Re: UBSAN: Undefined behaviour in drivers/tty/vt/keyboard.c

[Kyungtae Kim]

On Mon, Mar 23, 2020 at 07:46:16AM +0100, Greg KH wrote:
> On Sun, Mar 22, 2020 at 11:34:01PM -0400, Kyungtae Kim wrote:
> > We report a bug (in linux-5.5.11) found by FuzzUSB (modified version
> > of syzkaller)
> >
> > Seems the variable "npadch" has a very large value (i.e., 333333333)
> > as a result of multiple executions of the function "k_ascii" (keyboard.c:888)
> > while the variable "base" has 10.
> > So their multiplication at line 888 in "k_ascii" will become
> > larger than the max of type int, causing such an integer overflow.
> >
> > I believe this can be solved by checking for overflow ahead of operations
> > e.g., using check_mul_overflow().
> >
> > kernel config: https://kt0755.github.io/etc/config_v5.5.11
>
> Great, can you send a patch for this?
>
> thanks,
>
> greg k-h

I'm not sure the following works best.

diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c
index 15d33fa0c925..c1ae9d2e6970 100644
--- a/drivers/tty/vt/keyboard.c
+++ b/drivers/tty/vt/keyboard.c
@@ -869,6 +869,7 @@ 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;
+       int bytes, res;

        if (up_flag)
                return;
@@ -884,6 +885,8 @@ static void k_ascii(struct vc_data *vc, unsigned
char value, char up_flag)

        if (npadch == -1)
                npadch = value;
+       else if (check_mul_overflow(npadch, base, &bytes) ||
check_add_overflow(bytes, value, &res))
+               return;
        else
                npadch = npadch * base + value;
 }

Thanks,
Kyungtae Kim