2022-02-23 07:56:47

by Ard Biesheuvel

Subject: [PATCH v2] crypto: crypto_xor - use helpers for unaligned accesses

Dereferencing a misaligned pointer is undefined behavior in C, and may
result in codegen on architectures such as ARM that trigger alignments
traps and expensive fixups in software.

Instead, use the get_aligned()/put_aligned() accessors, which are cheap
or even completely free when CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS=y.

In the converse case, the prior alignment checks ensure that the casts
are safe, and so no unaligned accessors are necessary.

Signed-off-by: Ard Biesheuvel <[email protected]>
---
v2: fix issue in crypto_xor_cpy()

crypto/algapi.c | 24 +++++++++++++++++---
include/crypto/algapi.h | 10 ++++++--
2 files changed, 29 insertions(+), 5 deletions(-)

diff --git a/crypto/algapi.c b/crypto/algapi.c
index 9f15e11f5d73..a6a10f2ee0dc 100644
--- a/crypto/algapi.c
+++ b/crypto/algapi.c
@@ -1008,7 +1008,13 @@ void __crypto_xor(u8 *dst, const u8 *src1, const u8 *src2, unsigned int len)
}

while (IS_ENABLED(CONFIG_64BIT) && len >= 8 && !(relalign & 7)) {
- *(u64 *)dst = *(u64 *)src1 ^ *(u64 *)src2;
+ if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)) {
+ u64 l = get_unaligned((u64 *)src1) ^
+ get_unaligned((u64 *)src2);
+ put_unaligned(l, (u64 *)dst);
+ } else {
+ *(u64 *)dst = *(u64 *)src1 ^ *(u64 *)src2;
+ }
dst += 8;
src1 += 8;
src2 += 8;
@@ -1016,7 +1022,13 @@ void __crypto_xor(u8 *dst, const u8 *src1, const u8 *src2, unsigned int len)
}

while (len >= 4 && !(relalign & 3)) {
- *(u32 *)dst = *(u32 *)src1 ^ *(u32 *)src2;
+ if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)) {
+ u32 l = get_unaligned((u32 *)src1) ^
+ get_unaligned((u32 *)src2);
+ put_unaligned(l, (u32 *)dst);
+ } else {
+ *(u32 *)dst = *(u32 *)src1 ^ *(u32 *)src2;
+ }
dst += 4;
src1 += 4;
src2 += 4;
@@ -1024,7 +1036,13 @@ void __crypto_xor(u8 *dst, const u8 *src1, const u8 *src2, unsigned int len)
}

while (len >= 2 && !(relalign & 1)) {
- *(u16 *)dst = *(u16 *)src1 ^ *(u16 *)src2;
+ if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)) {
+ u16 l = get_unaligned((u16 *)src1) ^
+ get_unaligned((u16 *)src2);
+ put_unaligned(l, (u16 *)dst);
+ } else {
+ *(u16 *)dst = *(u16 *)src1 ^ *(u16 *)src2;
+ }
dst += 2;
src1 += 2;
src2 += 2;
diff --git a/include/crypto/algapi.h b/include/crypto/algapi.h
index f76ec723ceae..f50c5d1725da 100644
--- a/include/crypto/algapi.h
+++ b/include/crypto/algapi.h
@@ -13,6 +13,8 @@
#include <linux/list.h>
#include <linux/types.h>

+#include <asm/unaligned.h>
+
/*
* Maximum values for blocksize and alignmask, used to allocate
* static buffers that are big enough for any combination of
@@ -154,9 +156,11 @@ static inline void crypto_xor(u8 *dst, const u8 *src, unsigned int size)
(size % sizeof(unsigned long)) == 0) {
unsigned long *d = (unsigned long *)dst;
unsigned long *s = (unsigned long *)src;
+ unsigned long l;

while (size > 0) {
- *d++ ^= *s++;
+ l = get_unaligned(d) ^ get_unaligned(s++);
+ put_unaligned(l, d++);
size -= sizeof(unsigned long);
}
} else {
@@ -173,9 +177,11 @@ static inline void crypto_xor_cpy(u8 *dst, const u8 *src1, const u8 *src2,
unsigned long *d = (unsigned long *)dst;
unsigned long *s1 = (unsigned long *)src1;
unsigned long *s2 = (unsigned long *)src2;
+ unsigned long l;

while (size > 0) {
- *d++ = *s1++ ^ *s2++;
+ l = get_unaligned(s1++) ^ get_unaligned(s2++);
+ put_unaligned(l, d++);
size -= sizeof(unsigned long);
}
} else {
--
2.30.2

2022-03-03 00:43:35

by Herbert Xu

Subject: Re: [PATCH v2] crypto: crypto_xor - use helpers for unaligned accesses

On Wed, Feb 23, 2022 at 08:07:01AM +0100, Ard Biesheuvel wrote:
> Dereferencing a misaligned pointer is undefined behavior in C, and may
> result in codegen on architectures such as ARM that trigger alignments
> traps and expensive fixups in software.
>
> Instead, use the get_aligned()/put_aligned() accessors, which are cheap
> or even completely free when CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS=y.
>
> In the converse case, the prior alignment checks ensure that the casts
> are safe, and so no unaligned accessors are necessary.
>
> Signed-off-by: Ard Biesheuvel <[email protected]>
> ---
> v2: fix issue in crypto_xor_cpy()
>
> crypto/algapi.c | 24 +++++++++++++++++---
> include/crypto/algapi.h | 10 ++++++--
> 2 files changed, 29 insertions(+), 5 deletions(-)

Patch applied. Thanks.
--
Email: Herbert Xu <[email protected]>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt