[PATCH 1/2] big key: get rid of stack array allocation

[PATCH 1/2] big key: get rid of stack array allocation

[Tycho Andersen]

We're interested in getting rid of all of the stack allocated arrays in the
kernel [1]. This patch removes one in keys by switching to malloc/free.
Note that we use kzalloc, to avoid leaking the nonce. I'm not sure this is
really necessary, but extra paranoia seems prudent.

Manually tested using the program from the add_key man page to trigger
big_key.

[1]: https://lkml.org/lkml/2018/3/7/621

Signed-off-by: Tycho Andersen <tycho@tycho.ws>
CC: David Howells <dhowells@redhat.com>
CC: James Morris <jmorris@namei.org>
CC: "Serge E. Hallyn" <serge@hallyn.com>
CC: Jason A. Donenfeld <Jason@zx2c4.com>
---
 security/keys/big_key.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/security/keys/big_key.c b/security/keys/big_key.c
index fa728f662a6f..70f9f785c59d 100644
--- a/security/keys/big_key.c
+++ b/security/keys/big_key.c
@@ -108,13 +108,18 @@ static int big_key_crypt(enum big_key_op op, struct big_key_buf *buf, size_t dat
 	 * an .update function, so there's no chance we'll wind up reusing the
 	 * key to encrypt updated data. Simply put: one key, one encryption.
 	 */
-	u8 zero_nonce[crypto_aead_ivsize(big_key_aead)];
+	u8 *zero_nonce;
+
+	zero_nonce = kzalloc(crypto_aead_ivsize(big_key_aead), GFP_KERNEL);
+	if (!zero_nonce)
+		return -ENOMEM;
 
 	aead_req = aead_request_alloc(big_key_aead, GFP_KERNEL);
-	if (!aead_req)
+	if (!aead_req) {
+		kfree(zero_nonce);
 		return -ENOMEM;
+	}
 
-	memset(zero_nonce, 0, sizeof(zero_nonce));
 	aead_request_set_crypt(aead_req, buf->sg, buf->sg, datalen, zero_nonce);
 	aead_request_set_callback(aead_req, CRYPTO_TFM_REQ_MAY_SLEEP, NULL, NULL);
 	aead_request_set_ad(aead_req, 0);
@@ -131,6 +136,7 @@ static int big_key_crypt(enum big_key_op op, struct big_key_buf *buf, size_t dat
 error:
 	mutex_unlock(&big_key_aead_lock);
 	aead_request_free(aead_req);
+	kzfree(zero_nonce);
 	return ret;
 }
 
-- 
2.15.1

[PATCH 2/2] dh key: get rid of stack array allocation

[Tycho Andersen]

Similarly to the previous patch, we would like to get rid of stack
allocated arrays: https://lkml.org/lkml/2018/3/7/621

In this case, we can also use a malloc style approach to free the temporary
buffer, being careful to also use kzfree to free them (indeed, at least one
of these has a memzero_explicit, but it seems like maybe they both
should?).

Signed-off-by: Tycho Andersen <tycho@tycho.ws>
CC: David Howells <dhowells@redhat.com>
CC: James Morris <jmorris@namei.org>
CC: "Serge E. Hallyn" <serge@hallyn.com>
---
 security/keys/dh.c | 27 +++++++++++++++++++++------
 1 file changed, 21 insertions(+), 6 deletions(-)

diff --git a/security/keys/dh.c b/security/keys/dh.c
index d1ea9f325f94..f02261b24759 100644
--- a/security/keys/dh.c
+++ b/security/keys/dh.c
@@ -162,19 +162,27 @@ static int kdf_ctr(struct kdf_sdesc *sdesc, const u8 *src, unsigned int slen,
 			goto err;
 
 		if (zlen && h) {
-			u8 tmpbuffer[h];
+			u8 *tmpbuffer;
 			size_t chunk = min_t(size_t, zlen, h);
-			memset(tmpbuffer, 0, chunk);
+
+			err = -ENOMEM;
+			tmpbuffer = kzalloc(chunk, GFP_KERNEL);
+			if (!tmpbuffer)
+				goto err;
 
 			do {
 				err = crypto_shash_update(desc, tmpbuffer,
 							  chunk);
-				if (err)
+				if (err) {
+					kzfree(tmpbuffer);
 					goto err;
+				}
 
 				zlen -= chunk;
 				chunk = min_t(size_t, zlen, h);
 			} while (zlen);
+
+			kzfree(tmpbuffer);
 		}
 
 		if (src && slen) {
@@ -184,13 +192,20 @@ static int kdf_ctr(struct kdf_sdesc *sdesc, const u8 *src, unsigned int slen,
 		}
 
 		if (dlen < h) {
-			u8 tmpbuffer[h];
+			u8 *tmpbuffer;
+
+			err = -ENOMEM;
+			tmpbuffer = kzalloc(h, GFP_KERNEL);
+			if (!tmpbuffer)
+				goto err;
 
 			err = crypto_shash_final(desc, tmpbuffer);
-			if (err)
+			if (err) {
+				kzfree(tmpbuffer);
 				goto err;
+			}
 			memcpy(dst, tmpbuffer, dlen);
-			memzero_explicit(tmpbuffer, h);
+			kzfree(tmpbuffer);
 			return 0;
 		} else {
 			err = crypto_shash_final(desc, dst);
-- 
2.15.1

Re: [PATCH 2/2] dh key: get rid of stack array allocation

[Serge E. Hallyn]

Quoting Tycho Andersen (tycho@tycho.ws):
> Similarly to the previous patch, we would like to get rid of stack
> allocated arrays: https://lkml.org/lkml/2018/3/7/621
> 
> In this case, we can also use a malloc style approach to free the temporary
> buffer, being careful to also use kzfree to free them (indeed, at least one
> of these has a memzero_explicit, but it seems like maybe they both
> should?).
> 
> Signed-off-by: Tycho Andersen <tycho@tycho.ws>
> CC: David Howells <dhowells@redhat.com>
> CC: James Morris <jmorris@namei.org>
> CC: "Serge E. Hallyn" <serge@hallyn.com>

Acked-by: Serge Hallyn <serge@hallyn.com>

for both, thanks.

> ---
>  security/keys/dh.c | 27 +++++++++++++++++++++------
>  1 file changed, 21 insertions(+), 6 deletions(-)
> 
> diff --git a/security/keys/dh.c b/security/keys/dh.c
> index d1ea9f325f94..f02261b24759 100644
> --- a/security/keys/dh.c
> +++ b/security/keys/dh.c
> @@ -162,19 +162,27 @@ static int kdf_ctr(struct kdf_sdesc *sdesc, const u8 *src, unsigned int slen,
>  			goto err;
>  
>  		if (zlen && h) {
> -			u8 tmpbuffer[h];
> +			u8 *tmpbuffer;
>  			size_t chunk = min_t(size_t, zlen, h);
> -			memset(tmpbuffer, 0, chunk);
> +
> +			err = -ENOMEM;
> +			tmpbuffer = kzalloc(chunk, GFP_KERNEL);
> +			if (!tmpbuffer)
> +				goto err;
>  
>  			do {
>  				err = crypto_shash_update(desc, tmpbuffer,
>  							  chunk);
> -				if (err)
> +				if (err) {
> +					kzfree(tmpbuffer);
>  					goto err;
> +				}
>  
>  				zlen -= chunk;
>  				chunk = min_t(size_t, zlen, h);
>  			} while (zlen);
> +
> +			kzfree(tmpbuffer);
>  		}
>  
>  		if (src && slen) {
> @@ -184,13 +192,20 @@ static int kdf_ctr(struct kdf_sdesc *sdesc, const u8 *src, unsigned int slen,
>  		}
>  
>  		if (dlen < h) {
> -			u8 tmpbuffer[h];
> +			u8 *tmpbuffer;
> +
> +			err = -ENOMEM;
> +			tmpbuffer = kzalloc(h, GFP_KERNEL);
> +			if (!tmpbuffer)
> +				goto err;
>  
>  			err = crypto_shash_final(desc, tmpbuffer);
> -			if (err)
> +			if (err) {
> +				kzfree(tmpbuffer);
>  				goto err;
> +			}
>  			memcpy(dst, tmpbuffer, dlen);
> -			memzero_explicit(tmpbuffer, h);
> +			kzfree(tmpbuffer);
>  			return 0;
>  		} else {
>  			err = crypto_shash_final(desc, dst);
> -- 
> 2.15.1

Re: [PATCH 1/2] big key: get rid of stack array allocation

[Eric Biggers]

On Mon, Mar 12, 2018 at 10:29:06PM -0600, Tycho Andersen wrote:
> We're interested in getting rid of all of the stack allocated arrays in the
> kernel [1]. This patch removes one in keys by switching to malloc/free.
> Note that we use kzalloc, to avoid leaking the nonce. I'm not sure this is
> really necessary, but extra paranoia seems prudent.
> 
> Manually tested using the program from the add_key man page to trigger
> big_key.
> 
> [1]: https://lkml.org/lkml/2018/3/7/621
> 
> Signed-off-by: Tycho Andersen <tycho@tycho.ws>
> CC: David Howells <dhowells@redhat.com>
> CC: James Morris <jmorris@namei.org>
> CC: "Serge E. Hallyn" <serge@hallyn.com>
> CC: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
>  security/keys/big_key.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/security/keys/big_key.c b/security/keys/big_key.c
> index fa728f662a6f..70f9f785c59d 100644
> --- a/security/keys/big_key.c
> +++ b/security/keys/big_key.c
> @@ -108,13 +108,18 @@ static int big_key_crypt(enum big_key_op op, struct big_key_buf *buf, size_t dat
>  	 * an .update function, so there's no chance we'll wind up reusing the
>  	 * key to encrypt updated data. Simply put: one key, one encryption.
>  	 */
> -	u8 zero_nonce[crypto_aead_ivsize(big_key_aead)];
> +	u8 *zero_nonce;
> +
> +	zero_nonce = kzalloc(crypto_aead_ivsize(big_key_aead), GFP_KERNEL);
> +	if (!zero_nonce)
> +		return -ENOMEM;
>  
>  	aead_req = aead_request_alloc(big_key_aead, GFP_KERNEL);
> -	if (!aead_req)
> +	if (!aead_req) {
> +		kfree(zero_nonce);
>  		return -ENOMEM;
> +	}
>  
> -	memset(zero_nonce, 0, sizeof(zero_nonce));
>  	aead_request_set_crypt(aead_req, buf->sg, buf->sg, datalen, zero_nonce);
>  	aead_request_set_callback(aead_req, CRYPTO_TFM_REQ_MAY_SLEEP, NULL, NULL);
>  	aead_request_set_ad(aead_req, 0);
> @@ -131,6 +136,7 @@ static int big_key_crypt(enum big_key_op op, struct big_key_buf *buf, size_t dat
>  error:
>  	mutex_unlock(&big_key_aead_lock);
>  	aead_request_free(aead_req);
> +	kzfree(zero_nonce);
>  	return ret;

A dynamic allocation here doesn't make sense -- the algorithm is hard-coded to
AES-GCM, so the IV size is fixed.  You should just include <crypto/gcm.h> and
use GCM_AES_IV_LEN.  As a sanity check you can add
'BUG_ON(crypto_aead_ivsize(big_key_aead) != GCM_AES_IV_LEN' to big_key_init().

kzfree() also doesn't make sense since the nonce is not secret information.

