[PATCH 0/5] KEYS: fixes for new keyctl_dh_compute() KDF extension

[PATCH 0/5] KEYS: fixes for new keyctl_dh_compute() KDF extension

[Eric Biggers]

This patch series fixes several bugs in the KDF extension to
keyctl_dh_compute() currently sitting in keys-next: a way userspace could
cause an infinite loop, two ways userspace could cause the use of
uninitialized memory, a misalignment, and missing __user annotations.

Eric Biggers (5):
  KEYS: DH: forbid using digest_null as the KDF hash
  KEYS: DH: don't feed uninitialized "otherinfo" into KDF
  KEYS: DH: don't feed uninitialized result memory into KDF
  KEYS: DH: ensure the KDF counter is properly aligned
  KEYS: DH: add __user annotations to keyctl_kdf_params

 include/uapi/linux/keyctl.h |  4 ++--
 security/keys/dh.c          | 50 ++++++++++++++++++++++-----------------------
 2 files changed, 26 insertions(+), 28 deletions(-)

-- 
2.12.2

[PATCH 1/5] KEYS: DH: forbid using digest_null as the KDF hash

[Eric Biggers]

From: Eric Biggers <ebiggers@google.com>

Requesting "digest_null" in the keyctl_kdf_params caused an infinite
loop in kdf_ctr() because the "null" hash has a digest size of 0.  Fix
it by rejecting hash algorithms with a digest size of 0.

Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 security/keys/dh.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/security/keys/dh.c b/security/keys/dh.c
index e603bd912e4c..8abc70ebe22d 100644
--- a/security/keys/dh.c
+++ b/security/keys/dh.c
@@ -89,6 +89,7 @@ static int kdf_alloc(struct kdf_sdesc **sdesc_ret, char *hashname)
 	struct crypto_shash *tfm;
 	struct kdf_sdesc *sdesc;
 	int size;
+	int err;
 
 	/* allocate synchronous hash */
 	tfm = crypto_alloc_shash(hashname, 0, 0);
@@ -97,16 +98,25 @@ static int kdf_alloc(struct kdf_sdesc **sdesc_ret, char *hashname)
 		return PTR_ERR(tfm);
 	}
 
+	err = -EINVAL;
+	if (crypto_shash_digestsize(tfm) == 0)
+		goto out_free_tfm;
+
+	err = -ENOMEM;
 	size = sizeof(struct shash_desc) + crypto_shash_descsize(tfm);
 	sdesc = kmalloc(size, GFP_KERNEL);
 	if (!sdesc)
-		return -ENOMEM;
+		goto out_free_tfm;
 	sdesc->shash.tfm = tfm;
 	sdesc->shash.flags = 0x0;
 
 	*sdesc_ret = sdesc;
 
 	return 0;
+
+out_free_tfm:
+	crypto_free_shash(tfm);
+	return err;
 }
 
 static void kdf_dealloc(struct kdf_sdesc *sdesc)
-- 
2.12.2

[PATCH 2/5] KEYS: DH: don't feed uninitialized "otherinfo" into KDF

[Eric Biggers]

From: Eric Biggers <ebiggers@google.com>

If userspace called KEYCTL_DH_COMPUTE with kdf_params containing NULL
otherinfo but nonzero otherinfolen, the kernel would allocate a buffer
for the otherinfo, then feed it into the KDF without initializing it.
Fix this by always doing the copy from userspace (which will fail with
EFAULT in this scenario).

Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 security/keys/dh.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/security/keys/dh.c b/security/keys/dh.c
index 8abc70ebe22d..1c1cac677041 100644
--- a/security/keys/dh.c
+++ b/security/keys/dh.c
@@ -317,7 +317,7 @@ long __keyctl_dh_compute(struct keyctl_dh_params __user *params,
 	 * Concatenate SP800-56A otherinfo past DH shared secret -- the
 	 * input to the KDF is (DH shared secret || otherinfo)
 	 */
-	if (kdfcopy && kdfcopy->otherinfo &&
+	if (kdfcopy &&
 	    copy_from_user(kbuf + resultlen, kdfcopy->otherinfo,
 			   kdfcopy->otherinfolen) != 0) {
 		ret = -EFAULT;
-- 
2.12.2

[PATCH 3/5] KEYS: DH: don't feed uninitialized result memory into KDF

[Eric Biggers]

From: Eric Biggers <ebiggers@google.com>

The result of the Diffie-Hellman computation may be shorter than the
input prime number.  Only calculate the KDF over the actual result;
don't include additional uninitialized memory.

Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 security/keys/dh.c | 22 ++++++++++------------
 1 file changed, 10 insertions(+), 12 deletions(-)

diff --git a/security/keys/dh.c b/security/keys/dh.c
index 1c1cac677041..a3a8607107f5 100644
--- a/security/keys/dh.c
+++ b/security/keys/dh.c
@@ -313,17 +313,6 @@ long __keyctl_dh_compute(struct keyctl_dh_params __user *params,
 		goto error4;
 	}
 
-	/*
-	 * Concatenate SP800-56A otherinfo past DH shared secret -- the
-	 * input to the KDF is (DH shared secret || otherinfo)
-	 */
-	if (kdfcopy &&
-	    copy_from_user(kbuf + resultlen, kdfcopy->otherinfo,
-			   kdfcopy->otherinfolen) != 0) {
-		ret = -EFAULT;
-		goto error5;
-	}
-
 	ret = do_dh(result, base, private, prime);
 	if (ret)
 		goto error5;
@@ -333,8 +322,17 @@ long __keyctl_dh_compute(struct keyctl_dh_params __user *params,
 		goto error5;
 
 	if (kdfcopy) {
+		/*
+		 * Concatenate SP800-56A otherinfo past DH shared secret -- the
+		 * input to the KDF is (DH shared secret || otherinfo)
+		 */
+		if (copy_from_user(kbuf + nbytes, kdfcopy->otherinfo,
+				   kdfcopy->otherinfolen) != 0) {
+			ret = -EFAULT;
+			goto error5;
+		}
 		ret = keyctl_dh_compute_kdf(sdesc, buffer, buflen, kbuf,
-					    resultlen + kdfcopy->otherinfolen);
+					    nbytes + kdfcopy->otherinfolen);
 	} else {
 		ret = nbytes;
 		if (copy_to_user(buffer, kbuf, nbytes) != 0)
-- 
2.12.2

Re: [PATCH 3/5] KEYS: DH: don't feed uninitialized result memory into KDF

[Stephan Müller]

Am Donnerstag, 20. April 2017, 07:46:31 CEST schrieb Eric Biggers:

Hi Eric,

> From: Eric Biggers <ebiggers@google.com>
> 
> The result of the Diffie-Hellman computation may be shorter than the
> input prime number.  Only calculate the KDF over the actual result;
> don't include additional uninitialized memory.

Thank you for catching that (and all the rest). But I think this patch is not 
correct. If the DH operation results in a shorter value, the trailing part 
must be set to null and the KDF calculated over the entire prime length.

Thus, if the DH result is shorter than the prime, the memory should look like 
DH result || 0x00 <as often as needed to make it prime length> || otherinfo.

