[PATCH 2/5] lib/crypto: arm/poly1305: Fix register corruption in no-SIMD contexts

[Eric Biggers <ebiggers@kernel.org>]

Restore the SIMD usability check that was removed by commit 773426f4771b
("crypto: arm/poly1305 - Add block-only interface").

This safety check is cheap and is well worth eliminating a footgun.
While the Poly1305 functions *should* be called only where SIMD
registers are usable, if they are anyway, they should just do the right
thing instead of corrupting random tasks' registers and/or computing
incorrect MACs.  Fixing this is also needed for poly1305_kunit to pass.

Just use may_use_simd() instead of the original crypto_simd_usable(),
since poly1305_kunit won't rely on crypto_simd_disabled_for_test.

Fixes: 773426f4771b ("crypto: arm/poly1305 - Add block-only interface")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
---
 lib/crypto/arm/poly1305-glue.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/lib/crypto/arm/poly1305-glue.c b/lib/crypto/arm/poly1305-glue.c
index 5b65b840c166..2d86c78af883 100644
--- a/lib/crypto/arm/poly1305-glue.c
+++ b/lib/crypto/arm/poly1305-glue.c
@@ -5,10 +5,11 @@
  * Copyright (C) 2019 Linaro Ltd. <ard.biesheuvel@linaro.org>
  */
 
 #include <asm/hwcap.h>
 #include <asm/neon.h>
+#include <asm/simd.h>
 #include <crypto/internal/poly1305.h>
 #include <linux/cpufeature.h>
 #include <linux/jump_label.h>
 #include <linux/kernel.h>
 #include <linux/module.h>
@@ -32,11 +33,11 @@ static __ro_after_init DEFINE_STATIC_KEY_FALSE(have_neon);
 void poly1305_blocks_arch(struct poly1305_block_state *state, const u8 *src,
 			  unsigned int len, u32 padbit)
 {
 	len = round_down(len, POLY1305_BLOCK_SIZE);
 	if (IS_ENABLED(CONFIG_KERNEL_MODE_NEON) &&
-	    static_branch_likely(&have_neon)) {
+	    static_branch_likely(&have_neon) && likely(may_use_simd())) {
 		do {
 			unsigned int todo = min_t(unsigned int, len, SZ_4K);
 
 			kernel_neon_begin();
 			poly1305_blocks_neon(state, src, todo, padbit);
-- 
2.50.0