bit(k, key_down))
				continue;

			sym = U(key_maps[0][k]);
			if (KTYP(sym) != KT_SHIFT && KTYP(sym) != KT_SLOCK)
				continue;

			val = KVAL(sym);
			if (val == KVAL(K_CAPSSHIFT))
				val = KVAL(K_SHIFT);

			shift_down[val]++;
			shift_state |= (1 << val);
		}
	}
}

Best regards,
									Pavel

> [ 2662.777727] CPU: 2 PID: 30576 Comm: syz-executor Tainted: G    B           4.6.0-next-20160527-sasha-00024-g6ab0dc9-dirty #3098
> 
> [ 2662.777792]  1ffff1001635bde7 00000000dc4d92d7 ffff8800b1adefc0 ffffffffa3fd0b37
> 
> [ 2662.777826]  ffffffff00000002 fffffbfff5deeda4 0000000041b58ab3 ffffffffae8dd830
> 
> [ 2662.777862]  ffffffffa3fd09c8 ffffffffa23fa2e8 ffff8800b1adef98 ffff8801d42a20f8
> 
> [ 2662.777868] Call Trace:
> 
> [ 2662.777935] dump_stack (lib/dump_stack.c:53)
> [ 2662.778136] kasan_report_error (include/linux/kasan.h:28 mm/kasan/report.c:211 mm/kasan/report.c:277)
> [ 2662.778265] __asan_report_load2_noabort (mm/kasan/report.c:317)
> [ 2662.778318] do_compute_shiftstate (drivers/tty/vt/keyboard.c:386)
> [ 2662.778332] fn_null (drivers/tty/vt/keyboard.c:625)
> [ 2662.778345] k_spec (drivers/tty/vt/keyboard.c:645)
> [ 2662.778361] kbd_event (drivers/tty/vt/keyboard.c:1459 drivers/tty/vt/keyboard.c:1475)
> [ 2662.778907] input_to_handler (drivers/input/input.c:120 (discriminator 3))
> [ 2662.778926] input_pass_values (drivers/input/input.c:148)
> [ 2662.778944] input_handle_event (drivers/input/input.c:406)
> [ 2662.779015] input_inject_event (include/linux/rcupdate.h:910 drivers/input/input.c:467)
> [ 2662.779034] evdev_do_ioctl (drivers/input/evdev.c:1102)
> [ 2662.779271] evdev_ioctl_handler (drivers/input/evdev.c:1302)
> [ 2662.779305] evdev_ioctl (drivers/input/evdev.c:1312)
> [ 2662.779320] do_vfs_ioctl (fs/ioctl.c:44 fs/ioctl.c:674)
> [ 2662.779463] SyS_ioctl (fs/ioctl.c:689 fs/ioctl.c:680)
> [ 2662.779511] do_syscall_64 (arch/x86/entry/common.c:350)
> [ 2662.779530] entry_SYSCALL64_slow_path (arch/x86/entry/entry_64.S:251)
> [ 2662.779535] Memory state around the buggy address:
> 
> [ 2662.779566]  ffffffffb2e68580: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> 
> [ 2662.779579]  ffffffffb2e68600: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> 
> [ 2662.779589] >ffffffffb2e68680: 00 00 00 00 fa fa fa fa 00 00 00 00 00 00 00 00
> 
> [ 2662.779594]                                ^
> 
> [ 2662.779604]  ffffffffb2e68700: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> 
> [ 2662.779615]  ffffffffb2e68780: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> 
> 
> Thanks,
> Sasha

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