Thus, instead of this patch, I would think that the kmalloc call should be 
changed to a kzalloc.
> 
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
>  security/keys/dh.c | 22 ++++++++++------------
>  1 file changed, 10 insertions(+), 12 deletions(-)
> 
> diff --git a/security/keys/dh.c b/security/keys/dh.c
> index 1c1cac677041..a3a8607107f5 100644
> --- a/security/keys/dh.c
> +++ b/security/keys/dh.c
> @@ -313,17 +313,6 @@ long __keyctl_dh_compute(struct keyctl_dh_params __user
> *params, goto error4;
>  	}
> 
> -	/*
> -	 * Concatenate SP800-56A otherinfo past DH shared secret -- the
> -	 * input to the KDF is (DH shared secret || otherinfo)
> -	 */
> -	if (kdfcopy &&
> -	    copy_from_user(kbuf + resultlen, kdfcopy->otherinfo,
> -			   kdfcopy->otherinfolen) != 0) {
> -		ret = -EFAULT;
> -		goto error5;
> -	}
> -
>  	ret = do_dh(result, base, private, prime);
>  	if (ret)
>  		goto error5;
> @@ -333,8 +322,17 @@ long __keyctl_dh_compute(struct keyctl_dh_params __user
> *params, goto error5;
> 
>  	if (kdfcopy) {
> +		/*
> +		 * Concatenate SP800-56A otherinfo past DH shared secret -- the
> +		 * input to the KDF is (DH shared secret || otherinfo)
> +		 */
> +		if (copy_from_user(kbuf + nbytes, kdfcopy->otherinfo,
> +				   kdfcopy->otherinfolen) != 0) {
> +			ret = -EFAULT;
> +			goto error5;
> +		}
>  		ret = keyctl_dh_compute_kdf(sdesc, buffer, buflen, kbuf,
> -					    resultlen + kdfcopy->otherinfolen);
> +					    nbytes + kdfcopy->otherinfolen);
>  	} else {
>  		ret = nbytes;
>  		if (copy_to_user(buffer, kbuf, nbytes) != 0)



Ciao
Stephan

Re: [PATCH 3/5] KEYS: DH: don't feed uninitialized result memory into KDF

[Eric Biggers]

Hi Stephan,

On Thu, Apr 20, 2017 at 03:27:17PM +0200, Stephan Müller wrote:
> Am Donnerstag, 20. April 2017, 07:46:31 CEST schrieb Eric Biggers:
> 
> Hi Eric,
> 
> > From: Eric Biggers <ebiggers@google.com>
> > 
> > The result of the Diffie-Hellman computation may be shorter than the
> > input prime number.  Only calculate the KDF over the actual result;
> > don't include additional uninitialized memory.
> 
> Thank you for catching that (and all the rest). But I think this patch is not 
> correct. If the DH operation results in a shorter value, the trailing part 
> must be set to null and the KDF calculated over the entire prime length.
> 
> Thus, if the DH result is shorter than the prime, the memory should look like 
> DH result || 0x00 <as often as needed to make it prime length> || otherinfo.
> 
> Thus, instead of this patch, I would think that the kmalloc call should be 
> changed to a kzalloc.
> > 

Is this in the standard?  And is it the user-specified length of the prime
number, or the length after stripping leading zeroes?  Also, note that the
numbers are being represented in big endian format; is that required, or just
coincidental?  With big endian numbers leading zeroes go at the beginning, not
the end, otherwise their value will be changed...

By the way: do we really need this in the kernel at all, given that it's just
doing some math on data which userspace has access to?

- Eric

Re: [PATCH 3/5] KEYS: DH: don't feed uninitialized result memory into KDF

[Stephan Müller]

Am Donnerstag, 20. April 2017, 19:46:02 CEST schrieb Eric Biggers:

Hi Eric,

> Hi Stephan,
> 
> On Thu, Apr 20, 2017 at 03:27:17PM +0200, Stephan Müller wrote:
> > Am Donnerstag, 20. April 2017, 07:46:31 CEST schrieb Eric Biggers:
> > 
> > Hi Eric,
> > 
> > > From: Eric Biggers <ebiggers@google.com>
> > > 
> > > The result of the Diffie-Hellman computation may be shorter than the
> > > input prime number.  Only calculate the KDF over the actual result;
> > > don't include additional uninitialized memory.
> > 
> > Thank you for catching that (and all the rest). But I think this patch is
> > not correct. If the DH operation results in a shorter value, the trailing
> > part must be set to null and the KDF calculated over the entire prime
> > length.
> > 
> > Thus, if the DH result is shorter than the prime, the memory should look
> > like DH result || 0x00 <as often as needed to make it prime length> ||
> > otherinfo.
> > 
> > Thus, instead of this patch, I would think that the kmalloc call should be
> > changed to a kzalloc.
> 
> Is this in the standard? 

