[PATCH] ima: dynamically allocate shash_desc

[PATCH] ima: dynamically allocate shash_desc

[Arnd Bergmann]

On 32-bit ARM, we get a warning about excessive stack usage when
building with clang.

security/integrity/ima/ima_crypto.c:504:5: error: stack frame size of 1152 bytes in function 'ima_calc_field_array_hash' [-Werror,-Wframe-larger-than=]

Using kmalloc to get the descriptor reduces this to 320 bytes.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 security/integrity/ima/ima_crypto.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/security/integrity/ima/ima_crypto.c b/security/integrity/ima/ima_crypto.c
index d4c7b8e1b083..8a66bab4c435 100644
--- a/security/integrity/ima/ima_crypto.c
+++ b/security/integrity/ima/ima_crypto.c
@@ -461,16 +461,21 @@ static int ima_calc_field_array_hash_tfm(struct ima_field_data *field_data,
 					 struct ima_digest_data *hash,
 					 struct crypto_shash *tfm)
 {
-	SHASH_DESC_ON_STACK(shash, tfm);
+	struct shash_desc *shash;
 	int rc, i;
 
+	shash = kmalloc(sizeof(struct shash_desc) + crypto_shash_descsize(tfm),
+			GFP_KERNEL);
+	if (!shash)
+		return -ENOMEM;
+
 	shash->tfm = tfm;
 
 	hash->length = crypto_shash_digestsize(tfm);
 
 	rc = crypto_shash_init(shash);
 	if (rc != 0)
-		return rc;
+		goto out;
 
 	for (i = 0; i < num_fields; i++) {
 		u8 buffer[IMA_EVENT_NAME_LEN_MAX + 1] = { 0 };
@@ -497,7 +502,8 @@ static int ima_calc_field_array_hash_tfm(struct ima_field_data *field_data,
 
 	if (!rc)
 		rc = crypto_shash_final(shash, hash->digest);
-
+out:
+	kfree(shash);
 	return rc;
 }
 
-- 
2.20.0

Re: [PATCH] ima: dynamically allocate shash_desc

[Mimi Zohar]

On Mon, 2019-06-17 at 13:20 +0200, Arnd Bergmann wrote:
> On 32-bit ARM, we get a warning about excessive stack usage when
> building with clang.
> 
> security/integrity/ima/ima_crypto.c:504:5: error: stack frame size
> of 1152 bytes in function 'ima_calc_field_array_hash' [-Werror,-
> Wframe-larger-than=]

I'm definitely not seeing this.  Is this problem a result of non
upstreamed patches?  For sha1, currently the only possible hash
algorithm, I'm seeing 664.

Mimi

> 
> Using kmalloc to get the descriptor reduces this to 320 bytes.
> 
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  security/integrity/ima/ima_crypto.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/security/integrity/ima/ima_crypto.c b/security/integrity/ima/ima_crypto.c
> index d4c7b8e1b083..8a66bab4c435 100644
> --- a/security/integrity/ima/ima_crypto.c
> +++ b/security/integrity/ima/ima_crypto.c
> @@ -461,16 +461,21 @@ static int ima_calc_field_array_hash_tfm(struct ima_field_data *field_data,
>  					 struct ima_digest_data *hash,
>  					 struct crypto_shash *tfm)
>  {
> -	SHASH_DESC_ON_STACK(shash, tfm);
> +	struct shash_desc *shash;
>  	int rc, i;
>  
> +	shash = kmalloc(sizeof(struct shash_desc) + crypto_shash_descsize(tfm),
> +			GFP_KERNEL);
> +	if (!shash)
> +		return -ENOMEM;
> +
>  	shash->tfm = tfm;
>  
>  	hash->length = crypto_shash_digestsize(tfm);
>  
>  	rc = crypto_shash_init(shash);
>  	if (rc != 0)
> -		return rc;
> +		goto out;
>  
>  	for (i = 0; i < num_fields; i++) {
>  		u8 buffer[IMA_EVENT_NAME_LEN_MAX + 1] = { 0 };
> @@ -497,7 +502,8 @@ static int ima_calc_field_array_hash_tfm(struct ima_field_data *field_data,
>  
>  	if (!rc)
>  		rc = crypto_shash_final(shash, hash->digest);
> -
> +out:
> +	kfree(shash);
>  	return rc;
>  }
>  

Re: [PATCH] ima: dynamically allocate shash_desc

[Mimi Zohar]

On Mon, 2019-06-17 at 11:55 -0400, Mimi Zohar wrote:
> On Mon, 2019-06-17 at 13:20 +0200, Arnd Bergmann wrote:
> > On 32-bit ARM, we get a warning about excessive stack usage when
> > building with clang.
> > 
> > security/integrity/ima/ima_crypto.c:504:5: error: stack frame size
> > of 1152 bytes in function 'ima_calc_field_array_hash' [-Werror,-
> > Wframe-larger-than=]
> 
> I'm definitely not seeing this.  Is this problem a result of non
> upstreamed patches?  For sha1, currently the only possible hash
> algorithm, I'm seeing 664.

Every time a measurement is added to the measurement list, the memory
would be allocated/freed.  The frequency of new measurements is policy
dependent.  For performance reasons, I'd prefer if the allocation
remains on the stack.

Mimi

Re: [PATCH] ima: dynamically allocate shash_desc

[Arnd Bergmann]

On Mon, Jun 17, 2019 at 8:08 PM Mimi Zohar <zohar@linux.ibm.com> wrote:
>
> On Mon, 2019-06-17 at 11:55 -0400, Mimi Zohar wrote:
> > On Mon, 2019-06-17 at 13:20 +0200, Arnd Bergmann wrote:
> > > On 32-bit ARM, we get a warning about excessive stack usage when
> > > building with clang.
> > >
> > > security/integrity/ima/ima_crypto.c:504:5: error: stack frame size
> > > of 1152 bytes in function 'ima_calc_field_array_hash' [-Werror,-
> > > Wframe-larger-than=]
> >
> > I'm definitely not seeing this.  Is this problem a result of non
> > upstreamed patches?  For sha1, currently the only possible hash
> > algorithm, I'm seeing 664.

You won't see it with gcc, only with clang in some randconfig builds,
I suppose only when KASAN is enabled.

> Every time a measurement is added to the measurement list, the memory
> would be allocated/freed.  The frequency of new measurements is policy
> dependent.  For performance reasons, I'd prefer if the allocation
> remains on the stack.

Is there a way to preallocate the shash_desc instead? That would
avoid the overhead.

        Arnd

Re: [PATCH] ima: dynamically allocate shash_desc

[Mimi Zohar]

On Mon, 2019-06-17 at 22:08 +0200, Arnd Bergmann wrote:
> On Mon, Jun 17, 2019 at 8:08 PM Mimi Zohar <zohar@linux.ibm.com> wrote:
> >
> > On Mon, 2019-06-17 at 11:55 -0400, Mimi Zohar wrote:
> > > On Mon, 2019-06-17 at 13:20 +0200, Arnd Bergmann wrote:
> > > > On 32-bit ARM, we get a warning about excessive stack usage when
> > > > building with clang.
> > > >
> > > > security/integrity/ima/ima_crypto.c:504:5: error: stack frame size
> > > > of 1152 bytes in function 'ima_calc_field_array_hash' [-Werror,-
> > > > Wframe-larger-than=]
> > >
> > > I'm definitely not seeing this.  Is this problem a result of non
> > > upstreamed patches?  For sha1, currently the only possible hash
> > > algorithm, I'm seeing 664.
> 
> You won't see it with gcc, only with clang in some randconfig builds,
> I suppose only when KASAN is enabled.
> 
> > Every time a measurement is added to the measurement list, the memory
> > would be allocated/freed.  The frequency of new measurements is policy
> > dependent.  For performance reasons, I'd prefer if the allocation
> > remains on the stack.
> 
> Is there a way to preallocate the shash_desc instead? That would
> avoid the overhead.

There are 3 other SHASH_DESC_ON_STACK definitions in just
ima_crypto.c, with a total of ~55 other places in the kernel.  Before
fixing this particular function, I'd like to know if the "excessive
stack usage" warning is limited to ima_calc_field_array_hash_tfm().
 If so, what is so special about its usage of SHASH_DESC_ON_STACK?

thanks,

Mimi

Re: [PATCH] ima: dynamically allocate shash_desc

[Arnd Bergmann]

On Tue, Jun 18, 2019 at 3:55 PM Mimi Zohar <zohar@linux.ibm.com> wrote:
>
> On Mon, 2019-06-17 at 22:08 +0200, Arnd Bergmann wrote:
> > On Mon, Jun 17, 2019 at 8:08 PM Mimi Zohar <zohar@linux.ibm.com> wrote:
> > >
> > > On Mon, 2019-06-17 at 11:55 -0400, Mimi Zohar wrote:
> > > > On Mon, 2019-06-17 at 13:20 +0200, Arnd Bergmann wrote:
> > > > > On 32-bit ARM, we get a warning about excessive stack usage when
> > > > > building with clang.
> > > > >
> > > > > security/integrity/ima/ima_crypto.c:504:5: error: stack frame size
> > > > > of 1152 bytes in function 'ima_calc_field_array_hash' [-Werror,-
> > > > > Wframe-larger-than=]
> > > >
> > > > I'm definitely not seeing this.  Is this problem a result of non
> > > > upstreamed patches?  For sha1, currently the only possible hash
> > > > algorithm, I'm seeing 664.
> >
> > You won't see it with gcc, only with clang in some randconfig builds,
> > I suppose only when KASAN is enabled.
> >
> > > Every time a measurement is added to the measurement list, the memory
> > > would be allocated/freed.  The frequency of new measurements is policy
> > > dependent.  For performance reasons, I'd prefer if the allocation
> > > remains on the stack.
> >
> > Is there a way to preallocate the shash_desc instead? That would
> > avoid the overhead.
>
> There are 3 other SHASH_DESC_ON_STACK definitions in just
> ima_crypto.c, with a total of ~55 other places in the kernel.  Before
> fixing this particular function, I'd like to know if the "excessive
> stack usage" warning is limited to ima_calc_field_array_hash_tfm().
>  If so, what is so special about its usage of SHASH_DESC_ON_STACK?

