linux-hardening - Re: [PATCH v2] tools: Fix use-after-free for realloc(..., 0)

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Date:   Tue, 15 Feb 2022 14:58:33 -0600
From:   Justin Forbes <jmforbes@...uxtx.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Valdis Klētnieks <valdis.kletnieks@...edu>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] tools: Fix use-after-free for realloc(..., 0)

On Sun, Feb 13, 2022 at 10:24:43AM -0800, Kees Cook wrote:
> GCC 12 was correctly reporting a potential use-after-free condition in
> the xrealloc helper. Fix the warning by avoiding an implicit "free(ptr)"
> when size == 0:
> 
> In file included from help.c:12:
> In function 'xrealloc',
>     inlined from 'add_cmdname' at help.c:24:2: subcmd-util.h:56:23: error: pointer may be used after 'realloc' [-Werror=use-after-free]
>    56 |                 ret = realloc(ptr, size);
>       |                       ^~~~~~~~~~~~~~~~~~
> subcmd-util.h:52:21: note: call to 'realloc' here
>    52 |         void *ret = realloc(ptr, size);
>       |                     ^~~~~~~~~~~~~~~~~~
> subcmd-util.h:58:31: error: pointer may be used after 'realloc' [-Werror=use-after-free]
>    58 |                         ret = realloc(ptr, 1);
>       |                               ^~~~~~~~~~~~~~~
> subcmd-util.h:52:21: note: call to 'realloc' here
>    52 |         void *ret = realloc(ptr, size);
>       |                     ^~~~~~~~~~~~~~~~~~
> 
> Reported-by: "Valdis Klētnieks" <valdis.kletnieks@...edu>
> Fixes: 2f4ce5ec1d44 ("perf tools: Finalize subcmd independence")
> Cc: Josh Poimboeuf <jpoimboe@...hat.com>
> Signed-off-by: Kees Cook <keescook@...omium.org>

Tested this patch with the Fedora rawhide builds, and this does indeed
fix the issue.

Tested-by: Justin M. Forbes <jforbes@...oraproject.org>