That is the gotcha that is not explicitly written in the standard. However, 
based on my experience with other implementations and tests like ECDSA and RSA 
CAVS testing, the standards seem to always interpreted to use the full allowed 
length. If the mathematical result is shorter than the defined length, it 
should be zero-padded.

> And is it the user-specified length of the prime
> number, or the length after stripping leading zeroes?

It should be the length of the prime used for the DH operation. As the prime 
is given with the DH parameters, the DH parameter set defines the length of 
the prime.

I cannot say about the stripping of leading zeros of user-provided primes 
because I have no idea where that is defined.

I would need to do some further research on this matter and check with the 
authors of the standard.

> Also, note that the
> numbers are being represented in big endian format; is that required, or
> just coincidental?  With big endian numbers leading zeroes go at the
> beginning, not the end, otherwise their value will be changed...

The numbers are MPI and are therefore big endian formats. Also the counter in 
the KDF is a big endian format which is mandated in the spec.

You are right that the zeros go to the beginning of the number and I made a 
wrong statement in my initial remark regarding the zero value.
> 
> - Ericinning and my initial remark regarding where the zeros are is wrong. 
Yet, IMHO we should not stip the zeros before applying the KDF as this would 
imply that the KDF result is different.
> 
> By the way: do we really need this in the kernel at all, given that it's
> just doing some math on data which userspace has access to?

It is the question about how we want the keys subsystem to operate. The DH 
shared secret shall not be used as a key. But the DH operation is part of the 
key subsystem. If there is never a case where the result of the DH operation 
is used in the kernel, then the KDF can be removed and my patches could be 
reverted. However, in this case, the entire DH business could be questioned as 
this can easily be done in user space as well.

Ciao
Stephan

Re: [PATCH 3/5] KEYS: DH: don't feed uninitialized result memory into KDF

[Eric Biggers]

Hi Stephan,

On Thu, Apr 20, 2017 at 08:38:30PM +0200, Stephan Müller wrote:
> > 
> > By the way: do we really need this in the kernel at all, given that it's
> > just doing some math on data which userspace has access to?
> 
> It is the question about how we want the keys subsystem to operate. The DH 
> shared secret shall not be used as a key. But the DH operation is part of the 
> key subsystem. If there is never a case where the result of the DH operation 
> is used in the kernel, then the KDF can be removed and my patches could be 
> reverted. However, in this case, the entire DH business could be questioned as 
> this can easily be done in user space as well.
> 

Well, who exactly is asking for Diffie-Hellman in the kernel at all?  If it can
be done in userspace then it should be done there.  Having it in the kernel
means having yet another API that's callable by unprivileged users and needs to
be audited for security vulnerabilities.  Just because the kernel can support
doing hashes or has an arbitrary-precision arithmetic library or whatever
doesn't mean it's the right place to do random crypto stuff.

- Eric

Re: [PATCH 3/5] KEYS: DH: don't feed uninitialized result memory into KDF

[David Howells]

Eric Biggers <ebiggers3@gmail.com> wrote:

> > > By the way: do we really need this in the kernel at all, given that it's
> > > just doing some math on data which userspace has access to?
> > 
> > It is the question about how we want the keys subsystem to operate. The DH
> > shared secret shall not be used as a key. But the DH operation is part of
> > the key subsystem. If there is never a case where the result of the DH
> > operation is used in the kernel, then the KDF can be removed and my
> > patches could be reverted. However, in this case, the entire DH business
> > could be questioned as this can easily be done in user space as well.
> > 
> 
> Well, who exactly is asking for Diffie-Hellman in the kernel at all?  If it
> can be done in userspace then it should be done there.  Having it in the
> kernel means having yet another API that's callable by unprivileged users
> and needs to be audited for security vulnerabilities.  Just because the
> kernel can support doing hashes or has an arbitrary-precision arithmetic
> library or whatever doesn't mean it's the right place to do random crypto
> stuff.

