[PATCH 5.4 22/34] vt: keyboard: avoid signed integer overflow in k_ascii

Greg Kroah-Hartman
2020-06-09T17:54:57Z

From: Dmitry Torokhov <dmitry.torokhov@gmail.com>

commit b86dab054059b970111b5516ae548efaae5b3aae upstream.

When k_ascii is invoked several times in a row there is a potential for
signed integer overflow:

UBSAN: Undefined behaviour in drivers/tty/vt/keyboard.c:888:19 signed integer overflow:
10 * 1111111111 cannot be represented in type 'int'
CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.6.11 #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
Call Trace:
<IRQ>
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0xce/0x128 lib/dump_stack.c:118
ubsan_epilogue+0xe/0x30 lib/ubsan.c:154
handle_overflow+0xdc/0xf0 lib/ubsan.c:184
__ubsan_handle_mul_overflow+0x2a/0x40 lib/ubsan.c:205
k_ascii+0xbf/0xd0 drivers/tty/vt/keyboard.c:888
kbd_keycode drivers/tty/vt/keyboard.c:1477 [inline]
kbd_event+0x888/0x3be0 drivers/tty/vt/keyboard.c:1495

While it can be worked around by using check_mul_overflow()/
check_add_overflow(), it is better to introduce a separate flag to
signal that number pad is being used to compose a symbol, and
change type of the accumulator from signed to unsigned, thus
avoiding undefined behavior when it overflows.

Reported-by: Kyungtae Kim <kt0755@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200525232740.GA262061@dtor-ws
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/tty/vt/keyboard.c | 26 ++++++++++++++++----------
1 file changed, 16 insertions(+), 10 deletions(-)

--- a/drivers/tty/vt/keyboard.c
+++ b/drivers/tty/vt/keyboard.c
@@ -127,7 +127,11 @@ static DEFINE_SPINLOCK(func_buf_lock); /
static unsigned long key_down[BITS_TO_LONGS(KEY_CNT)]; /* keyboard key bitmap */
static unsigned char shift_down[NR_SHIFT]; /* shift state counters.. */
static bool dead_key_next;
-static int npadch = -1; /* -1 or number assembled on pad */
+
+/* Handles a number being assembled on the number pad */
+static bool npadch_active;
+static unsigned int npadch_value;
+
static unsigned int diacr;
static char rep; /* flag telling character repeat */

@@ -845,12 +849,12 @@ static void k_shift(struct vc_data *vc,
shift_state &= ~(1 << value);

/* kludge */
- if (up_flag && shift_state != old_state && npadch != -1) {
+ if (up_flag && shift_state != old_state && npadch_active) {
if (kbd->kbdmode == VC_UNICODE)
- to_utf8(vc, npadch);
+ to_utf8(vc, npadch_value);
else
- put_queue(vc, npadch & 0xff);
- npadch = -1;
+ put_queue(vc, npadch_value & 0xff);
+ npadch_active = false;
}
}

@@ -868,7 +872,7 @@ static void k_meta(struct vc_data *vc, u

static void k_ascii(struct vc_data *vc, unsigned char value, char up_flag)
{
- int base;
+ unsigned int base;

if (up_flag)
return;
@@ -882,10 +886,12 @@ static void k_ascii(struct vc_data *vc,
base = 16;
}

- if (npadch == -1)
- npadch = value;
- else
- npadch = npadch * base + value;
+ if (!npadch_active) {
+ npadch_value = 0;
+ npadch_active = true;
+ }
+
+ npadch_value = npadch_value * base + value;
}

static void k_lock(struct vc_data *vc, unsigned char value, char up_flag)