[PATCH v3 0/3] Fixes and rework for aes_s390

[PATCH v3 0/3] Fixes and rework for aes_s390

[Harald Freudenberger]

Fix and rework some issues around arch/s390/aes_s390.c:

- Fix skcipher_walk return code handling in aes_s390
- Add scrub of some temp buffers
- Rework AES CTR to be prepared for context analysis

For more details please see patch headers.

Changelog:

v1: initial version
v2: Fix missing scrub on a temp buffer used to process left over bytes
    in CTR mode.
v3: - Yet another buffer scrubbing was missing. Added to the 2nd 
      patch and rephrased commit header.
    - Added patch to fix one warning with context analysis regarding
      the handling of a mutex with the AES CTR implementation.

Harald Freudenberger (3):
  s390/crypto: Fix skcipher_walk return code handling in aes_s390
  s390/crypto: Fix missing scrub of temp buffers with AES ctr and gcm
    algorithm
  s390/crypto: Rewrite AES ctr mode to be prepared for context analysis

 arch/s390/crypto/aes_s390.c | 82 +++++++++++++++++++++++--------------
 1 file changed, 52 insertions(+), 30 deletions(-)


base-commit: f5098b6bae761e346ebcd9da7f95622c04733cff
--
2.43.0

[PATCH v3 1/3] s390/crypto: Fix skcipher_walk return code handling in aes_s390

[Harald Freudenberger]

The return codes from skcipher_walk_virt() were not properly checked
before entering the processing loops in ecb_aes_crypt() and ctr_aes_crypt().
If skcipher_walk_virt() fails, the walk structure may be in an undefined
state, and attempting to process data could lead to incorrect behavior
or accessing uninitialized memory.

Add proper return code checking to ensure correct handling of the walk
initialization and walk advance and eventually return to the caller
with that return code.

Fixes: 7988fb2c03c8 ("crypto: s390/aes - convert to skcipher API")
Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Cc: stable@vger.kernel.org # 5.5+
---
 arch/s390/crypto/aes_s390.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/arch/s390/crypto/aes_s390.c b/arch/s390/crypto/aes_s390.c
index 62edc66d5478..366ce22d3623 100644
--- a/arch/s390/crypto/aes_s390.c
+++ b/arch/s390/crypto/aes_s390.c
@@ -129,7 +129,7 @@ static int ecb_aes_crypt(struct skcipher_request *req, unsigned long modifier)
 		return fallback_skcipher_crypt(sctx, req, modifier);
 
 	ret = skcipher_walk_virt(&walk, req, false);
-	while ((nbytes = walk.nbytes) != 0) {
+	while (!ret && ((nbytes = walk.nbytes) != 0)) {
 		/* only use complete blocks */
 		n = nbytes & ~(AES_BLOCK_SIZE - 1);
 		cpacf_km(sctx->fc | modifier, sctx->key,
@@ -233,7 +233,7 @@ static int cbc_aes_crypt(struct skcipher_request *req, unsigned long modifier)
 		return ret;
 	memcpy(param.iv, walk.iv, AES_BLOCK_SIZE);
 	memcpy(param.key, sctx->key, sctx->key_len);
-	while ((nbytes = walk.nbytes) != 0) {
+	while (!ret && ((nbytes = walk.nbytes) != 0)) {
 		/* only use complete blocks */
 		n = nbytes & ~(AES_BLOCK_SIZE - 1);
 		cpacf_kmc(sctx->fc | modifier, &param,
@@ -359,7 +359,7 @@ static int xts_aes_crypt(struct skcipher_request *req, unsigned long modifier)
 	memcpy(xts_param.key + offset, xts_ctx->key, xts_ctx->key_len);
 	memcpy(xts_param.init, pcc_param.xts, 16);
 
