linux-kernel - Re: [PATCH 1/4] lib/vsprintf.c: handle invalid format specifiers more robustly

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Date:	Mon, 28 Sep 2015 11:08:11 +0300
From:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
	Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 1/4] lib/vsprintf.c: handle invalid format specifiers
 more robustly

On Fri, 2015-09-25 at 19:41 +0200, Rasmus Villemoes wrote:
> If we meet any invalid or unsupported format specifier, 'handling' it
> by just printing it as a literal string is not safe: Presumably the
> format string and the arguments passed gcc's type checking, but that
> means something like sprintf(buf, "%n %pd", &intvar, dentry) would 
> end
> up interpreting &intvar as a struct dentry*.
> 
> When the offending specifier was %n it used to be at the end of the
> format string, but we can't rely on that always being the case. Also,
> gcc doesn't complain about some more or less exotic qualifiers (or
> 'length modifiers' in posix-speak) such as 'j' or 'q', but being
> unrecognized by the kernel's printf implementation, they'd be
> interpreted as unknown specifiers, and the rest of arguments would be
> interpreted wrongly.
> 
> So let's complain about anything we don't understand, not just %n, 
> and
> stop pretending that we'd be able to make sense of the rest of the
> format/arguments. If the offending specifier is in a printk() call we
> unfortunately only get a "BUG: recent printk recursion!", but at 
> least
> direct users of the sprintf family will be caught.

I like the fix (I noticed the same issue after commented out Martin's
patch).

Few minor comments below.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>

> 
> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
> ---
>  lib/vsprintf.c | 31 +++++++++++++++++++++----------
>  1 file changed, 21 insertions(+), 10 deletions(-)
> 
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index 95cd63b43b99..f2590a80937f 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -1769,14 +1769,14 @@ qualifier:
>  
>  	case 'n':
>  		/*
> -		 * Since %n poses a greater security risk than 
> utility, treat
> -		 * it as an invalid format specifier. Warn about its 
> use so
> -		 * that new instances don't get added.
> +		 * Since %n poses a greater security risk than

Any reason to wrap first string?

> +		 * utility, treat it as any other invalid or
> +		 * unsupported format specifier.
>  		 */
> -		WARN_ONCE(1, "Please remove ignored %%n in '%s'\n", 
> fmt);
>  		/* Fall-through */
>  
>  	default:
> +		WARN_ONCE(1, "Please remove unsupported %%%c in 
> format string\n", *fmt);
>  		spec->type = FORMAT_TYPE_INVALID;
>  		return fmt - start;
>  	}
> @@ -1944,10 +1944,15 @@ int vsnprintf(char *buf, size_t size, const 
> char *fmt, va_list args)
>  			break;
>  
>  		case FORMAT_TYPE_INVALID:
> -			if (str < end)
> -				*str = '%';
> -			++str;
> -			break;
> +			/*
> +			 * Presumably the arguments passed gcc's 
> type
> +			 * checking, but there is no safe or sane 
> way
> +			 * for us to continue parsing the format and
> +			 * fetching from the va_list; the remaining
> +			 * specifiers and arguments would be out of
> +			 * sync.

Could we use wider strings in the commentary here?

> +			 */
> +			goto out;
>  
>  		default:
>  			switch (spec.type) {
> @@ -1992,6 +1997,7 @@ int vsnprintf(char *buf, size_t size, const 
> char *fmt, va_list args)
>  		}
>  	}
>  
> +out:
>  	if (size > 0) {
>  		if (str < end)
>  			*str = '\0';
> @@ -2189,9 +2195,10 @@ do {						> 	> 	> 	> \
> 
>  
>  		switch (spec.type) {
>  		case FORMAT_TYPE_NONE:
> -		case FORMAT_TYPE_INVALID:
>  		case FORMAT_TYPE_PERCENT_CHAR:
>  			break;
> +		case FORMAT_TYPE_INVALID:
> +			goto out;
>  
>  		case FORMAT_TYPE_WIDTH:
>  		case FORMAT_TYPE_PRECISION:
> @@ -2253,6 +2260,7 @@ do {						> 	> 	> 	> \
> 
>  		}
>  	}
>  
> +out:
>  	return (u32 *)(PTR_ALIGN(str, sizeof(u32))) - bin_buf;
>  #undef save_arg
>  }
> @@ -2375,12 +2383,14 @@ int bstr_printf(char *buf, size_t size, const 
> char *fmt, const u32 *bin_buf)
>  			break;
>  
>  		case FORMAT_TYPE_PERCENT_CHAR:
> -		case FORMAT_TYPE_INVALID:
>  			if (str < end)
>  				*str = '%';
>  			++str;
>  			break;
>  
> +		case FORMAT_TYPE_INVALID:
> +			goto out;
> +
>  		default: {
>  			unsigned long long num;
>  
> @@ -2423,6 +2433,7 @@ int bstr_printf(char *buf, size_t size, const 
> char *fmt, const u32 *bin_buf)
>  		} /* switch(spec.type) */
>  	} /* while(*fmt) */
>  
> +out:
>  	if (size > 0) {
>  		if (str < end)
>  			*str = '\0';

-- 
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Intel Finland Oy
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