Re: [PATCH] ocfs2: mount fails with buffer overflow in strlen

From: Joseph Qi
Date: Wed Sep 29 2021 - 05:12:55 EST




On 9/29/21 2:24 PM, Valentin Vidić wrote:
> On Wed, Sep 29, 2021 at 10:38:59AM +0800, Joseph Qi wrote:
>> Okay, you are right, strlen(src) is indeed wrong here.
>>
>> But please note that in strlcpy():
>> size_t ret = strlen(src);
>> if (size) {
>> size_t len = (ret >= size) ? size - 1 : ret;
>> memcpy(dest, src, len);
>> dest[len] = '\0';
>> }
>>
>> Take ci_stack "o2cb" for example, strlen("o2cb") may return wrong if the
>> coming byte is not null, say it is 10.
>> The input size is 5, so len will finally be 4.
>> So dest is still correct ending with null byte. No overflow happens.
>> So the problem here is the wrong return value, but it is discarded in
>> ocfs2_initialize_super().
>
> strlcpy starts with a call to strlen(src) and this is where the read overflow
> happens. If the kernel is compiled with CONFIG_FORTIFY_SOURCE this gets
> executed instead (include/linux/fortify-string.h):
>
> __FORTIFY_INLINE __kernel_size_t strlen(const char *p)
> {
> __kernel_size_t ret;
> size_t p_size = __builtin_object_size(p, 1);
>
> /* Work around gcc excess stack consumption issue */
> if (p_size == (size_t)-1 ||
> (__builtin_constant_p(p[p_size - 1]) && p[p_size - 1] == '\0'))
> return __underlying_strlen(p);
> ret = strnlen(p, p_size);
> if (p_size <= ret)
> fortify_panic(__func__);
> return ret;
> }
>
> So while strlcpy did work before this fortify check, it is probably not the
> best option anymore due to the missing null terminator in the source.
>
Got it, it really triggers panic in strlen().
So could you please update the commit log? I think CONFIG_FORTIFY_SOURCE
is necessary information since it is not default enabled.
And add comments with your changes, e.g.

/*
* ci_stack and ci_cluster in ocfs2_cluster_info may not null
* terminated, make sure no overflow happens here.
*/

BTW, since we use kzalloc to alloc osb, so we don't have to manually
set the last null byte.

Thanks,
Joseph