-	while ((nbytes = walk.nbytes) != 0) {
+	while (!ret && ((nbytes = walk.nbytes) != 0)) {
 		/* only use complete blocks */
 		n = nbytes & ~(AES_BLOCK_SIZE - 1);
 		cpacf_km(xts_ctx->fc | modifier, xts_param.key + offset,
@@ -487,7 +487,7 @@ static int fullxts_aes_crypt(struct skcipher_request *req,  unsigned long modifi
 	memcpy(fxts_param.tweak, req->iv, AES_BLOCK_SIZE);
 	fxts_param.nap[0] = 0x01; /* initial alpha power (1, little-endian) */
 
-	while ((nbytes = walk.nbytes) != 0) {
+	while (!ret && ((nbytes = walk.nbytes) != 0)) {
 		/* only use complete blocks */
 		n = nbytes & ~(AES_BLOCK_SIZE - 1);
 		cpacf_km(xts_ctx->fc | modifier, fxts_param.key + offset,
@@ -577,7 +577,7 @@ static int ctr_aes_crypt(struct skcipher_request *req)
 	locked = mutex_trylock(&ctrblk_lock);
 
 	ret = skcipher_walk_virt(&walk, req, false);
-	while ((nbytes = walk.nbytes) >= AES_BLOCK_SIZE) {
+	while (!ret && ((nbytes = walk.nbytes) >= AES_BLOCK_SIZE)) {
 		n = AES_BLOCK_SIZE;
 
 		if (nbytes >= 2*AES_BLOCK_SIZE && locked)
@@ -596,7 +596,7 @@ static int ctr_aes_crypt(struct skcipher_request *req)
 	/*
 	 * final block may be < AES_BLOCK_SIZE, copy only nbytes
 	 */
-	if (nbytes) {
+	if (!ret && nbytes) {
 		memset(buf, 0, AES_BLOCK_SIZE);
 		memcpy(buf, walk.src.virt.addr, nbytes);
 		cpacf_kmctr(sctx->fc, sctx->key, buf, buf,
-- 
2.43.0

Re: [PATCH v3 1/3] s390/crypto: Fix skcipher_walk return code handling in aes_s390

[sashiko-bot]

> The return codes from skcipher_walk_virt() were not properly checked
> before entering the processing loops in ecb_aes_crypt() and ctr_aes_crypt().
> If skcipher_walk_virt() fails, the walk structure may be in an undefined
> state, and attempting to process data could lead to incorrect behavior
> or accessing uninitialized memory.
> 
> Add proper return code checking to ensure correct handling of the walk
> initialization and walk advance and eventually return to the caller
> with that return code.
> 
> Fixes: 7988fb2c03c8 ("crypto: s390/aes - convert to skcipher API")
> Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260807135440.35888-1-freude@linux.ibm.com?part=1

[PATCH v3 2/3] s390/crypto: Fix missing scrub of temp buffers with AES ctr and gcm algorithm

[Harald Freudenberger]

In function ctr_aes_crypt() there is a buffer used to process
remaining bytes < AES_BLOCK_SIZE. This buffer was not scrubbed and
thus could lead to expose of unwanted data. When the buffer is used
explicitly scrub it at the end of the code block to avoid exposure of
maybe sensitive data.

In a similar way the function gcm_aes_crypt() hat an error path where
the CPACF param block was not scrubbed. Instead of return early now
these error paths go to end of function where explicit scrubbing is
done.

Fixes: d07f951903fa ("crypto: s390/aes - Fix buffer overread in CTR mode")
Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Cc: stable@vger.kernel.org # 6.8+
---
 arch/s390/crypto/aes_s390.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/arch/s390/crypto/aes_s390.c b/arch/s390/crypto/aes_s390.c
index 366ce22d3623..976f6f7257d5 100644
--- a/arch/s390/crypto/aes_s390.c
+++ b/arch/s390/crypto/aes_s390.c
@@ -604,6 +604,7 @@ static int ctr_aes_crypt(struct skcipher_request *req)
 		memcpy(walk.dst.virt.addr, buf, nbytes);
 		crypto_inc(walk.iv, AES_BLOCK_SIZE);
 		ret = skcipher_walk_done(&walk, 0);
+		memzero_explicit(buf, sizeof(buf));
 	}
 
 	return ret;
@@ -895,10 +896,14 @@ static int gcm_aes_crypt(struct aead_request *req, unsigned int flags)
 			  gw_in.ptr, aad_bytes);
 
