linux-kernel - Re: [PATCH v4 13/14] rxrpc: Prepare to remove VLA usage for SKCIPHER_REQUEST_ON

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Date:   Thu, 12 Jul 2018 13:30:33 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Eric Biggers <ebiggers@...gle.com>,
        Alasdair Kergon <agk@...hat.com>,
        Giovanni Cabiddu <giovanni.cabiddu@...el.com>,
        Lars Persson <larper@...s.com>,
        Mike Snitzer <snitzer@...hat.com>,
        Rabin Vincent <rabinv@...s.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        "David S. Miller" <davem@...emloft.net>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@...r.kernel.org>, qat-linux@...el.com,
        dm-devel@...hat.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Howells <dhowells@...hat.com>
Subject: Re: [PATCH v4 13/14] rxrpc: Prepare to remove VLA usage for SKCIPHER_REQUEST_ON_STACK

On Thu, Jul 12, 2018 at 1:23 PM, Kees Cook <keescook@...omium.org> wrote:
> On Thu, Jul 12, 2018 at 8:11 AM, Arnd Bergmann <arnd@...db.de> wrote:
>> On Wed, Jul 11, 2018 at 10:36 PM, Kees Cook <keescook@...omium.org> wrote:
>>> Two uses 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 bumps the affected objects by 20% to silence the warnings while
>>> still providing coverage is anything grows even more.
>>>
>>> Signed-off-by: Kees Cook <keescook@...omium.org>
>>
>> (adding David Howells to cc)
>>
>> I don't think these are in a fast path, it should be possible to just use
>> skcipher_alloc_req() instead of SKCIPHER_REQUEST_ON_STACK() here.
>> From what I can tell, neither of the two are called in atomic context, so
>> you should be able to use a GFP_KERNEL allocation.
>
> Sure, I can do that instead.

Actually, I think this can actually be adjusted to just re-use the
stack allocation, since rxkad_verify_packet() finishes one before
doing another in rxkad_verify_packet_1():

diff --git a/net/rxrpc/rxkad.c b/net/rxrpc/rxkad.c
index 278ac0807a60..d6a2e7cab384 100644
--- a/net/rxrpc/rxkad.c
+++ b/net/rxrpc/rxkad.c
@@ -316,10 +316,10 @@ static int rxkad_secure_packet(struct rxrpc_call *call,
  */
 static int rxkad_verify_packet_1(struct rxrpc_call *call, struct sk_buff *skb,
                                 unsigned int offset, unsigned int len,
-                                rxrpc_seq_t seq)
+                                rxrpc_seq_t seq,
+                                struct skcipher_request *req)
 {
        struct rxkad_level1_hdr sechdr;
-       SKCIPHER_REQUEST_ON_STACK(req, call->conn->cipher);
        struct rxrpc_crypt iv;
        struct scatterlist sg[16];
        struct sk_buff *trailer;
@@ -549,7 +549,7 @@ static int rxkad_verify_packet(struct rxrpc_call
*call, struct sk_buff *skb,
        case RXRPC_SECURITY_PLAIN:
                return 0;
        case RXRPC_SECURITY_AUTH:
-               return rxkad_verify_packet_1(call, skb, offset, len, seq);
+               return rxkad_verify_packet_1(call, skb, offset, len, seq, req);
        case RXRPC_SECURITY_ENCRYPT:
                return rxkad_verify_packet_2(call, skb, offset, len, seq);
        default:


-Kees

-- 
Kees Cook
Pixel Security