linux-kernel - Re: [PATCH] modpost: fix segfault in sym

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Date:	Thu, 3 Dec 2009 12:07:02 +0800
From:	Américo Wang <xiyou.wangcong@...il.com>
To:	Mike Frysinger <vapier@...too.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] modpost: fix segfault in sym_is() with prefixed arches

On Thu, Dec 3, 2009 at 11:50 AM, Mike Frysinger <vapier@...too.org> wrote:
> The sym_is() compares a symbol in an attempt to automatically skip symbol
> prefixes.  It does this first by searching the real symbol with the normal
> unprefixed symbol.  But then it uses the length of the original symbol to
> check the end of the substring instead of the length of the symbol it is
> looking for.  On non-prefixed arches, this is effectively the same thing,
> so there is no problem.  On prefixed-arches, since this is exceeds by just
> one byte, a crash is rare and it is usually a NUL byte anyways.  But every
> once in a blue moon, you get the right page alignment and it segfaults.
>
> For example, on the Blackfin arch, sym_is() will be called with the real
> symbol "___mod_usb_device_table" as "symbol" when looking for the normal
> symbol "__mod_usb_device_table" as "name".  The substring will thus return
> one byte into "symbol" and store it into "match".  But then "match" will
> be indexed with the length of "symbol" instead of "name" and so we will
> exceed the storage.  i.e. the code ends up doing:
>        char foo[] = "abc"; return foo[strlen(foo)+1] == '\0';
>
> Signed-off-by: Mike Frysinger <vapier@...too.org>
> ---
>  scripts/mod/file2alias.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/scripts/mod/file2alias.c b/scripts/mod/file2alias.c
> index 40e0045..1ffd1e4 100644
> --- a/scripts/mod/file2alias.c
> +++ b/scripts/mod/file2alias.c
> @@ -726,7 +726,7 @@ static inline int sym_is(const char *symbol, const char *name)
>        match = strstr(symbol, name);
>        if (!match)
>                return 0;
> -       return match[strlen(symbol)] == '\0';
> +       return match[strlen(name)] == '\0';
>  }
>
>  static void do_table(void *symval, unsigned long size,

Nice catch!

Acked-by: WANG Cong <xiyou.wangcong@...il.com>
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