On Wed, Oct 10, 2012 at 1:46 PM, Andrew Morton
<a...@linux-foundation.org> wrote:
> On Tue, 9 Oct 2012 15:54:01 -0700
> Kees Cook <keesc...@chromium.org> wrote:
>
>> Calling uname() with the UNAME26 personality set allows a leak of kernel
>> stack contents. This fixes it by initializing the stack buffer to zero,
>> defensively calculating the length of copy_to_user() call, and making
>> the len argument unsigned.
>>
>> ...
>>
>> --- a/kernel/sys.c
>> +++ b/kernel/sys.c
>> @@ -1265,13 +1265,13 @@ 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];
>>
>> if (current->personality & UNAME26) {
>> - char *rest = UTS_RELEASE;
>> + const char *rest = UTS_RELEASE;
>> + char buf[65] = { 0 };
>> int ndots = 0;
>> unsigned v;
>>
>> @@ -1283,7 +1283,9 @@ static int override_release(char __user *release, int
>> len)
>> rest++;
>> }
>> v = ((LINUX_VERSION_CODE >> 8) & 0xff) + 40;
>> - snprintf(buf, len, "2.6.%u%s", v, rest);
>> + snprintf(buf, sizeof(buf), "2.6.%u%s", v, rest);
>> + if (sizeof(buf) < len)
>> + len = sizeof(buf);
>> ret = copy_to_user(release, buf, len);
>> }
>> return ret;
>
> 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);
> (This code is application #11,493 for the sprintf_user() which we don't have)
Indeed.
-Kees
--
Kees Cook
Chrome OS Security
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