[PATCH] ima: avoid appraise error for hash calc interrupt

[PATCH] ima: avoid appraise error for hash calc interrupt

[Patrick Callaghan]

The integrity_kernel_read() call in ima_calc_file_hash_tfm() can return
a value of 0 before all bytes of the file are read. A value of 0 would
normally indicate an EOF. This has been observed if a user process is
causing a file appraisal and is terminated with a SIGTERM signal. The
most common occurrence of seeing the problem is if a shutdown or systemd
reload is initiated while files are being appraised.

The problem is similar to commit <f5e1040196db> (ima: always return
negative code for error) that fixed the problem in
ima_calc_file_hash_atfm().

Suggested-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Patrick Callaghan <patrickc@linux.ibm.com>
---
 security/integrity/ima/ima_crypto.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/security/integrity/ima/ima_crypto.c b/security/integrity/ima/ima_crypto.c
index 73044fc..7967a69 100644
--- a/security/integrity/ima/ima_crypto.c
+++ b/security/integrity/ima/ima_crypto.c
@@ -362,8 +362,10 @@ static int ima_calc_file_hash_tfm(struct file *file,
 			rc = rbuf_len;
 			break;
 		}
-		if (rbuf_len == 0)
+		if (rbuf_len == 0) {	/* unexpected EOF */
+			rc = -EINVAL;
 			break;
+		}
 		offset += rbuf_len;
 
 		rc = crypto_shash_update(shash, rbuf, rbuf_len);
-- 
2.8.3

Re: [PATCH] ima: avoid appraise error for hash calc interrupt

[Lakshmi Ramasubramanian]

On 11/11/19 11:23 AM, Patrick Callaghan wrote:

> -		if (rbuf_len == 0)
> +		if (rbuf_len == 0) {	/* unexpected EOF */
> +			rc = -EINVAL;
>   			break;
> +		}
>   		offset += rbuf_len;

Should there be an additional check to validate that (offset + rbuf_len) 
is less than i_size before calling cypto_shash_update (since rbuf_len is 
one of the parameters for this call)?

                if ((rbuf_len == 0) || (offset + rbuf_len >= i_size)) {
                         rc = -EINVAL;
                         break;
                }
                offset += rbuf_len;

>   	       rc = crypto_shash_update(shash, rbuf, rbuf_len);

  -lakshmi

Re: [PATCH] ima: avoid appraise error for hash calc interrupt

[Mimi Zohar]

On Mon, 2019-11-11 at 14:29 -0800, Lakshmi Ramasubramanian wrote:
> On 11/11/19 11:23 AM, Patrick Callaghan wrote:
> 
> > -		if (rbuf_len == 0)
> > +		if (rbuf_len == 0) {	/* unexpected EOF */
> > +			rc = -EINVAL;
> >   			break;
> > +		}
> >   		offset += rbuf_len;
> 
> Should there be an additional check to validate that (offset + rbuf_len) 
> is less than i_size before calling cypto_shash_update (since rbuf_len is 
> one of the parameters for this call)?

The "while" statement enforces that.

Mimi

> 
>                 if ((rbuf_len == 0) || (offset + rbuf_len >= i_size)) {
>                          rc = -EINVAL;
>                          break;
>                 }
>                 offset += rbuf_len;
> 
> >   	       rc = crypto_shash_update(shash, rbuf, rbuf_len);
> 
>   -lakshmi
> 

Re: [PATCH] ima: avoid appraise error for hash calc interrupt

[Lakshmi Ramasubramanian]

On 11/12/2019 9:14 AM, Mimi Zohar wrote:

> On Mon, 2019-11-11 at 14:29 -0800, Lakshmi Ramasubramanian wrote:
>> On 11/11/19 11:23 AM, Patrick Callaghan wrote:
>>
>>> -		if (rbuf_len == 0)
>>> +		if (rbuf_len == 0) {	/* unexpected EOF */
>>> +			rc = -EINVAL;
>>>    			break;
>>> +		}
>>>    		offset += rbuf_len;
>>
>> Should there be an additional check to validate that (offset + rbuf_len)
>> is less than i_size before calling cypto_shash_update (since rbuf_len is
>> one of the parameters for this call)?
> 
> The "while" statement enforces that.
> 
> Mimi

Yes - but that check happens after the call to crypto_shash_update().

Perhaps integrity_kernel_read() will never return (rbuf_len) that will
  => violate the check in the "while" statement.
  => number of bytes read that is greater than the memory allocated for 
rbuf even in error conditions.

