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

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Date:   Wed, 29 Sep 2021 08:24:34 +0200
From:   Valentin Vidić <vvidic@...entin-vidic.from.hr>
To:     Joseph Qi <joseph.qi@...ux.alibaba.com>
Cc:     Mark Fasheh <mark@...heh.com>, Joel Becker <jlbec@...lplan.org>,
        ocfs2-devel@....oracle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ocfs2: mount fails with buffer overflow in strlen

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.

-- 
Valentin