Thanks,

Eric

Re: [PATCH 2/2] dh key: get rid of stack array allocation

[Eric Biggers]

On Mon, Mar 12, 2018 at 10:29:07PM -0600, Tycho Andersen wrote:
> Similarly to the previous patch, we would like to get rid of stack
> allocated arrays: https://lkml.org/lkml/2018/3/7/621
> 
> In this case, we can also use a malloc style approach to free the temporary
> buffer, being careful to also use kzfree to free them (indeed, at least one
> of these has a memzero_explicit, but it seems like maybe they both
> should?).
> 
> Signed-off-by: Tycho Andersen <tycho@tycho.ws>
> CC: David Howells <dhowells@redhat.com>
> CC: James Morris <jmorris@namei.org>
> CC: "Serge E. Hallyn" <serge@hallyn.com>
> ---
>  security/keys/dh.c | 27 +++++++++++++++++++++------
>  1 file changed, 21 insertions(+), 6 deletions(-)
> 
> diff --git a/security/keys/dh.c b/security/keys/dh.c
> index d1ea9f325f94..f02261b24759 100644
> --- a/security/keys/dh.c
> +++ b/security/keys/dh.c
> @@ -162,19 +162,27 @@ static int kdf_ctr(struct kdf_sdesc *sdesc, const u8 *src, unsigned int slen,
>  			goto err;
>  
>  		if (zlen && h) {
> -			u8 tmpbuffer[h];
> +			u8 *tmpbuffer;
>  			size_t chunk = min_t(size_t, zlen, h);
> -			memset(tmpbuffer, 0, chunk);
> +
> +			err = -ENOMEM;
> +			tmpbuffer = kzalloc(chunk, GFP_KERNEL);
> +			if (!tmpbuffer)
> +				goto err;
>  
>  			do {
>  				err = crypto_shash_update(desc, tmpbuffer,
>  							  chunk);
> -				if (err)
> +				if (err) {
> +					kzfree(tmpbuffer);
>  					goto err;
> +				}
>  
>  				zlen -= chunk;
>  				chunk = min_t(size_t, zlen, h);
>  			} while (zlen);
> +
> +			kzfree(tmpbuffer);
>  		}

This is just hashing zeroes.  Why not use the zeroes at the end of the 'src'
buffer which was allocated as 'outbuf' in __keyctl_dh_compute()?  It's already
the right size.  It might even simplify the code a bit since
crypto_shash_update() would no longer need to be in a loop.

>  
>  		if (src && slen) {
> @@ -184,13 +192,20 @@ static int kdf_ctr(struct kdf_sdesc *sdesc, const u8 *src, unsigned int slen,
>  		}
>  
>  		if (dlen < h) {
> -			u8 tmpbuffer[h];
> +			u8 *tmpbuffer;
> +
> +			err = -ENOMEM;
> +			tmpbuffer = kzalloc(h, GFP_KERNEL);
> +			if (!tmpbuffer)
> +				goto err;
>  
>  			err = crypto_shash_final(desc, tmpbuffer);
> -			if (err)
> +			if (err) {
> +				kzfree(tmpbuffer);
>  				goto err;
> +			}
>  			memcpy(dst, tmpbuffer, dlen);
> -			memzero_explicit(tmpbuffer, h);
> +			kzfree(tmpbuffer);
>  			return 0;
>  		} else {
>  			err = crypto_shash_final(desc, dst);
> -- 

Why not instead round the allocated size of 'outbuf' in keyctl_dh_compute_kdf()
up to the next 'crypto_shash_digestsize()'-boundary?  Then this temporary buffer
wouldn't be needed at all.

It would be nice if people thought about how to properly solve the problems when
doing these VLA conversions, rather than mindlessly replacing them with
kmalloc...

