linux-kernel - Re: [kernel-hardening] [PATCH] lib/vsprintf: add default case to 'i' specifier

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Message-ID: <20171009151609.xcfushhda2mfc3ff@smitten>
Date:   Mon, 9 Oct 2017 09:16:09 -0600
From:   Tycho Andersen <tycho@...ker.com>
To:     "Tobin C. Harding" <me@...in.cc>
Cc:     kernel-hardening@...ts.openwall.com, linux-kernel@...r.kernel.org
Subject: Re: [kernel-hardening] [PATCH] lib/vsprintf: add default case to 'i'
 specifier

On Mon, Oct 09, 2017 at 01:59:05PM +1100, Tobin C. Harding wrote:
> %pi leaks kernel addresses if incorrectly specified.
> 
> Currently the printk specifier %pi (%pI) contains a switch statement
> without a default clause. The %pi specifier requires a subsequent
> character (4, 6, or S) controlling the output. If the specifier is
> incomplete the switch statement will fall through and print the variable
> argument address in hex instead of the value of the argument (as an IP
> address).
> 
> If uncaught this leaks kernel addresses into dmesg. We can return an
> error string to make the bug visible and stop addresses leaking.
> 
> Add a default clause returning an error string, stops leaking addresses
> and makes the buggy code

...? :)

> Signed-off-by: Tobin C. Harding <me@...in.cc>
> ---
>  lib/vsprintf.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index 86c3385b9eb3..155702f05b14 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -1775,6 +1775,8 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
>  			default:
>  				return string(buf, end, "(invalid address)", spec);
>  			}}
> +		default:

Maybe a WARN(1, "invalid pointer format")? That way it'll be easy for
people to figure out where to fix.

Cheers,

Tycho

> +			return string(buf, end, "(invalid specifier, form: %pi4)", spec);
>  		}
>  		break;
>  	case 'E':
> -- 
> 2.7.4
>