Re: [PATCH] tpm: add buffer access validation in tpm2_get_pcr_allocation()

[Jarkko Sakkinen]

On Sun, Jan 29, 2017 at 10:48:39PM +0530, Nayna wrote:
> 
> 
> On 01/29/2017 08:10 PM, Jarkko Sakkinen wrote:
> > On Fri, Jan 27, 2017 at 10:25:49AM -0500, Nayna Jain wrote:
> > > This patch add validation in tpm2_get_pcr_allocation to avoid
> > > access beyond response buffer length.
> > > 
> > > Suggested-by: Stefan Berger <stefanb@xxxxxxxxxxxxxxxxxx>
> > > Signed-off-by: Nayna Jain <nayna@xxxxxxxxxxxxxxxxxx>
> > 
> > This validation looks broken to me.
> > 
> > > ---
> > >   drivers/char/tpm/tpm2-cmd.c | 28 +++++++++++++++++++++++-----
> > >   1 file changed, 23 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
> > > index 4aad84c..02c1ea7 100644
> > > --- a/drivers/char/tpm/tpm2-cmd.c
> > > +++ b/drivers/char/tpm/tpm2-cmd.c
> > > @@ -1008,9 +1008,13 @@ static ssize_t tpm2_get_pcr_allocation(struct tpm_chip *chip)
> > >   	struct tpm2_pcr_selection pcr_selection;
> > >   	struct tpm_buf buf;
> > >   	void *marker;
> > > -	unsigned int count = 0;
> > > +	void *end;
> > > +	void *pcr_select_offset;
> > > +	unsigned int count;
> > > +	u32 sizeof_pcr_selection;
> > > +	u32 resp_len;
> > 
> > Very cosmetic but we almos almost universally use the acronym 'rsp' in
> > the TPM driver.
> 
> Sure will update.
> 
> > 
> > >   	int rc;
> > > -	int i;
> > > +	int i = 0;
> > 
> > Why do you need to initialize it?
> 
> Because in out: count is replaced with i.
> And it is replaced because  now for loop can break even before reaching
> count, because of new buffer checks.
> > 
> > > 
> > >   	rc = tpm_buf_init(&buf, TPM2_ST_NO_SESSIONS, TPM2_CC_GET_CAPABILITY);
> > >   	if (rc)
> > > @@ -1034,15 +1038,29 @@ static ssize_t tpm2_get_pcr_allocation(struct tpm_chip *chip)
> > >   	}
> > > 
> > >   	marker = &buf.data[TPM_HEADER_SIZE + 9];
> > > +
> > > +	resp_len = be32_to_cpup((__be32 *)&buf.data[2]);
> > > +	end = &buf.data[resp_len];
> > 
> > What if the response contains larger length than the buffer size?
> 
> Isn't this check need to be done in tpm_transmit_cmd for all responses ?
> Though, it seems it is not done there as well.
> 
> And to understand what do we expect max buffer length. PAGE_SIZE or
> TPM_BUFSIZE ?

Oops. You are correct it is done there:

if (len != be32_to_cpu(header->length))
	return -EFAULT;

So need to do this.

/Jarkko

/Jarkko