crypto: scatterwalk - use kmap_local() not kmap_atomic()

kmap_atomic() is used to create short-lived mappings of pages that may
not be accessible via the kernel direct map. This is only needed on
32-bit architectures that implement CONFIG_HIGHMEM, but it can be used
on 64-bit other architectures too, where the returned mapping is simply
the kernel direct address of the page.

However, kmap_atomic() does not support migration on CONFIG_HIGHMEM
configurations, due to the use of per-CPU kmap slots, and so it disables
preemption on all architectures, not just the 32-bit ones. This implies
that all scatterwalk based crypto routines essentially execute with
preemption disabled all the time, which is less than ideal.

So let's switch scatterwalk_map/_unmap and the shash/ahash routines to
kmap_local() instead, which serves a similar purpose, but without the
resulting impact on preemption on architectures that have no need for
CONFIG_HIGHMEM.

Cc: Eric Biggers <ebiggers@kernel.org>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "Elliott, Robert (Servers)" <elliott@hpe.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 2 insertions, 2 deletions

diff --git a/crypto/shash.c b/crypto/shash.c
index 868b6ba2b3b7..58b46f198449 100644
--- a/crypto/shash.c
+++ b/crypto/shash.c
@@ -320,10 +320,10 @@ int shash_ahash_digest(struct ahash_request *req, struct shash_desc *desc)
 	     nbytes <= min(sg->length, ((unsigned int)(PAGE_SIZE)) - offset))) {
 		void *data;
 
-		data = kmap_atomic(sg_page(sg));
+		data = kmap_local_page(sg_page(sg));
 		err = crypto_shash_digest(desc, data + offset, nbytes,
 					  req->result);
-		kunmap_atomic(data);
+		kunmap_local(data);
 	} else
 		err = crypto_shash_init(desc) ?:
 		      shash_ahash_finup(req, desc);