SHASH_DESC_ON_STACK() uses at least 512 bytes of stack
everywhere, which is half of the warning limit for a function on
32 bit kernels.

With KASAN, a small redzone is put around it so we can detect out
of bounds access to a variable that is passed by reference.
clang makes that buffer larger than gcc, so we end up with something
like 768 bytes for each instance of SHASH_DESC_ON_STACK().

Most other users still stay below the 1024 byte warning level though,
because typical functions only use a few bytes of stack space.
In case of ima_calc_field_array_hash_tfm(), the is also the buffer[]
array of 255 bytes that gets another large redzone.

I fixed up all the (randconfig) warnings I get for arm32, arm64 and
x86 kernels, and I think there were four to five that were because of
SHASH_DESC_ON_STACK(). It might make sense to convert all
three instances in ima to preallocate the descriptor if we do it for
one of them, even when it's not actually needed.

     Arnd

Re: [PATCH] ima: dynamically allocate shash_desc

[Mimi Zohar]

On Tue, 2019-06-18 at 20:06 +0200, Arnd Bergmann wrote:
> On Tue, Jun 18, 2019 at 3:55 PM Mimi Zohar <zohar@linux.ibm.com> wrote:
> >
> > On Mon, 2019-06-17 at 22:08 +0200, Arnd Bergmann wrote:
> > > On Mon, Jun 17, 2019 at 8:08 PM Mimi Zohar <zohar@linux.ibm.com> wrote:
> > > >
> > > > On Mon, 2019-06-17 at 11:55 -0400, Mimi Zohar wrote:
> > > > > On Mon, 2019-06-17 at 13:20 +0200, Arnd Bergmann wrote:
> > > > > > On 32-bit ARM, we get a warning about excessive stack usage when
> > > > > > building with clang.
> > > > > >
> > > > > > security/integrity/ima/ima_crypto.c:504:5: error: stack frame size
> > > > > > of 1152 bytes in function 'ima_calc_field_array_hash' [-Werror,-
> > > > > > Wframe-larger-than=]
> > > > >
> > > > > I'm definitely not seeing this.  Is this problem a result of non
> > > > > upstreamed patches?  For sha1, currently the only possible hash
> > > > > algorithm, I'm seeing 664.
> > >
> > > You won't see it with gcc, only with clang in some randconfig builds,
> > > I suppose only when KASAN is enabled.
> > >
> > > > Every time a measurement is added to the measurement list, the memory
> > > > would be allocated/freed.  The frequency of new measurements is policy
> > > > dependent.  For performance reasons, I'd prefer if the allocation
> > > > remains on the stack.
> > >
> > > Is there a way to preallocate the shash_desc instead? That would
> > > avoid the overhead.
> >
> > There are 3 other SHASH_DESC_ON_STACK definitions in just
> > ima_crypto.c, with a total of ~55 other places in the kernel.  Before
> > fixing this particular function, I'd like to know if the "excessive
> > stack usage" warning is limited to ima_calc_field_array_hash_tfm().
> >  If so, what is so special about its usage of SHASH_DESC_ON_STACK?
> 
> SHASH_DESC_ON_STACK() uses at least 512 bytes of stack
> everywhere, which is half of the warning limit for a function on
> 32 bit kernels.
> 
> With KASAN, a small redzone is put around it so we can detect out
> of bounds access to a variable that is passed by reference.
> clang makes that buffer larger than gcc, so we end up with something
> like 768 bytes for each instance of SHASH_DESC_ON_STACK().
> 
> Most other users still stay below the 1024 byte warning level though,
> because typical functions only use a few bytes of stack space.
> In case of ima_calc_field_array_hash_tfm(), the is also the buffer[]
> array of 255 bytes that gets another large redzone.
> 
> I fixed up all the (randconfig) warnings I get for arm32, arm64 and
> x86 kernels, and I think there were four to five that were because of
> SHASH_DESC_ON_STACK(). It might make sense to convert all
> three instances in ima to preallocate the descriptor if we do it for
> one of them, even when it's not actually needed.

"buffer" is only used for the original "ima" template format, which is
limited to sha1.  Rather than allocating shash, I would prefer
"buffer" be allocated, if needed, and only the first time.

Mimi