Re: [PATCH v2] crypto: prefix module autoloading with "crypto-"

From: Kees Cook
Date: Mon Nov 17 2014 - 19:45:34 EST

On Mon, Nov 17, 2014 at 3:20 PM, Mathias Krause <minipli@xxxxxxxxxxxxxx> wrote:
> On 17 November 2014 21:02, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> This prefixes all crypto module loading with "crypto-" so we never run
>> the risk of exposing module auto-loading to userspace via a crypto API,
>> as demonstrated by Mathias Krause:
>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>> ---
>> v2:
>> - added missing #include, thanks to minipli
>> - built with allmodconfig
>> [...snip...]
>> diff --git a/include/linux/crypto.h b/include/linux/crypto.h
>> index d45e949699ea..d14230f6e977 100644
>> --- a/include/linux/crypto.h
>> +++ b/include/linux/crypto.h
>> @@ -26,6 +26,13 @@
>> #include <linux/uaccess.h>
>> /*
>> + * Autoloaded crypto modules should only use a prefixed name to avoid allowing
>> + * arbitrary modules to be loaded.
>> + */
>> +#define MODULE_ALIAS_CRYPTO(name) \
>> + MODULE_ALIAS("crypto-" name)
> This would break userland relying on the old aliases, e.g. 'modprobe
> aes' no longer works.
> Why not have both aliases, one with the "crypto-" prefix for on-demand
> loading within the crypto API and one without for manual loading from
> userland? E.g., something like this:
> #define MODULE_ALIAS_CRYPTO(name) \
> MODULE_ALIAS(name); \
> MODULE_ALIAS("crypto-" name)
> That would prevent the userland breakage and still achieves the goal
> of restricting the request_module() call offered by the means of the

That was my intention originally, and I should go back to it. The
trouble is with the use of __UNIQUE_ID in the MODULE_ALIAS macro. It
uses __LINE__ to produce the id, so the suggested macro expansion
(which is what I started with) won't work on non-gcc compilers.

I haven't found any solutions for C89 version of gcc's __COUNTER__,
and I haven't found any C89 ways to force a macro to be expanded as
being multi-line.

I'd like to avoid having to open-code both MODULE_ALIAS and
MODULE_ALIAS_CRYPTO in each module's source.

Anyone see some sneaky way to accomplish this?


Kees Cook
Chrome OS Security
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