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From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Kees Cook <keescook@chromium.org>
Cc: "Valdis Klētnieks" <valdis.kletnieks@vt.edu>,
	"Justin Forbes" <jmforbes@linuxtx.org>,
	"Arnaldo Carvalho de Melo" <acme@redhat.com>,
	linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
Subject: Re: [PATCH] tools: Fix use-after-free for realloc(..., 0)
Date: Sat, 12 Feb 2022 17:02:57 -0800	[thread overview]
Message-ID: <20220213010257.yllouthdbjzawil2@treble> (raw)
In-Reply-To: <20220212181855.3460176-1-keescook@chromium.org>

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


      reply	other threads:[~2022-02-13  1:03 UTC|newest]

Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-02-12 18:18 [PATCH] tools: Fix use-after-free for realloc(..., 0) Kees Cook
2022-02-13  1:02 ` Josh Poimboeuf [this message]

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