vt: keyboard: avoid signed integer overflow in k_ascii

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>

diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c
index 15d33fa0c925de3d5ab03b55e031c54ebd41eceb..568b2171f335966b15d7e6cb9295577d54d8c3d8 100644 (file)
--- a/drivers/tty/vt/keyboard.c
+++ b/drivers/tty/vt/keyboard.c
@@ -127,7 +127,11 @@ static DEFINE_SPINLOCK(func_buf_lock); /* guard 'func_buf'  and friends */
 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, unsigned char value, char up_flag)
                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, unsigned char value, char up_flag)
 
 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, unsigned char value, char up_flag)
                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)