Just making sure.

thanks,
  -lakshmi

> 
>>
>>                  if ((rbuf_len == 0) || (offset + rbuf_len >= i_size)) {
>>                           rc = -EINVAL;
>>                           break;
>>                  }
>>                  offset += rbuf_len;
>>
>>>    	       rc = crypto_shash_update(shash, rbuf, rbuf_len);

Re: [PATCH] ima: avoid appraise error for hash calc interrupt

[Mimi Zohar]

On Tue, 2019-11-12 at 09:33 -0800, Lakshmi Ramasubramanian wrote:
> On 11/12/2019 9:14 AM, Mimi Zohar wrote:
> 
> > On Mon, 2019-11-11 at 14:29 -0800, Lakshmi Ramasubramanian wrote:
> >> On 11/11/19 11:23 AM, Patrick Callaghan wrote:
> >>
> >>> -		if (rbuf_len == 0)
> >>> +		if (rbuf_len == 0) {	/* unexpected EOF */
> >>> +			rc = -EINVAL;
> >>>    			break;
> >>> +		}
> >>>    		offset += rbuf_len;
> >>
> >> Should there be an additional check to validate that (offset + rbuf_len)
> >> is less than i_size before calling cypto_shash_update (since rbuf_len is
> >> one of the parameters for this call)?
> > 
> > The "while" statement enforces that.
> > 
> > Mimi
> 
> Yes - but that check happens after the call to crypto_shash_update().
> 
> Perhaps integrity_kernel_read() will never return (rbuf_len) that will
>   => violate the check in the "while" statement.
>   => number of bytes read that is greater than the memory allocated for 
> rbuf even in error conditions.
> 
> Just making sure.

integrity_kernel_read() returns an error (< 0) or the number of bytes
read.  The while statement ensures that there is more data to read, so
returning 0 is always an error.

Mimi

Re: [PATCH] ima: avoid appraise error for hash calc interrupt

[Patrick Callaghan]

On Tue, 2019-11-12 at 13:12 -0500, Mimi Zohar wrote:
> On Tue, 2019-11-12 at 09:33 -0800, Lakshmi Ramasubramanian wrote:
> > On 11/12/2019 9:14 AM, Mimi Zohar wrote:
> > 
> > > On Mon, 2019-11-11 at 14:29 -0800, Lakshmi Ramasubramanian wrote:
> > > > On 11/11/19 11:23 AM, Patrick Callaghan wrote:
> > > > 
> > > > > -		if (rbuf_len == 0)
> > > > > +		if (rbuf_len == 0) {	/* unexpected EOF */
> > > > > +			rc = -EINVAL;
> > > > >    			break;
> > > > > +		}
> > > > >    		offset += rbuf_len;
> > > > 
> > > > Should there be an additional check to validate that (offset +
> > > > rbuf_len)
> > > > is less than i_size before calling cypto_shash_update (since
> > > > rbuf_len is
> > > > one of the parameters for this call)?
> > > 
> > > The "while" statement enforces that.
> > > 
> > > Mimi
> > 
> > Yes - but that check happens after the call to
> > crypto_shash_update().
> > 
> > Perhaps integrity_kernel_read() will never return (rbuf_len) that
> > will
> >   => violate the check in the "while" statement.
> >   => number of bytes read that is greater than the memory allocated
> > for 
> > rbuf even in error conditions.
> > 
> > Just making sure.
> 
> integrity_kernel_read() returns an error (< 0) or the number of bytes
> read.  The while statement ensures that there is more data to read,
> so
> returning 0 is always an error.
> 
> Mimi
Hello Laks,
You suggested that the if statement of the patch change to the
following:

if ((rbuf_len == 0) || (offset + rbuf_len >= i_size)) {

Unless the file size changed between the time that i_size was set in
ima_calc_file_hash_tfm() and an i_size_read() call was subsequently
issued in a function downstream of the integrity_kernel_read() call,
the rbuf_len returned on the integrity_kernel_read() call will not be
more than i_size - offset. I do not think that it is possible for the
file size to change during this window but nonetheless, if it can, this
would be a different problem and I would not want to include this in my
patch. That said, I do appreciate you taking time to review this patch.

Re: [PATCH] ima: avoid appraise error for hash calc interrupt

[Lakshmi Ramasubramanian]

On 11/14/19 5:55 AM, Patrick Callaghan wrote:

Hi Patrick,

> Hello Laks,
> You suggested that the if statement of the patch change to the
> following:
> 
> if ((rbuf_len == 0) || (offset + rbuf_len >= i_size)) {
> 
> Unless the file size changed between the time that i_size was set in
> ima_calc_file_hash_tfm() and an i_size_read() call was subsequently
> issued in a function downstream of the integrity_kernel_read() call,
> the rbuf_len returned on the integrity_kernel_read() call will not be
> more than i_size - offset. I do not think that it is possible for the
> file size to change during this window but nonetheless, if it can, this
> would be a different problem and I would not want to include this in my
> patch. That said, I do appreciate you taking time to review this patch.

You are right - unless the file size changes between the calls this 
problem would not occur. I agree - that issue, even if it can occur, 
should be addressed separately.