I understood that there is the possibility of offloading this to hardware.

David

Re: [PATCH 3/5] KEYS: DH: don't feed uninitialized result memory into KDF

[Eric Biggers]

On Thu, Apr 27, 2017 at 04:15:19PM +0100, David Howells wrote:
> Eric Biggers <ebiggers3@gmail.com> wrote:
> 
> > > > By the way: do we really need this in the kernel at all, given that it's
> > > > just doing some math on data which userspace has access to?
> > > 
> > > It is the question about how we want the keys subsystem to operate. The DH
> > > shared secret shall not be used as a key. But the DH operation is part of
> > > the key subsystem. If there is never a case where the result of the DH
> > > operation is used in the kernel, then the KDF can be removed and my
> > > patches could be reverted. However, in this case, the entire DH business
> > > could be questioned as this can easily be done in user space as well.
> > > 
> > 
> > Well, who exactly is asking for Diffie-Hellman in the kernel at all?  If it
> > can be done in userspace then it should be done there.  Having it in the
> > kernel means having yet another API that's callable by unprivileged users
> > and needs to be audited for security vulnerabilities.  Just because the
> > kernel can support doing hashes or has an arbitrary-precision arithmetic
> > library or whatever doesn't mean it's the right place to do random crypto
> > stuff.
> 
> I understood that there is the possibility of offloading this to hardware.
> 

Okay, but where is this hardware, where are drivers for it, and how do we know
this API is actually going to be compatible with it?  Will it be just for
performance, or for other reasons too?  None of this seems to have been
explained.

Eric

[PATCH 4/5] KEYS: DH: ensure the KDF counter is properly aligned

[Eric Biggers]

From: Eric Biggers <ebiggers@google.com>

Accessing a 'u8[4]' through a '__be32 *' violates alignment rules.  Just
make the counter a __be32 instead.

Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 security/keys/dh.c | 16 +++-------------
 1 file changed, 3 insertions(+), 13 deletions(-)

diff --git a/security/keys/dh.c b/security/keys/dh.c
index a3a8607107f5..e6b757c33289 100644
--- a/security/keys/dh.c
+++ b/security/keys/dh.c
@@ -130,14 +130,6 @@ static void kdf_dealloc(struct kdf_sdesc *sdesc)
 	kzfree(sdesc);
 }
 
-/* convert 32 bit integer into its string representation */
-static inline void crypto_kw_cpu_to_be32(u32 val, u8 *buf)
-{
-	__be32 *a = (__be32 *)buf;
-
-	*a = cpu_to_be32(val);
-}
-
 /*
  * Implementation of the KDF in counter mode according to SP800-108 section 5.1
  * as well as SP800-56A section 5.8.1 (Single-step KDF).
@@ -154,16 +146,14 @@ static int kdf_ctr(struct kdf_sdesc *sdesc, const u8 *src, unsigned int slen,
 	unsigned int h = crypto_shash_digestsize(desc->tfm);
 	int err = 0;
 	u8 *dst_orig = dst;
-	u32 i = 1;
-	u8 iteration[sizeof(u32)];
+	__be32 counter = cpu_to_be32(1);
 
 	while (dlen) {
 		err = crypto_shash_init(desc);
 		if (err)
 			goto err;
 
-		crypto_kw_cpu_to_be32(i, iteration);
-		err = crypto_shash_update(desc, iteration, sizeof(u32));
+		err = crypto_shash_update(desc, (u8 *)&counter, sizeof(__be32));
 		if (err)
 			goto err;
 
@@ -189,7 +179,7 @@ static int kdf_ctr(struct kdf_sdesc *sdesc, const u8 *src, unsigned int slen,
 
 			dlen -= h;
 			dst += h;
-			i++;
+			counter = cpu_to_be32(be32_to_cpu(counter) + 1);
 		}
 	}
 
