Re: crypto ahash requests on the stack

[Harald Freudenberger <freude@linux.ibm.com>]

On 2025-08-25 16:23, Mikulas Patocka wrote:
> Hi
> 
> I'd like to ask about this condition in crypto_ahash_digest:
> 	if (ahash_req_on_stack(req) && ahash_is_async(tfm))
> 		return -EAGAIN;
> 
> Can it be removed? Or, is there some reason why you can't have
> asynchronous requests on the stack (such as inability of doing DMA to
> virtually mapped stack)?
> 
> Or, should I just clear the flag CRYPTO_TFM_REQ_ON_STACK in my code?
> 
> I'm modifying dm-integrity to use asynchronous API so that Harald
> Freudenberger can use it on mainframes (the reason is that his
> implementation only provides asynchronous API) and I would prefer to 
> place
> ahash requests on the stack (and wait for them before the function 
> exits).
> 
> The commit 04bfa4c7d5119ca38f8133bfdae7957a60c8b221 says that we should
> clone the request with HASH_REQUEST_CLONE, but that is not usable in
> dm-integrity, because dm-integrity must work even when the system is 
> out
> of memory.
> 
> Mikulas

The problem with this 'on the stack' is also with the buffer addresses.
The asynch implementations get scatterlists. By playing around there,
I found out that the addresses in scatterlists are checked:

scatterlist.h:

static inline void sg_set_buf(struct scatterlist *sg, const void *buf,
			      unsigned int buflen)
{
#ifdef CONFIG_DEBUG_SG
	BUG_ON(!virt_addr_valid(buf));
#endif
	sg_set_page(sg, virt_to_page(buf), buflen, offset_in_page(buf));
}

and virt_addr_valid(x) fails on stack addresses (!). I talked with the
s390 subsystem maintainer and he confirms this. So stack addresses can't
be used for scatterlists even when there is no DMA involved.

Harald Freudenberger