Eric

Re: [PATCH 2/2] dh key: get rid of stack array allocation

[Tycho Andersen]

Hi Eric,

On Wed, Mar 14, 2018 at 07:21:12PM -0700, Eric Biggers wrote:
> On Mon, Mar 12, 2018 at 10:29:07PM -0600, Tycho Andersen wrote:
> > Similarly to the previous patch, we would like to get rid of stack
> > allocated arrays: https://lkml.org/lkml/2018/3/7/621
> > 
> > In this case, we can also use a malloc style approach to free the temporary
> > buffer, being careful to also use kzfree to free them (indeed, at least one
> > of these has a memzero_explicit, but it seems like maybe they both
> > should?).
> > 
> > Signed-off-by: Tycho Andersen <tycho@tycho.ws>
> > CC: David Howells <dhowells@redhat.com>
> > CC: James Morris <jmorris@namei.org>
> > CC: "Serge E. Hallyn" <serge@hallyn.com>
> > ---
> >  security/keys/dh.c | 27 +++++++++++++++++++++------
> >  1 file changed, 21 insertions(+), 6 deletions(-)
> > 
> > diff --git a/security/keys/dh.c b/security/keys/dh.c
> > index d1ea9f325f94..f02261b24759 100644
> > --- a/security/keys/dh.c
> > +++ b/security/keys/dh.c
> > @@ -162,19 +162,27 @@ static int kdf_ctr(struct kdf_sdesc *sdesc, const u8 *src, unsigned int slen,
> >  			goto err;
> >  
> >  		if (zlen && h) {
> > -			u8 tmpbuffer[h];
> > +			u8 *tmpbuffer;
> >  			size_t chunk = min_t(size_t, zlen, h);
> > -			memset(tmpbuffer, 0, chunk);
> > +
> > +			err = -ENOMEM;
> > +			tmpbuffer = kzalloc(chunk, GFP_KERNEL);
> > +			if (!tmpbuffer)
> > +				goto err;
> >  
> >  			do {
> >  				err = crypto_shash_update(desc, tmpbuffer,
> >  							  chunk);
> > -				if (err)
> > +				if (err) {
> > +					kzfree(tmpbuffer);
> >  					goto err;
> > +				}
> >  
> >  				zlen -= chunk;
> >  				chunk = min_t(size_t, zlen, h);
> >  			} while (zlen);
> > +
> > +			kzfree(tmpbuffer);
> >  		}
> 
> This is just hashing zeroes.  Why not use the zeroes at the end of the 'src'
> buffer which was allocated as 'outbuf' in __keyctl_dh_compute()?  It's already
> the right size.  It might even simplify the code a bit since
> crypto_shash_update() would no longer need to be in a loop.

Can you clarify what you mean by the "end" here? It looks like the end
is copied over with the user string just before it's passed into
keyctl_dh_compute_kdf().

In any case, I agree that it's dumb to do this allocation in a loop
now. What if instead we just do one big long allocation of zlen, hash
it, and then free it? This has the advantage that it's not allocated
two functions away from where it's used...

> >  
> >  		if (src && slen) {
> > @@ -184,13 +192,20 @@ static int kdf_ctr(struct kdf_sdesc *sdesc, const u8 *src, unsigned int slen,
> >  		}
> >  
> >  		if (dlen < h) {
> > -			u8 tmpbuffer[h];
> > +			u8 *tmpbuffer;
> > +
> > +			err = -ENOMEM;
> > +			tmpbuffer = kzalloc(h, GFP_KERNEL);
> > +			if (!tmpbuffer)
> > +				goto err;
> >  
> >  			err = crypto_shash_final(desc, tmpbuffer);
> > -			if (err)
> > +			if (err) {
> > +				kzfree(tmpbuffer);
> >  				goto err;
> > +			}
> >  			memcpy(dst, tmpbuffer, dlen);
> > -			memzero_explicit(tmpbuffer, h);
> > +			kzfree(tmpbuffer);
> >  			return 0;
> >  		} else {
> >  			err = crypto_shash_final(desc, dst);
> > -- 
> 
> Why not instead round the allocated size of 'outbuf' in keyctl_dh_compute_kdf()
> up to the next 'crypto_shash_digestsize()'-boundary?  Then this temporary buffer
> wouldn't be needed at all.

