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

Jarkko Sakkinen
2017-01-29T21:20:47Z

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@linux.vnet.ibm.com>
> > > Signed-off-by: Nayna Jain <nayna@linux.vnet.ibm.com>
> >
> > 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