-- 
2.12.2

[PATCH 5/5] KEYS: DH: add __user annotations to keyctl_kdf_params

[Eric Biggers]

From: Eric Biggers <ebiggers@google.com>

Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 include/uapi/linux/keyctl.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/keyctl.h b/include/uapi/linux/keyctl.h
index 201c6644b237..ef16df06642a 100644
--- a/include/uapi/linux/keyctl.h
+++ b/include/uapi/linux/keyctl.h
@@ -70,8 +70,8 @@ struct keyctl_dh_params {
 };
 
 struct keyctl_kdf_params {
-	char *hashname;
-	char *otherinfo;
+	char __user *hashname;
+	char __user *otherinfo;
 	__u32 otherinfolen;
 	__u32 __spare[8];
 };
-- 
2.12.2

Re: [PATCH 0/5] KEYS: fixes for new keyctl_dh_compute() KDF extension

[David Howells]

Stephan,

Eric Biggers <ebiggers3@gmail.com> wrote:

> This patch series fixes several bugs in the KDF extension to
> keyctl_dh_compute() currently sitting in keys-next: a way userspace could
> cause an infinite loop, two ways userspace could cause the use of
> uninitialized memory, a misalignment, and missing __user annotations.

Do you want to ack these or do they need fixing?

David

Re: [PATCH 0/5] KEYS: fixes for new keyctl_dh_compute() KDF extension

[Stephan Müller]

Am Freitag, 28. April 2017, 17:53:00 CEST schrieb David Howells:

Hi David,

> Stephan,
> 
> Eric Biggers <ebiggers3@gmail.com> wrote:
> > This patch series fixes several bugs in the KDF extension to
> > keyctl_dh_compute() currently sitting in keys-next: a way userspace could
> > cause an infinite loop, two ways userspace could cause the use of
> > uninitialized memory, a misalignment, and missing __user annotations.
> 
> Do you want to ack these or do they need fixing?

All patches except patch 3/5:

Acked-by: Stephan Mueller <smueller@chronox.de>

For the patch 3/5 I would send an updated patch shortly which changes the 
kmalloc to kzalloc.

Ciao
Stephan

Re: [PATCH 0/5] KEYS: fixes for new keyctl_dh_compute() KDF extension

[Stephan Müller]

Am Freitag, 28. April 2017, 17:53:00 CEST schrieb David Howells:

Hi David,

> Stephan,
> 
> Eric Biggers <ebiggers3@gmail.com> wrote:
> > This patch series fixes several bugs in the KDF extension to
> > keyctl_dh_compute() currently sitting in keys-next: a way userspace could
> > cause an infinite loop, two ways userspace could cause the use of
> > uninitialized memory, a misalignment, and missing __user annotations.
> 
> Do you want to ack these or do they need fixing?

Please see the replacement for patch 3/5 below.

Thanks
Stephan

---8<---

>From 7229546f5ca0eed032208c03e7aef47f6b52ca18 Mon Sep 17 00:00:00 2001
From: Stephan Mueller <smueller@chronox.de>
Date: Mon, 1 May 2017 16:45:22 +0200
Subject: [PATCH] keys: SP800-56A - preserve leading zeros for shared secret

The shared secret that is to be processed shall be the unchanged result
of the DH mathematical primitive. The leading zeros shall be preserved.

In addition, the kernel memory that is used as input to the KDF is
zeroized to ensure that no leaks exist.

