Re: [PATCH v2] fix stack memory content leak via UNAME26

[Kees Cook]

On Wed, Oct 10, 2012 at 3:46 PM, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, 10 Oct 2012 15:31:07 -0700
> Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
>> > This looks unecessarily complicated.  Is there a reason to be copying
>> > all 65 bytes out to userspace?
>> >
>> > If not, then couldn't we just do
>> >
>> >         len = scnprintf(...);
>> >         ret = copy_to_user(..., len + 1);
>> >
>> > ?
>>
>> As it is, nothing calls override_release with crazy "len" values, but,
>> to make the code less fragile, there should be checking for
>> sizeof(buf) vs len. In the patch I sent, bounding the sprintf was
>> sizeof(buf), and the copy_to_user was bounded by effectively
>> min(sizeof(buf), len). If you wanted to use scnprintf, you'd have to
>> reorganize the checks and explicitly handle len == 0:
>>
>> if (!len)
>>     return -EFAULT;
>> if (sizeof(buf) < len)
>>     len = sizeof(buf)
>> len = scnprintf(buf, len, "2.6.%u%s", v, rest);
>> ret = copy_to_user(release, buf, len + 1);
>
> It would be pretty absurd for someone to call override_release() with
> len==0?  All callers use sizeof() on some pretty well-defined array.
>
> So I'd have thought that something like
>
> --- a/kernel/sys.c~a
> +++ a/kernel/sys.c
> @@ -1265,7 +1265,7 @@ DECLARE_RWSEM(uts_sem);
>   * Work around broken programs that cannot handle "Linux 3.0".
>   * Instead we map 3.x to 2.6.40+x, so e.g. 3.0 would be 2.6.40
>   */
> -static int override_release(char __user *release, int len)
> +static int override_release(char __user *release, size_t len)
>  {
>         int ret = 0;
>         char buf[65];
> @@ -1274,6 +1274,7 @@ static int override_release(char __user
>                 char *rest = UTS_RELEASE;
>                 int ndots = 0;
>                 unsigned v;
> +               size_t copy;
>
>                 while (*rest) {
>                         if (*rest == '.' && ++ndots >= 3)
> @@ -1283,8 +1284,9 @@ static int override_release(char __user
>                         rest++;
>                 }
>                 v = ((LINUX_VERSION_CODE >> 8) & 0xff) + 40;
> -               snprintf(buf, len, "2.6.%u%s", v, rest);
> -               ret = copy_to_user(release, buf, len);
> +               copy = scnprintf(buf, min(len, sizeof(buf)),
> +                                 "2.6.%u%s", v, rest);
> +               ret = copy_to_user(release, buf, copy + 1);
>         }
>         return ret;
>  }
>
> would suffice?
>
> Not a big deal I guess, but copying out stuff beyond the NUL is a bit odd.

Sure, that looks good to me.

Thanks!

-Kees

-- 
Kees Cook
Chrome OS Security
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