Re: [PATCH v4 13/14] rxrpc: Prepare to remove VLA usage for SKCIPHER_REQUEST_ON_STACK

[Kees Cook]

On Thu, Jul 12, 2018 at 1:23 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Thu, Jul 12, 2018 at 8:11 AM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>> On Wed, Jul 11, 2018 at 10:36 PM, Kees Cook <keescook@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
>>
>> (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