Another one (again - am not saying this needs to be addressed in this 
patch, but just wanted to point out)

	rbuf = kzalloc(PAGE_SIZE, GFP_KERNEL);
	...
	rbuf_len = integrity_kernel_read(file, offset, rbuf, PAGE_SIZE);
	...
	rc = crypto_shash_update(shash, rbuf, rbuf_len);

rbuf is of size PAGE_SIZE, but rbuf_len, returned by 
integrity_kernel_read() is passed as buffer size to 
crypto_shash_update() without any validation (rbuf_len <= PAGE_SIZE)

It is assumed here that integrity_kernel_read() would not return a 
length greater than rbuf size and hence crypto_shash_update() would 
never access beyond the given buffer.

thanks,
  -lakshmi

Re: [PATCH] ima: avoid appraise error for hash calc interrupt

[Patrick Callaghan]

On Thu, 2019-11-14 at 10:45 -0800, Lakshmi Ramasubramanian wrote:
> On 11/14/19 5:55 AM, Patrick Callaghan wrote:
> 
> Hi Patrick,
> 
> > Hello Laks,
> > You suggested that the if statement of the patch change to the
> > following:
> > 
> > if ((rbuf_len == 0) || (offset + rbuf_len >= i_size)) {
> > 
> > Unless the file size changed between the time that i_size was set
> > in
> > ima_calc_file_hash_tfm() and an i_size_read() call was subsequently
> > issued in a function downstream of the integrity_kernel_read()
> > call,
> > the rbuf_len returned on the integrity_kernel_read() call will not
> > be
> > more than i_size - offset. I do not think that it is possible for
> > the
> > file size to change during this window but nonetheless, if it can,
> > this
> > would be a different problem and I would not want to include this
> > in my
> > patch. That said, I do appreciate you taking time to review this
> > patch.
> 
> You are right - unless the file size changes between the calls this 
> problem would not occur. I agree - that issue, even if it can occur, 
> should be addressed separately.
> 
> Another one (again - am not saying this needs to be addressed in
> this 
> patch, but just wanted to point out)
> 
> 	rbuf = kzalloc(PAGE_SIZE, GFP_KERNEL);
> 	...
> 	rbuf_len = integrity_kernel_read(file, offset, rbuf,
> PAGE_SIZE);
> 	...
> 	rc = crypto_shash_update(shash, rbuf, rbuf_len);
> 
> rbuf is of size PAGE_SIZE, but rbuf_len, returned by 
> integrity_kernel_read() is passed as buffer size to 
> crypto_shash_update() without any validation (rbuf_len <= PAGE_SIZE)
> 
> It is assumed here that integrity_kernel_read() would not return a 
> length greater than rbuf size and hence crypto_shash_update() would 
> never access beyond the given buffer.
> 
> thanks,
>   -lakshmi
> 
> 
Hello Laks,
Agreed. The assumption is that integrity_kernel_read() function does
not return a value greater than the fourth parameter passed to it (i.e.
does not read more bytes from the file than the size of the buffer
passed to it). I tried to validate that this assumption was true by
following the code but felt I could not prove it with my current
knowledge of the code. If this assumption is not true then I believe
that any code change for this problem should go into a different
patch. 

Thank you.

Re: [PATCH] ima: avoid appraise error for hash calc interrupt

[Lakshmi Ramasubramanian]

On 11/15/19 7:25 AM, Patrick Callaghan wrote:

> Hello Laks,
> Agreed. The assumption is that integrity_kernel_read() function does
> not return a value greater than the fourth parameter passed to it (i.e.
> does not read more bytes from the file than the size of the buffer
> passed to it). I tried to validate that this assumption was true by
> following the code but felt I could not prove it with my current
> knowledge of the code. If this assumption is not true then I believe
> that any code change for this problem should go into a different
> patch.

I agree Patrick - not a blocker for this patch set.

thanks,
  -lakshmi

Re: [PATCH] ima: avoid appraise error for hash calc interrupt

[Sascha Hauer]

On Mon, Nov 11, 2019 at 02:23:48PM -0500, Patrick Callaghan wrote:
> The integrity_kernel_read() call in ima_calc_file_hash_tfm() can return
> a value of 0 before all bytes of the file are read. A value of 0 would
> normally indicate an EOF. This has been observed if a user process is
> causing a file appraisal and is terminated with a SIGTERM signal. The
> most common occurrence of seeing the problem is if a shutdown or systemd
> reload is initiated while files are being appraised.
> 
> The problem is similar to commit <f5e1040196db> (ima: always return
> negative code for error) that fixed the problem in
> ima_calc_file_hash_atfm().
> 
> Suggested-by: Mimi Zohar <zohar@linux.ibm.com>
> Signed-off-by: Patrick Callaghan <patrickc@linux.ibm.com>
> ---
>  security/integrity/ima/ima_crypto.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/security/integrity/ima/ima_crypto.c b/security/integrity/ima/ima_crypto.c
> index 73044fc..7967a69 100644
> --- a/security/integrity/ima/ima_crypto.c
> +++ b/security/integrity/ima/ima_crypto.c
> @@ -362,8 +362,10 @@ static int ima_calc_file_hash_tfm(struct file *file,
>  			rc = rbuf_len;
>  			break;
>  		}
> -		if (rbuf_len == 0)
> +		if (rbuf_len == 0) {	/* unexpected EOF */
> +			rc = -EINVAL;
>  			break;
> +		}

There's no point in calling crypto_shash_final() on incomplete data, so
setting rc to an error to avoid that seems the right thing to do to me,
so:

Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>

Sascha

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