[PATCH v5] crypto: rsassa-pkcs1 - Avoid cacheline sharing with underlying driver

[PATCH v5] crypto: rsassa-pkcs1 - Avoid cacheline sharing with underlying driver

[Changwei Zou]

out_buf is used as a DMA buffer for the RSA verification operation.
If it is not aligned to CRYPTO_DMA_ALIGN, cacheline sharing
problems (data corruption) would occur on CPUs with DMA-incoherent caches,
leading to -EKEYREJECTED.

Rename out_buf to buf, as it serves as both the input and output buffer.
Add a buf_ptr pointer to track its position.

Allocate the buffer separately via kmalloc(), which guarantees cacheline
alignment on architectures without fully coherent DMA.
This acts as a defensive measure, and avoids the need for an extra copy
in the underlying driver, which should check alignment before supplying
buffers to the hardware.

The intermittent error 'Key was rejected by service' on i.MX8 with CAAM
can be triggered when loading signed kernel modules.

    for i in $(seq 1 100); do
        sudo modprobe xfs 2>&1 && echo "SUCCESS on attempt $i" \
        && sudo rmmod xfs || echo "FAILED on attempt $i"
    done

Signed-off-by: Changwei Zou <changwei.zou@canonical.com>
---
 crypto/rsassa-pkcs1.c | 31 ++++++++++++++++---------------
 1 file changed, 16 insertions(+), 15 deletions(-)

diff --git a/crypto/rsassa-pkcs1.c b/crypto/rsassa-pkcs1.c
index 94fa5e9600e7..b1fb5111b6af 100644
--- a/crypto/rsassa-pkcs1.c
+++ b/crypto/rsassa-pkcs1.c
@@ -223,11 +223,12 @@ static int rsassa_pkcs1_verify(struct crypto_sig *tfm,
 	struct rsassa_pkcs1_ctx *ctx = crypto_sig_ctx(tfm);
 	unsigned int child_reqsize = crypto_akcipher_reqsize(ctx->child);
 	struct akcipher_request *child_req __free(kfree_sensitive) = NULL;
+	u8 *buf __free(kfree_sensitive) = NULL;
 	struct crypto_wait cwait;
 	struct scatterlist sg;
 	unsigned int dst_len;
 	unsigned int pos;
-	u8 *out_buf;
+	u8 *buf_ptr;
 	int err;
 
 	/* RFC 8017 sec 8.2.2 step 1 - length checking */
@@ -237,16 +238,16 @@ static int rsassa_pkcs1_verify(struct crypto_sig *tfm,
 		return -EINVAL;
 
 	/* RFC 8017 sec 8.2.2 step 2 - RSA verification */
-	child_req = kmalloc(sizeof(*child_req) + child_reqsize + ctx->key_size,
-			    GFP_KERNEL);
-	if (!child_req)
+	child_req = kmalloc(sizeof(*child_req) + child_reqsize, GFP_KERNEL);
+	buf = kmalloc(ctx->key_size, GFP_KERNEL);
+	if (!child_req || !buf)
 		return -ENOMEM;
 
-	out_buf = (u8 *)(child_req + 1) + child_reqsize;
-	memcpy(out_buf, src, slen);
+	buf_ptr = buf;
+	memcpy(buf_ptr, src, slen);
 
 	crypto_init_wait(&cwait);
-	sg_init_one(&sg, out_buf, slen);
+	sg_init_one(&sg, buf_ptr, slen);
 	akcipher_request_set_tfm(child_req, ctx->child);
 	akcipher_request_set_crypt(child_req, &sg, &sg, slen, slen);
 	akcipher_request_set_callback(child_req, CRYPTO_TFM_REQ_MAY_SLEEP,
@@ -263,35 +264,35 @@ static int rsassa_pkcs1_verify(struct crypto_sig *tfm,
 		return -EINVAL;
 
 	if (dst_len == ctx->key_size) {
-		if (out_buf[0] != 0x00)
+		if (buf_ptr[0] != 0x00)
 			/* Encrypted value had no leading 0 byte */
 			return -EINVAL;
 
 		dst_len--;
-		out_buf++;
+		buf_ptr++;
 	}
 
-	if (out_buf[0] != 0x01)
+	if (buf_ptr[0] != 0x01)
 		return -EBADMSG;
 
 	for (pos = 1; pos < dst_len; pos++)
-		if (out_buf[pos] != 0xff)
+		if (buf_ptr[pos] != 0xff)
 			break;
 