Reported-by: Eric Biggers <ebiggers3@gmail.com>
Signed-off-by: Stephan Mueller <smueller@chronox.de>
---
 include/linux/mpi.h |  2 +-
 lib/mpi/mpicoder.c  | 10 ++++++----
 security/keys/dh.c  |  4 ++--
 3 files changed, 9 insertions(+), 7 deletions(-)

diff --git a/include/linux/mpi.h b/include/linux/mpi.h
index 1cc5ffb..1f679b6 100644
--- a/include/linux/mpi.h
+++ b/include/linux/mpi.h
@@ -78,7 +78,7 @@ int mpi_fromstr(MPI val, const char *str);
 u32 mpi_get_keyid(MPI a, u32 *keyid);
 void *mpi_get_buffer(MPI a, unsigned *nbytes, int *sign);
 int mpi_read_buffer(MPI a, uint8_t *buf, unsigned buf_len, unsigned *nbytes,
-		    int *sign);
+		    int *sign, bool skip_lzeros);
 void *mpi_get_secure_buffer(MPI a, unsigned *nbytes, int *sign);
 int mpi_write_to_sgl(MPI a, struct scatterlist *sg, unsigned nbytes,
 		     int *sign);
diff --git a/lib/mpi/mpicoder.c b/lib/mpi/mpicoder.c
index 5a0f75a..659d787 100644
--- a/lib/mpi/mpicoder.c
+++ b/lib/mpi/mpicoder.c
@@ -137,11 +137,12 @@ static int count_lzeros(MPI a)
  *		the data to-be-written on -EOVERFLOW in case buf_len was too
  *		small.
  * @sign:	if not NULL, it will be set to the sign of a.
+ * @skip_lzeros:Skip the leading zeros of the MPI before writing to buffer.
  *
  * Return:	0 on success or error code in case of error
  */
 int mpi_read_buffer(MPI a, uint8_t *buf, unsigned buf_len, unsigned *nbytes,
-		    int *sign)
+		    int *sign, bool skip_lzeros)
 {
 	uint8_t *p;
 #if BYTES_PER_MPI_LIMB == 4
@@ -152,7 +153,7 @@ int mpi_read_buffer(MPI a, uint8_t *buf, unsigned buf_len, unsigned *nbytes,
 #error please implement for this limb size.
 #endif
 	unsigned int n = mpi_get_size(a);
-	int i, lzeros;
+	int i, lzeros = 0;
 
 	if (!buf || !nbytes)
 		return -EINVAL;
@@ -160,7 +161,8 @@ int mpi_read_buffer(MPI a, uint8_t *buf, unsigned buf_len, unsigned *nbytes,
 	if (sign)
 		*sign = a->sign;
 
-	lzeros = count_lzeros(a);
+	if (skip_lzeros)
+		lzeros = count_lzeros(a);
 
 	if (buf_len < n - lzeros) {
 		*nbytes = n - lzeros;
@@ -219,7 +221,7 @@ void *mpi_get_buffer(MPI a, unsigned *nbytes, int *sign)
 	if (!buf)
 		return NULL;
 
-	ret = mpi_read_buffer(a, buf, n, nbytes, sign);
+	ret = mpi_read_buffer(a, buf, n, nbytes, sign, true);
 
 	if (ret) {
 		kfree(buf);
diff --git a/security/keys/dh.c b/security/keys/dh.c
index e603bd9..80089dd 100644
--- a/security/keys/dh.c
+++ b/security/keys/dh.c
@@ -296,7 +296,7 @@ long __keyctl_dh_compute(struct keyctl_dh_params __user *params,
 	}
 
 	/* allocate space for DH shared secret and SP800-56A otherinfo */
-	kbuf = kmalloc(kdfcopy ? (resultlen + kdfcopy->otherinfolen) : resultlen,
+	kbuf = kzalloc(kdfcopy ? (resultlen + kdfcopy->otherinfolen) : resultlen,
 		       GFP_KERNEL);
 	if (!kbuf) {
 		ret = -ENOMEM;
@@ -318,7 +318,7 @@ long __keyctl_dh_compute(struct keyctl_dh_params __user *params,
 	if (ret)
 		goto error5;
 
-	ret = mpi_read_buffer(result, kbuf, resultlen, &nbytes, NULL);
+	ret = mpi_read_buffer(result, kbuf, resultlen, &nbytes, NULL, false);
 	if (ret != 0)
 		goto error5;
 
-- 
2.9.3