 		n = aad_bytes + pc_bytes;
-		if (gcm_in_walk_done(&gw_in, n) != n)
-			return -ENOMEM;
-		if (gcm_out_walk_done(&gw_out, n) != n)
-			return -ENOMEM;
+		if (gcm_in_walk_done(&gw_in, n) != n) {
+			ret = -ENOMEM;
+			goto out;
+		}
+		if (gcm_out_walk_done(&gw_out, n) != n) {
+			ret = -ENOMEM;
+			goto out;
+		}
 		aadlen -= aad_bytes;
 		pclen -= pc_bytes;
 	} while (aadlen + pclen > 0);
@@ -910,6 +915,7 @@ static int gcm_aes_crypt(struct aead_request *req, unsigned int flags)
 	} else
 		scatterwalk_map_and_copy(param.t, req->dst, len, taglen, 1);
 
+out:
 	memzero_explicit(&param, sizeof(param));
 	return ret;
 }
-- 
2.43.0

Re: [PATCH v3 2/3] s390/crypto: Fix missing scrub of temp buffers with AES ctr and gcm algorithm

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:

Pre-existing issues:
- [High] The patch fails to scrub the temporary stack buffers `gw_in` and `gw_out` in `gcm_aes_crypt()`, leaving plaintext or ciphertext fragments on the kernel stack.
- [High] The `ctr_aes_crypt` function acquires a mutex (`ctrblk_lock`) using `mutex_trylock()`, which is illegal in atomic or softirq context.
--

--- Patch [2]: [PATCH v3 2/3] s390/crypto: Fix missing scrub of temp buffers with AES ctr and gcm algorithm ---
commit 13cf8c423603f630fdd6d78e2084476a4308ed55
Author: Harald Freudenberger <freude@linux.ibm.com>

s390/crypto: Fix missing scrub of temp buffers with AES ctr and gcm algorithm

This commit fixes missing scrubbing of temporary buffers in AES CTR and GCM
modes. It scrubs the buf array in ctr_aes_crypt() and uses an explicit error
path in gcm_aes_crypt() to ensure param is scrubbed before returning.

> diff --git a/arch/s390/crypto/aes_s390.c b/arch/s390/crypto/aes_s390.c
> index 366ce22d36237..976f6f7257d51 100644
> --- a/arch/s390/crypto/aes_s390.c
> +++ b/arch/s390/crypto/aes_s390.c
> @@ -604,6 +604,7 @@ static int ctr_aes_crypt(struct skcipher_request *req)

[Severity: High]
This is a pre-existing issue, but is it safe to use mutex_trylock() and
mutex_unlock() in this function?

arch/s390/crypto/aes_s390.c:ctr_aes_crypt() {
    ...
	if (mutex_trylock(&ctrblk_lock)) {
		ret = __ctr_aes_crypt(sctx, &walk, true);
		mutex_unlock(&ctrblk_lock);
	} else {
    ...
}

The ctr(aes) skcipher algorithm can be invoked from softirq context, such as
via the IPsec network stack in NET_RX_SOFTIRQ.

Calling mutex_trylock() and mutex_unlock() from softirq context is generally
prohibited as it causes lock context violations. On PREEMPT_RT kernels, it
triggers WARN_ON_ONCE(!in_task()), and mutex_unlock() may acquire the
wait_lock raw spinlock without disabling softirqs, potentially creating
a deadlock.

>  		memcpy(walk.dst.virt.addr, buf, nbytes);
>  		crypto_inc(walk.iv, AES_BLOCK_SIZE);
>  		ret = skcipher_walk_done(&walk, 0);
> +		memzero_explicit(buf, sizeof(buf));
>  	}
>  
>  	return ret;

[ ... ]