-	if (pos < 9 || pos == dst_len || out_buf[pos] != 0x00)
+	if (pos < 9 || pos == dst_len || buf_ptr[pos] != 0x00)
 		return -EBADMSG;
 	pos++;
 
 	if (hash_prefix->size > dst_len - pos)
 		return -EBADMSG;
-	if (crypto_memneq(out_buf + pos, hash_prefix->data, hash_prefix->size))
+	if (crypto_memneq(buf_ptr + pos, hash_prefix->data, hash_prefix->size))
 		return -EBADMSG;
 	pos += hash_prefix->size;
 
-	/* RFC 8017 sec 8.2.2 step 4 - comparison of digest with out_buf */
+	/* RFC 8017 sec 8.2.2 step 4 - comparison of digest with buf */
 	if (dlen != dst_len - pos)
 		return -EKEYREJECTED;
-	if (memcmp(digest, out_buf + pos, dlen) != 0)
+	if (memcmp(digest, buf_ptr + pos, dlen) != 0)
 		return -EKEYREJECTED;
 
 	return 0;
-- 
2.43.0

Re: [PATCH v5] crypto: rsassa-pkcs1 - Avoid cacheline sharing with underlying driver

[Herbert Xu]

On Fri, Jul 31, 2026 at 12:44:46PM +1000, Changwei Zou wrote:
> out_buf is used as a DMA buffer for the RSA verification operation.
> If it is not aligned to CRYPTO_DMA_ALIGN, cacheline sharing
> problems (data corruption) would occur on CPUs with DMA-incoherent caches,
> leading to -EKEYREJECTED.
> 
> Rename out_buf to buf, as it serves as both the input and output buffer.
> Add a buf_ptr pointer to track its position.
> 
> Allocate the buffer separately via kmalloc(), which guarantees cacheline
> alignment on architectures without fully coherent DMA.
> This acts as a defensive measure, and avoids the need for an extra copy
> in the underlying driver, which should check alignment before supplying
> buffers to the hardware.
> 
> The intermittent error 'Key was rejected by service' on i.MX8 with CAAM
> can be triggered when loading signed kernel modules.
> 
>     for i in $(seq 1 100); do
>         sudo modprobe xfs 2>&1 && echo "SUCCESS on attempt $i" \
>         && sudo rmmod xfs || echo "FAILED on attempt $i"
>     done
> 
> Signed-off-by: Changwei Zou <changwei.zou@canonical.com>
> ---
>  crypto/rsassa-pkcs1.c | 31 ++++++++++++++++---------------
>  1 file changed, 16 insertions(+), 15 deletions(-)

In the Crypto API we don't ask the user to provide aligned data.

In general it's the driver's responsibility to ensure proper
alignment.  Which driver are we talking about here?

Thanks,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Re: [PATCH v5] crypto: rsassa-pkcs1 - Avoid cacheline sharing with underlying driver

[Changwei Zou]

Hi Herbert,

Please see the CAAM driver shown below.
Thank you very much for your guidance and support.

	(gdb) bt
	#0  rsa_edesc_alloc () at drivers/crypto/caam/caampkc.c:294
	#1  caam_rsa_enc () at drivers/crypto/caam/caampkc.c:733
	#2  crypto_akcipher_encrypt () at include/crypto/akcipher.h:279
	#3  rsassa_pkcs1_verify () at crypto/rsassa-pkcs1.c:266

Kind regards,
Changwei

Re: [PATCH v5] crypto: rsassa-pkcs1 - Avoid cacheline sharing with underlying driver

[Changwei Zou]

Hi Herbert,

An orthogonal driver-level patch can be found at the following link.

https://lore.kernel.org/all/20260805041902.1575170-1-changwei.zou@canonical.com/T/#u

Thanks a lot.

Kind regards,
Changwei