linux-kernel - Re: [PATCH v3 1/6] tpm: Move buffer handling from static inlines to real functions

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Date:   Thu, 26 Oct 2023 13:55:55 -0400
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Jerry Snitselaar <jsnitsel@...hat.com>,
        Jarkko Sakkinen <jarkko@...nel.org>
Cc:     linux-integrity@...r.kernel.org, keyrings@...r.kernel.org,
        William Roberts <bill.c.roberts@...il.com>,
        Stefan Berger <stefanb@...ux.ibm.com>,
        David Howells <dhowells@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Peter Huewe <peterhuewe@....de>,
        Mario Limonciello <mario.limonciello@....com>,
        Julien Gomes <julien@...sta.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/6] tpm: Move buffer handling from static inlines to
 real functions

On Thu, 2023-10-26 at 10:10 -0700, Jerry Snitselaar wrote:
> On Wed, Oct 25, 2023 at 08:35:55PM +0300, Jarkko Sakkinen wrote:
> > On Wed Oct 25, 2023 at 12:03 PM EEST, Jerry Snitselaar wrote:
> > > Reviewed-by: Jerry Snitselaar <jsnitsel@...hat.com>
> > 
> > On Wed, 2023-10-25 at 02:03 -0700, Jerry Snitselaar wrote:
> > > Reviewed-by: Jerry Snitselaar <jsnitsel@...hat.com>
> > > 
> > 
> > Thanks I'll add it to the next round.
> > 
> > For the tpm_buf_read(), I was thinking along the lines of:
> > 
> > /**
> >  * tpm_buf_read() - Read from a TPM buffer
> >  * @buf:        &tpm_buf instance
> >  * @pos:        position within the buffer
> >  * @count:      the number of bytes to read
> >  * @output:     the output buffer
> >  *
> >  * Read bytes from a TPM buffer, and update the position. Returns
> > false when the
> >  * amount of bytes requested would overflow the buffer, which is
> > expected to
> >  * only happen in the case of hardware failure.
> >  */
> > static bool tpm_buf_read(const struct tpm_buf *buf, off_t *pos,
> > size_t count, void *output)
> > {
> >         off_t next = *pos + count;
> > 
> >         if (next >= buf->length) {
> >                 pr_warn("%s: %lu >= %lu\n", __func__, next,
> > *offset);
> >                 return false;
> >         }
> > 
> >         memcpy(output, &buf->data[*pos], count);
> >         *offset = next;
> >         return true;
> > }
> > 
> > BR, Jarkko
> > 
> 
> Then the callers will check, and return -EIO?

Really, no, why would we do that?

The initial buffer is a page and no TPM currently can have a command
that big, so if the buffer overflows, it's likely a programming error
(failure to terminate loop or something) rather than a runtime one (a
user actually induced a command that big and wanted it to be sent to
the TPM).  The only reason you might need to check is the no-alloc case
and you passed in a much smaller buffer, but even there, I would guess
it will come down to a coding fault not a possible runtime error.

James