> @@ -895,10 +896,14 @@ static int gcm_aes_crypt(struct aead_request *req, unsigned int flags)
>  			  gw_in.ptr, aad_bytes);
>  
>  		n = aad_bytes + pc_bytes;
> -		if (gcm_in_walk_done(&gw_in, n) != n)
> -			return -ENOMEM;
> -		if (gcm_out_walk_done(&gw_out, n) != n)
> -			return -ENOMEM;
> +		if (gcm_in_walk_done(&gw_in, n) != n) {
> +			ret = -ENOMEM;
> +			goto out;
> +		}
> +		if (gcm_out_walk_done(&gw_out, n) != n) {
> +			ret = -ENOMEM;
> +			goto out;
> +		}
>  		aadlen -= aad_bytes;
>  		pclen -= pc_bytes;
>  	} while (aadlen + pclen > 0);
> @@ -910,6 +915,7 @@ static int gcm_aes_crypt(struct aead_request *req, unsigned int flags)
>  	} else
>  		scatterwalk_map_and_copy(param.t, req->dst, len, taglen, 1);
>  
> +out:
>  	memzero_explicit(&param, sizeof(param));

[Severity: High]
This isn't a bug introduced by this patch, but since the goal is to prevent
exposure of sensitive data on the stack by scrubbing temporary buffers,
shouldn't gw_in and gw_out also be scrubbed?

The gcm_sg_walk structures contain a u8 buf[AES_BLOCK_SIZE] that is allocated
on the stack:

arch/s390/crypto/aes_s390.c:gcm_aes_crypt() {
    ...
    struct gcm_sg_walk gw_in, gw_out;
    ...
}

Any AEAD encryption/decryption with fragmented input scatterlists will use
gw_in.buf and gw_out.buf to bounce-buffer data, which will be left on the
kernel stack upon return.

>  	return ret;
>  }

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260807135440.35888-1-freude@linux.ibm.com?part=2

[PATCH v3 3/3] s390/crypto: Rewrite AES ctr mode to be prepared for context analysis

[Harald Freudenberger]

The AES ctr implementation produces a warning when used with clang and
CONTEXT_ANALYIS enabled:

arch/s390/crypto/aes_s390.c:585:13: warning: mutex 'ctrblk_lock' is not held on every path through here [-Wthread-safety-analysis]
  585 |                 ctrptr = (n > AES_BLOCK_SIZE) ? ctrblk : walk.iv;
      |                           ^
arch/s390/crypto/aes_s390.c:577:11: note: mutex acquired here
  577 |         locked = mutex_trylock(&ctrblk_lock);
      |                  ^

Rewrite and reorganize the code such that the clang compiler's needs
are fulfilled with keeping the compatibility, performance and
correctness of the crypto algorithm.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Suggested-by: Heiko Carstens <hca@linux.ibm.com>
---
 arch/s390/crypto/aes_s390.c | 60 +++++++++++++++++++++++--------------
 1 file changed, 38 insertions(+), 22 deletions(-)

diff --git a/arch/s390/crypto/aes_s390.c b/arch/s390/crypto/aes_s390.c
index 976f6f7257d5..e75f41e2bcb7 100644
--- a/arch/s390/crypto/aes_s390.c
+++ b/arch/s390/crypto/aes_s390.c
@@ -562,46 +562,62 @@ static unsigned int __ctrblk_init(u8 *ctrptr, u8 *iv, unsigned int nbytes)
 	return n;
 }
 
+static int __ctr_aes_crypt(struct s390_aes_ctx *sctx,
+			   struct skcipher_walk *walk, bool locked)
+{
+	unsigned int n, nbytes;
+	int ret = 0;
+	u8 *ctrptr;
+
+	while (!ret && ((nbytes = walk->nbytes) >= AES_BLOCK_SIZE)) {
+		n = AES_BLOCK_SIZE;
+		if (nbytes >= 2 * AES_BLOCK_SIZE && locked)
+			n = __ctrblk_init(ctrblk, walk->iv, nbytes);
+		ctrptr = (n > AES_BLOCK_SIZE) ? ctrblk : walk->iv;
+		cpacf_kmctr(sctx->fc, sctx->key, walk->dst.virt.addr,
+			    walk->src.virt.addr, n, ctrptr);
+		if (ctrptr == ctrblk) {
+			memcpy(walk->iv, ctrptr + n - AES_BLOCK_SIZE,
+			       AES_BLOCK_SIZE);
+		}
+		crypto_inc(walk->iv, AES_BLOCK_SIZE);
+		ret = skcipher_walk_done(walk, nbytes - n);
+	}
+
+	return ret;
+}
+
 static int ctr_aes_crypt(struct skcipher_request *req)
 {
 	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
 	struct s390_aes_ctx *sctx = crypto_skcipher_ctx(tfm);
-	u8 buf[AES_BLOCK_SIZE], *ctrptr;
 	struct skcipher_walk walk;
-	unsigned int n, nbytes;
-	int ret, locked;
+	u8 buf[AES_BLOCK_SIZE];
+	int ret;
 
