Re: [PATCH 2/2] vsprintf: Stop using obsolete simple_strtoul()

Andy Shevchenko
2018-12-11T18:05:06Z

On Tue, Dec 11, 2018 at 09:22:22AM -0800, Linus Torvalds wrote:
> On Tue, Dec 11, 2018 at 7:21 AM Thomas Preston
> <thomas.preston@codethink.co.uk> wrote:
> >
> > Stop using the obsolete functions simple_strtoul() and
> > simple_strtoull(). Instead, we should use the improved kstrtol() and
> > kstrtoll() functions. To do this, we must copy the current field into a
> > null-terminated tmpstr and advance the variable `next` manually.
>
> I see what you're trying to do, but this fix is much much worse than
> the bug was.
>
> > + if (field_width > 0) {
> > + char tmpstr[INT_BUF_LEN];
> > + int ret;
> > +
> > + strscpy(tmpstr, str, field_width+1);
>
> If field_width is larger than INT_BUF_LEN, you are now corrupting kernel stack.
>
> And no, you can't fix it by limiting field_width, since a large
> field_width is quite possible and might even be valid - and still fit
> in an int. Maybe the number is
>
> 000000000000000000000001
>
> or something?
>
> A fix might be to skip leading zeroes.
>
> Honestly, just do it by hand. Don't use kstrol and friends at all.
> Just do something like
>
> unsigned long long val = 0;
> p = str;
> for (;;) {
> int c;
> if (field_width > 0 && p - str >= field_width)
> break;
> c = hexval(*p++);
> if (c < 0 || c > base)
> break;
> val = val * base + c;
> // check for overflow

I think it's slightly more complicated, I run the following test case on glibc:

uint32_t hi, lo, t;

sscanf("00fafafafa0d0b0b0b0c000000", "%8x%8x%x", &hi, &lo, &t);

64-bit:
HI: 00fafafa LO: fa0d0b0b (c000000)
32-bit:
HI: 00fafafa LO: fa0d0b0b (ffffffff)

> }
> /* Now do "sign" and range checking on val */
> /* Ta-daa, all done */
>
> or similar. Treat the above as pseudo-code, I didn't fill in all the details.
>
> Linus

--
With Best Regards,
Andy Shevchenko