Re: [PATCH 2/3] crypto: hash - Use nth_page instead of doing it by hand

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Mar 11, 2025 at 06:20:31PM +0800, Herbert Xu wrote:
> Use nth_page instead of adding n to the page pointer.
> 
> This also fixes a real bug in shash_ahash_digest where the the
> check for continguous hash data may be incorrect in the presence
> of highmem.  This could result in an incorrect hash or worse.
> 
> Fixes: 5f7082ed4f48 ("crypto: hash - Export shash through hash")
> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
> ---
>  crypto/ahash.c | 38 +++++++++++++++++++++++++-------------
>  1 file changed, 25 insertions(+), 13 deletions(-)
> 
> diff --git a/crypto/ahash.c b/crypto/ahash.c
> index 9c26175c21a8..75d642897e36 100644
> --- a/crypto/ahash.c
> +++ b/crypto/ahash.c
> @@ -16,6 +16,7 @@
>  #include <linux/cryptouser.h>
>  #include <linux/err.h>
>  #include <linux/kernel.h>
> +#include <linux/mm.h>
>  #include <linux/module.h>
>  #include <linux/sched.h>
>  #include <linux/slab.h>
> @@ -79,7 +80,7 @@ static int hash_walk_new_entry(struct crypto_hash_walk *walk)
>  
>  	sg = walk->sg;
>  	walk->offset = sg->offset;
> -	walk->pg = sg_page(walk->sg) + (walk->offset >> PAGE_SHIFT);
> +	walk->pg = nth_page(sg_page(walk->sg), walk->offset >> PAGE_SHIFT);
>  	walk->offset = offset_in_page(walk->offset);
>  	walk->entrylen = sg->length;
>  
> @@ -201,25 +202,36 @@ int shash_ahash_digest(struct ahash_request *req, struct shash_desc *desc)
>  	unsigned int nbytes = req->nbytes;
>  	struct scatterlist *sg;
>  	unsigned int offset;
> +	struct page *page;
> +	void *data;
>  	int err;
>  
> -	if (ahash_request_isvirt(req))
> +	if (!nbytes || ahash_request_isvirt(req))
>  		return crypto_shash_digest(desc, req->svirt, nbytes,
>  					   req->result);
>  
> -	if (nbytes &&
> -	    (sg = req->src, offset = sg->offset,
> -	     nbytes <= min(sg->length, ((unsigned int)(PAGE_SIZE)) - offset))) {
> -		void *data;
> +	sg = req->src;
> +	if (nbytes > sg->length)
> +		return crypto_shash_init(desc) ?:
> +		       shash_ahash_finup(req, desc);
>  
> -		data = kmap_local_page(sg_page(sg));
> -		err = crypto_shash_digest(desc, data + offset, nbytes,
> -					  req->result);
> -		kunmap_local(data);
> -	} else
> -		err = crypto_shash_init(desc) ?:
> -		      shash_ahash_finup(req, desc);
> +	page = sg_page(sg);
> +	offset = sg->offset;
> +	page = nth_page(page, offset >> PAGE_SHIFT);
> +	offset = offset_in_page(offset);
>  
> +	if (!IS_ENABLED(CONFIG_HIGHMEM))
> +		return crypto_shash_digest(desc, page_address(page) + offset,
> +					   nbytes, req->result);
> +
> +	if (nbytes > (unsigned int)PAGE_SIZE - offset)
> +		return crypto_shash_init(desc) ?:
> +		       shash_ahash_finup(req, desc);
> +
> +	data = kmap_local_page(page);
> +	err = crypto_shash_digest(desc, data + offset, nbytes,
> +				  req->result);
> +	kunmap_local(data);
>  	return err;

I guess you think this is fixing a bug in the case where sg->offset > PAGE_SIZE?
Is that case even supported?  It is supposed to be the offset into a page.

Even if so, a simpler fix (1 line) would be to use:
'sg->length >= nbytes && sg->offset + nbytes <= PAGE_SIZE'

- Eric