Re: [PATCH] kernel: kallsyms: memory override issue, need check destination buffer length

[Rusty Russell]

Chen Gang <gang.chen@xxxxxxxxxxx> writes:
>   We don't export any symbols > 128 characters, but if we did then
>   kallsyms_expand_symbol() would overflow the buffer handed to it.
>   So we need check destination buffer length when copying.
>
>   the related test:
>     if we define an EXPORT function which name more than 128.
>     will panic when call kallsyms_lookup_name by init_kprobes on booting.
>     after check the length (provide this patch), it is ok.
>
>   Implementaion:
>     add additional destination buffer length parameter (maxlen)
>     if uncompressed string is too long (>= maxlen), it will be truncated.
>     not check the parameters whether valid, since it is a static function.
>
>
> Signed-off-by: Chen Gang <gang.chen@xxxxxxxxxxx>
> Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>

??!!!  I never signed this off!  I've never seen it before!

Minor comments below:

> -static unsigned int kallsyms_expand_symbol(unsigned int off, char *result)
> +static unsigned int kallsyms_expand_symbol(unsigned int off,
> +					   char *result, int maxlen)

'size_t maxlen' would be more explicit.

>  {
>  	int len, skipped_first = 0;
>  	const u8 *tptr, *data;
> +	char *begin = result;
>   	/* Get the compressed symbol length from the first symbol byte. */
>  	data = &kallsyms_names[off];
> @@ -113,14 +116,16 @@ static unsigned int kallsyms_expand_symbol(unsigned int off, char *result)
>   		while (*tptr) {
>  			if (skipped_first) {
> -				*result = *tptr;
> -				result++;
> +				*result++ = *tptr;
> +				if (result - begin == maxlen - 1)
> +					goto tail;

You could just decrement maxlen instead, and handle maxlen == 0 at the
same time:

                if (maxlen <= 1)
                        goto tail;
                *result = *tptr;
                result++;
                maxlen--;

> @@ -176,7 +181,7 @@ unsigned long kallsyms_lookup_name(const char *name)
>  	unsigned int off;
>   	for (i = 0, off = 0; i < kallsyms_num_syms; i++) {
> -		off = kallsyms_expand_symbol(off, namebuf);
> +		off = kallsyms_expand_symbol(off, namebuf, sizeof(namebuf));
>   		if (strcmp(namebuf, name) == 0)
>  			return kallsyms_addresses[i];

I prefer to use ARRAY_SIZE(namebuf) instead of sizeof(namebuf).  That
way we break compile if the declaration is changed from an array to a
pointer one day.

Same for the others.

Thanks,
Rusty.
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