linux-kernel - Re: [PATCH] vsprintf: ignore arguments to %n

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Date:	Mon, 27 Jan 2014 16:59:54 -0800
From:	Joe Perches <joe@...ches.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ryan Mallon <rmallon@...il.com>, Jiri Kosina <jkosina@...e.cz>,
	Al Viro <viro@...iv.linux.org.uk>,
	Olof Johansson <olof@...om.net>,
	Stepan Moskovchenko <stepanm@...eaurora.org>,
	Daniel Borkmann <dborkman@...hat.com>
Subject: Re: [PATCH] vsprintf: ignore arguments to %n

On Mon, 2014-01-27 at 16:39 -0800, Kees Cook wrote:

> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
[]
> @@ -1735,14 +1735,10 @@ int vsnprintf(char *buf, size_t size, const char *fmt, va_list args)
>  		case FORMAT_TYPE_NRCHARS: {
>  			/*
>  			 * Since %n poses a greater security risk than
> -			 * utility, ignore %n and skip its argument.
> +			 * utility, it should not be implemented. Instead,
> +			 * when encountering %n, ignore the arguments.
>  			 */
> -			void *skip_arg;
> -
> -			WARN_ONCE(1, "Please remove ignored %%n in '%s'\n",
> -					old_fmt);
> -
> -			skip_arg = va_arg(args, void *);
> +			WARN_ONCE(1, "Ignored %%n in '%s'\n", old_fmt);
>  			break;
>  		}

I don't think this is better either.

Why not tell people to fix the code?
checkpatch isn't run on quite a lot of patches.

anyway, here's a possible checkpatch test too.
---
 scripts/checkpatch.pl | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 1dbd6d1..95c3264 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -4069,6 +4069,15 @@ sub process {
 			}
 		}
 
+# check for unsupported uses of %n in what appear to be formats
+		if ($sline =~ /\b$logFunctions\s*\(.*"/) {
+			my $fmt = get_quoted_string($line, $rawline);
+			if ($fmt ne "" && $fmt =~ /[^\%]\%n/) {
+				WARN("PRINTF_CONTROL_N",
+				     "printf style use of \%n is not supported\n" . $herecurr);
+			}
+		}
+
 # Check for misused memsets
 		if ($^V && $^V ge 5.10.0 &&
 		    defined $stat &&


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