crypto: algapi - introduce the flag CRYPTO_ALG_ALLOCATES_MEMORY

Introduce a new algorithm flag CRYPTO_ALG_ALLOCATES_MEMORY.  If this
flag is set, then the driver allocates memory in its request routine.
Such drivers are not suitable for disk encryption because GFP_ATOMIC
allocation can fail anytime (causing random I/O errors) and GFP_KERNEL
allocation can recurse into the block layer, causing a deadlock.

For now, this flag is only implemented for some algorithm types.  We
also assume some usage constraints for it to be meaningful, since there
are lots of edge cases the crypto API allows (e.g., misaligned or
fragmented scatterlists) that mean that nearly any crypto algorithm can
allocate memory in some case.  See the comment for details.

Also add this flag to CRYPTO_ALG_INHERITED_FLAGS so that when a template
is instantiated, this flag is set on the template instance if it is set
on any algorithm the instance uses.

Based on a patch by Mikulas Patocka <mpatocka@redhat.com>
(https://lore.kernel.org/r/alpine.LRH.2.02.2006301414580.30526@file01.intranet.prod.int.rdu2.redhat.com).
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

include/crypto/algapi.h

@@ -246,7 +246,8 @@ static inline u32 crypto_requires_off(struct crypto_attr_type *algt, u32 off)
  * algorithm if any "inner" algorithm has them set.
  */
 #define CRYPTO_ALG_INHERITED_FLAGS	\
-	(CRYPTO_ALG_ASYNC | CRYPTO_ALG_NEED_FALLBACK)
+	(CRYPTO_ALG_ASYNC | CRYPTO_ALG_NEED_FALLBACK |	\
+	 CRYPTO_ALG_ALLOCATES_MEMORY)
 
 /*
  * Given the type and mask that specify the flags restrictions on a template

include/linux/crypto.h

@@ -100,6 +100,38 @@
  */
 #define CRYPTO_NOLOAD			0x00008000
 
+/*
+ * The algorithm may allocate memory during request processing, i.e. during
+ * encryption, decryption, or hashing.  Users can request an algorithm with this
+ * flag unset if they can't handle memory allocation failures.
+ *
+ * This flag is currently only implemented for algorithms of type "skcipher",
+ * "aead", "ahash", "shash", and "cipher".  Algorithms of other types might not
+ * have this flag set even if they allocate memory.
+ *
+ * In some edge cases, algorithms can allocate memory regardless of this flag.
+ * To avoid these cases, users must obey the following usage constraints:
+ *    skcipher:
+ *	- The IV buffer and all scatterlist elements must be aligned to the
+ *	  algorithm's alignmask.
+ *	- If the data were to be divided into chunks of size
+ *	  crypto_skcipher_walksize() (with any remainder going at the end), no
+ *	  chunk can cross a page boundary or a scatterlist element boundary.
+ *    aead:
+ *	- The IV buffer and all scatterlist elements must be aligned to the
+ *	  algorithm's alignmask.
+ *	- The first scatterlist element must contain all the associated data,
+ *	  and its pages must be !PageHighMem.
+ *	- If the plaintext/ciphertext were to be divided into chunks of size
+ *	  crypto_aead_walksize() (with the remainder going at the end), no chunk
+ *	  can cross a page boundary or a scatterlist element boundary.
+ *    ahash:
+ *	- The result buffer must be aligned to the algorithm's alignmask.
+ *	- crypto_ahash_finup() must not be used unless the algorithm implements
+ *	  ->finup() natively.
+ */
+#define CRYPTO_ALG_ALLOCATES_MEMORY	0x00010000
+
 /*
  * Transform masks and values (for crt_flags).
  */