linux-kernel - Re: [PATCH v6 9/9] vsprintf: Avoid confusion between invalid address and value

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Date:   Tue, 19 Feb 2019 12:03:46 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Tobin C . Harding" <me@...in.cc>, Joe Perches <joe@...ches.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.cz>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 9/9] vsprintf: Avoid confusion between invalid address
 and value

On (02/08/19 16:23), Petr Mladek wrote:
[..]
>  Plain Pointers
>  --------------
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index 3a95b4d1ca2e..e51cbc2be540 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -1510,7 +1510,7 @@ char *ip_addr_string(char *buf, char *end, const void *ptr,
>  		case AF_INET6:
>  			return ip6_addr_string_sa(buf, end, &sa->v6, spec, fmt);
>  		default:
> -			return string_nocheck(buf, end, "(invalid address)", spec);
> +			return string_nocheck(buf, end, "(einval)", spec);
>  		}}

Hmm... The original code looks "a bit" dangerous.

Suppose, in my driver I want to sprintf() IPv4 address. The longest
possible address is 3 * 4 (%d%d%d) + 3 bytes (dots) + terminating NULL.
E.g. 111.111.111.111

So I can allocate a 16-bytes buffer (stack or slab) and accidentally
do an %piS sprintf() on a corrupted in_addr struct:

		char buf[16];
		sprintf(buf, "%piS", in_addr);

forcing sprintf() to write "(invalid address)" to a 16-bytes buffer,
but the thing is - strlen("(invalid address)") > 16.


We might want to take this change out of this series.

	-ss