linux-hardening - Re: [PATCH] tools: Fix use-after-free for realloc(..., 0)

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Date:   Sat, 12 Feb 2022 17:02:57 -0800
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Valdis Klētnieks <valdis.kletnieks@...edu>,
        Justin Forbes <jmforbes@...uxtx.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] tools: Fix use-after-free for realloc(..., 0)

On Sat, Feb 12, 2022 at 10:18:55AM -0800, Kees Cook wrote:
>  static inline void *xrealloc(void *ptr, size_t size)
>  {
> -	void *ret = realloc(ptr, size);
> -	if (!ret && !size)
> -		ret = realloc(ptr, 1);
> +	void *ret;
> +
> +	/*
> +	 * Convert a zero-sized allocation into 1 byte, since
> +	 * realloc(ptr, 0) means free(ptr), but we don't want
> +	 * to release the memory. For a new allocation (when
> +	 * ptr == NULL), avoid triggering NULL-checking error
> +	 * conditions for zero-sized allocations.
> +	 */
> +	if (!size)
> +		size = 1;
> +	ret = realloc(ptr, size);
>  	if (!ret) {
> -		ret = realloc(ptr, size);
> -		if (!ret && !size)
> -			ret = realloc(ptr, 1);
> -		if (!ret)
> -			die("Out of memory, realloc failed");
> +		/*
> +		 * If realloc() fails, the original block is left untouched;
> +		 * it is not freed or moved.
> +		 */
> +		die("Out of memory, realloc failed");

xrealloc() only has two uses -- both via ALLOC_GROW() -- and they don't
rely on the 'size == 0' freeing anyway.  How about simplifying this
further to just:

 	ret = realloc(ptr, size);
  	if (!ret)
		die("Out of memory, realloc failed");

Much easier to grok, and as a bonus, we don't need the long comments :-)

-- 
Josh