netdev - Re: [PATCH] rxrpc: Reuse SKCIPHER_REQUEST_ON

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Date:   Wed, 18 Jul 2018 21:31:46 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Eric Biggers <ebiggers@...gle.com>
Cc:     David Howells <dhowells@...hat.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Arnd Bergmann <arnd@...db.de>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        "David S. Miller" <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rxrpc: Reuse SKCIPHER_REQUEST_ON_STACK buffer

On Mon, Jul 16, 2018 at 10:42 AM, Eric Biggers <ebiggers@...gle.com> wrote:
> On Sun, Jul 15, 2018 at 08:49:47PM -0700, Kees Cook wrote:
>> The use of SKCIPHER_REQUEST_ON_STACK() will trigger FRAME_WARN warnings
>> (when less than 2048) once the VLA is no longer hidden from the check:
>>
>> net/rxrpc/rxkad.c:398:1: warning: the frame size of 1152 bytes is larger than 1024 bytes [-Wframe-larger-than=]
>> net/rxrpc/rxkad.c:242:1: warning: the frame size of 1152 bytes is larger than 1024 bytes [-Wframe-larger-than=]
>>
>> This passes the initial SKCIPHER_REQUEST_ON_STACK allocation to the leaf
>> functions for reuse. Two requests allocated on the stack are not needed
>> when only one is used at a time.
>>
>> Signed-off-by: Kees Cook <keescook@...omium.org>
>> ---
>>  net/rxrpc/rxkad.c | 25 +++++++++++++------------
>>  1 file changed, 13 insertions(+), 12 deletions(-)
>> [...]
> How about doing the 'skcipher_request_set_tfm(req, call->conn->cipher)' and
> 'skcipher_request_zero(req);' just once, in the top-level function
> rxkad_verify_packet(), instead of before/after every time the request is used?

Hm. While that does sound reasonable, I like having it be
operationally unchanged. And changing this makes the caller function a
bit more weird. Right now it's doing direct returns, and we'd need to
cover error paths, etc. Having the "zero" _right_ after the
encrypt/decrypt seems more robust to me.

-Kees

-- 
Kees Cook
Pixel Security