Re: [PATCH v4 11/14] treewide: Prepare to remove VLA usage for AHASH_REQUEST_ON_STACK
From: Arnd Bergmann <arnd@arndb.de>
Date: 2018-07-18 15:33:14
Also in:
dm-devel, lkml
On Wed, Jul 18, 2018 at 5:19 PM, Ard Biesheuvel [off-list ref] wrote:
On 18 July 2018 at 23:50, Ard Biesheuvel [off-list ref] wrote:quoted
On 18 July 2018 at 05:59, Arnd Bergmann [off-list ref] wrote:quoted
On Sun, Jul 15, 2018 at 6:28 AM, Kees Cook [off-list ref] wrote:quoted
After my ahash to shash conversions, only ccm is left as an ahash user, since it actually uses sg. But with the hard-coded value reduced to 376, this doesn't trip the frame warnings any more. :) I'll send an updated series soon.Maybe we should get rid of that one as well then and remove AHASH_REQUEST_ON_STACK()? I see that Ard (now on Cc) added this usage only recently. Looking at the code some more, I also find that the descsize is probably much smaller than 376 for all possible cases of "cbcmac(*)", either alg->cra_blocksize plus a few bytes or sizeof(mac_desc_ctx) (i.e. 20) for arch/arm64/crypto/aes-glue.c. Walking the sglist here means open-coding a shash_ahash_update() implementation in crypto_ccm_auth(), that that doesn't seem to add much complexity over what it already has to do to chain the sglist today.It would be better to add a variably sized ahash request member to struct crypto_ccm_req_priv_ctx, the only problem is that the last member of that struct (skreq) is variably sized already, so it would involve having a struct ahash_request pointer pointing into the same struct, after the skreq member.Actually, I think the below should already do the trick: ahreq and skreq are not used at the same time, so we can stick them in a union, and take the max() of the reqsize to ensure there's enough empty space after it.
This looks very nice indeed.
quoted hunk ↗ jump to hunk
--------8<----------diff --git a/crypto/ccm.c b/crypto/ccm.c index 0a083342ec8c..b242fd0d3262 100644 --- a/crypto/ccm.c +++ b/crypto/ccm.c@@ -50,7 +50,10 @@ struct crypto_ccm_req_priv_ctx { u32 flags; struct scatterlist src[3]; struct scatterlist dst[3]; - struct skcipher_request skreq; + union { + struct ahash_request ahreq; + struct skcipher_request skreq; + }; };
And this structure is never put on the stack anywhere but
always dynamically allocated anyway, right?
Arnd