 	if (unlikely(!sctx->fc))
 		return fallback_skcipher_crypt(sctx, req, 0);
 
-	locked = mutex_trylock(&ctrblk_lock);
-
 	ret = skcipher_walk_virt(&walk, req, false);
-	while (!ret && ((nbytes = walk.nbytes) >= AES_BLOCK_SIZE)) {
-		n = AES_BLOCK_SIZE;
+	if (ret)
+		return ret;
 
-		if (nbytes >= 2*AES_BLOCK_SIZE && locked)
-			n = __ctrblk_init(ctrblk, walk.iv, nbytes);
-		ctrptr = (n > AES_BLOCK_SIZE) ? ctrblk : walk.iv;
-		cpacf_kmctr(sctx->fc, sctx->key, walk.dst.virt.addr,
-			    walk.src.virt.addr, n, ctrptr);
-		if (ctrptr == ctrblk)
-			memcpy(walk.iv, ctrptr + n - AES_BLOCK_SIZE,
-			       AES_BLOCK_SIZE);
-		crypto_inc(walk.iv, AES_BLOCK_SIZE);
-		ret = skcipher_walk_done(&walk, nbytes - n);
-	}
-	if (locked)
+	if (mutex_trylock(&ctrblk_lock)) {
+		ret = __ctr_aes_crypt(sctx, &walk, true);
 		mutex_unlock(&ctrblk_lock);
+	} else {
+		ret = __ctr_aes_crypt(sctx, &walk, false);
+	}
+
 	/*
 	 * final block may be < AES_BLOCK_SIZE, copy only nbytes
 	 */
-	if (!ret && nbytes) {
+	if (!ret && walk.nbytes) {
 		memset(buf, 0, AES_BLOCK_SIZE);
-		memcpy(buf, walk.src.virt.addr, nbytes);
+		memcpy(buf, walk.src.virt.addr, walk.nbytes);
 		cpacf_kmctr(sctx->fc, sctx->key, buf, buf,
 			    AES_BLOCK_SIZE, walk.iv);
-		memcpy(walk.dst.virt.addr, buf, nbytes);
+		memcpy(walk.dst.virt.addr, buf, walk.nbytes);
 		crypto_inc(walk.iv, AES_BLOCK_SIZE);
 		ret = skcipher_walk_done(&walk, 0);
 		memzero_explicit(buf, sizeof(buf));
-- 
2.43.0

Re: [PATCH v3 3/3] s390/crypto: Rewrite AES ctr mode to be prepared for context analysis

[sashiko-bot]

> The AES ctr implementation produces a warning when used with clang and
> CONTEXT_ANALYIS enabled:
> 
> arch/s390/crypto/aes_s390.c:585:13: warning: mutex 'ctrblk_lock' is not held on every path through here [-Wthread-safety-analysis]
>   585 |                 ctrptr = (n > AES_BLOCK_SIZE) ? ctrblk : walk.iv;
>       |                           ^
> arch/s390/crypto/aes_s390.c:577:11: note: mutex acquired here
>   577 |         locked = mutex_trylock(&ctrblk_lock);
>       |                  ^
> 
> Rewrite and reorganize the code such that the clang compiler's needs
> are fulfilled with keeping the compatibility, performance and
> correctness of the crypto algorithm.
> 
> Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260807135440.35888-1-freude@linux.ibm.com?part=3