On Mon, Jun 25, 2018 at 3:56 PM, Eric Biggers <ebiggers3@xxxxxxxxx> wrote: > On Mon, Jun 25, 2018 at 02:10:25PM -0700, Kees Cook wrote: >> In the quest to remove all stack VLA usage from the kernel[1], this caps >> the ahash request size similar to the other limits and adds a sanity >> check at registration. Unfortunately, these reqsizes can be huge. Looking >> at all callers of crypto_ahash_set_reqsize(), the largest appears to be >> 664 bytes, based on a combination of manual inspection and pahole output: >> >> 4 dcp_sha_req_ctx >> 40 crypto4xx_ctx >> 52 hash_req_ctx >> 80 ahash_request >> 88 rk_ahash_rctx >> 104 sun4i_req_ctx >> 200 mcryptd_hash_request_ctx >> 216 safexcel_ahash_req >> 228 sha1_hash_ctx >> 228 sha256_hash_ctx >> 248 img_hash_request_ctx >> 252 mtk_sha_reqctx >> 276 sahara_sha_reqctx >> 304 mv_cesa_ahash_req >> 316 iproc_reqctx_s >> 320 caam_hash_state >> 320 qce_sha_reqctx >> 356 sha512_hash_ctx >> 384 ahash_req_ctx >> 400 chcr_ahash_req_ctx >> 416 stm32_hash_request_ctx >> 432 talitos_ahash_req_ctx >> 462 atmel_sha_reqctx >> 496 ccp_aes_cmac_req_ctx >> 616 ccp_sha_req_ctx >> 664 artpec6_hash_request_context >> >> So, this chooses 720 as a larger "round" number for the max. >> > > This isn't accounting for the cases where a hash algorithm is "wrapped" with > another one, which increases the request size. For example, "sha512_mb" ends up > with a request size of > > sizeof(struct ahash_request) + > sizeof(struct mcryptd_hash_request_ctx) + > sizeof(struct ahash_request) + > sizeof(struct sha512_hash_ctx) > > == 808 bytes, on x86_64 with CONFIG_DEBUG_SG enabled. > > (Note also that structure sizes can vary depending on the architecture > and the kernel config.) > > So, with the self-tests enabled your new BUG_ON() is hit on boot: Ugh, right. Wow, that _really_ gets big. Which are likely to wrap which others? Looks like software case plus hardware case? i.e. mcryptd_hash_request_ctx with artpec6_hash_request_context is the largest we could get? So: 80 + 80 + 200 + 664 ? Oh, hilarious. That comes exactly to 1024. :P So ... 1024? -Kees -- Kees Cook Pixel Security