Thanks, I've made this change (and split it out into a separate patch)
for v2.

Tycho

Re: [PATCH 1/2] big key: get rid of stack array allocation

[Tycho Andersen]

Hi Eric,

On Wed, Mar 14, 2018 at 06:51:39PM -0700, Eric Biggers wrote:
> On Mon, Mar 12, 2018 at 10:29:06PM -0600, Tycho Andersen wrote:
> > We're interested in getting rid of all of the stack allocated arrays in the
> > kernel [1]. This patch removes one in keys by switching to malloc/free.
> > Note that we use kzalloc, to avoid leaking the nonce. I'm not sure this is
> > really necessary, but extra paranoia seems prudent.
> > 
> > Manually tested using the program from the add_key man page to trigger
> > big_key.
> > 
> > [1]: https://lkml.org/lkml/2018/3/7/621
> > 
> > Signed-off-by: Tycho Andersen <tycho@tycho.ws>
> > CC: David Howells <dhowells@redhat.com>
> > CC: James Morris <jmorris@namei.org>
> > CC: "Serge E. Hallyn" <serge@hallyn.com>
> > CC: Jason A. Donenfeld <Jason@zx2c4.com>
> > ---
> >  security/keys/big_key.c | 12 +++++++++---
> >  1 file changed, 9 insertions(+), 3 deletions(-)
> > 
> > diff --git a/security/keys/big_key.c b/security/keys/big_key.c
> > index fa728f662a6f..70f9f785c59d 100644
> > --- a/security/keys/big_key.c
> > +++ b/security/keys/big_key.c
> > @@ -108,13 +108,18 @@ static int big_key_crypt(enum big_key_op op, struct big_key_buf *buf, size_t dat
> >  	 * an .update function, so there's no chance we'll wind up reusing the
> >  	 * key to encrypt updated data. Simply put: one key, one encryption.
> >  	 */
> > -	u8 zero_nonce[crypto_aead_ivsize(big_key_aead)];
> > +	u8 *zero_nonce;
> > +
> > +	zero_nonce = kzalloc(crypto_aead_ivsize(big_key_aead), GFP_KERNEL);
> > +	if (!zero_nonce)
> > +		return -ENOMEM;
> >  
> >  	aead_req = aead_request_alloc(big_key_aead, GFP_KERNEL);
> > -	if (!aead_req)
> > +	if (!aead_req) {
> > +		kfree(zero_nonce);
> >  		return -ENOMEM;
> > +	}
> >  
> > -	memset(zero_nonce, 0, sizeof(zero_nonce));
> >  	aead_request_set_crypt(aead_req, buf->sg, buf->sg, datalen, zero_nonce);
> >  	aead_request_set_callback(aead_req, CRYPTO_TFM_REQ_MAY_SLEEP, NULL, NULL);
> >  	aead_request_set_ad(aead_req, 0);
> > @@ -131,6 +136,7 @@ static int big_key_crypt(enum big_key_op op, struct big_key_buf *buf, size_t dat
> >  error:
> >  	mutex_unlock(&big_key_aead_lock);
> >  	aead_request_free(aead_req);
> > +	kzfree(zero_nonce);
> >  	return ret;
> 
> A dynamic allocation here doesn't make sense -- the algorithm is hard-coded to
> AES-GCM, so the IV size is fixed.  You should just include <crypto/gcm.h> and
> use GCM_AES_IV_LEN.  As a sanity check you can add
> 'BUG_ON(crypto_aead_ivsize(big_key_aead) != GCM_AES_IV_LEN' to big_key_init().
> 
> kzfree() also doesn't make sense since the nonce is not secret information.

Thanks, I've fixed this for v2.

Cheers,

Tycho