Re: [PATCH v4 4/9] em28xx: fix em28xx_dvb_init for KASAN

[Andrey Ryabinin]

On 09/26/2017 09:47 AM, Arnd Bergmann wrote:
> On Mon, Sep 25, 2017 at 11:32 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>> On Mon, Sep 25, 2017 at 7:41 AM, David Laight <David.Laight@xxxxxxxxxx> wrote:
>>> From: Arnd Bergmann
>>>> Sent: 22 September 2017 22:29
>>> ...
>>>> It seems that this is triggered in part by using strlcpy(), which the
>>>> compiler doesn't recognize as copying at most 'len' bytes, since strlcpy
>>>> is not part of the C standard.
>>>
>>> Neither is strncpy().
>>>
>>> It'll almost certainly be a marker in a header file somewhere,
>>> so it should be possibly to teach it about other functions.
>>
>> I'm currently travelling and haven't investigated in detail, but from
>> taking a closer look here, I found that the hardened 'strlcpy()'
>> in include/linux/string.h triggers it. There is also a hardened
>> (much shorted) 'strncpy()' that doesn't trigger it in the same file,
>> and having only the extern declaration of strncpy also doesn't.
> 
> And a little more experimenting leads to this simple patch that fixes
> the problem:
> 
> --- a/include/linux/string.h
> +++ b/include/linux/string.h
> @@ -254,7 +254,7 @@ __FORTIFY_INLINE size_t strlcpy(char *p, const
> char *q, size_t size)
>         size_t q_size = __builtin_object_size(q, 0);
>         if (p_size == (size_t)-1 && q_size == (size_t)-1)
>                 return __real_strlcpy(p, q, size);
> -       ret = strlen(q);
> +       ret = __builtin_strlen(q);


I think this is not correct. Fortified strlen called here on purpose. If sizeof q is known at compile time
and 'q' contains not-null fortified strlen() will panic.


>         if (size) {
>                 size_t len = (ret >= size) ? size - 1 : ret;
>                 if (__builtin_constant_p(len) && len >= p_size)
> 
> The problem is apparently that the fortified strlcpy calls the fortified strlen,
> which in turn calls strnlen and that ends up calling the extern '__real_strnlen'
> that gcc cannot reduce to a constant expression for a constant input.


Per my observation, it's the code like this:
	if () 
		fortify_panic(__func__);


somehow prevent gcc to merge several "struct i2c_board_info info;" into one stack slot.
With the hack bellow, stack usage reduced to ~1,6K:

---
 include/linux/string.h | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/include/linux/string.h b/include/linux/string.h
index 54d21783e18d..9a96ff3ebf94 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -261,8 +261,6 @@ __FORTIFY_INLINE __kernel_size_t strlen(const char *p)
 	if (p_size == (size_t)-1)
 		return __builtin_strlen(p);
 	ret = strnlen(p, p_size);
-	if (p_size <= ret)
-		fortify_panic(__func__);
 	return ret;
 }
 
@@ -271,8 +269,6 @@ __FORTIFY_INLINE __kernel_size_t strnlen(const char *p, __kernel_size_t maxlen)
 {
 	size_t p_size = __builtin_object_size(p, 0);
 	__kernel_size_t ret = __real_strnlen(p, maxlen < p_size ? maxlen : p_size);
-	if (p_size <= ret && maxlen != ret)
-		fortify_panic(__func__);
 	return ret;
 }




> Not sure if that change is the best fix, but it seems to address the problem in
> this driver and probably leads to better code in other places as well.
> 

Probably it would be better to solve this on the strlcpy side, but I haven't found the way to do this right.
Alternative solutions:

 - use memcpy() instead of strlcpy(). All source strings are smaller than I2C_NAME_SIZE, so we could
   do something like this - memcpy(info.type, "si2168", sizeof("si2168"));
   Also this should be faster.

 - Move code under different "case:" in the switch(dev->model) to the separate function should help as well.
   But it might be harder to backport into stables.