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

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Message-ID: <ac616f9f1eef89e9c202b43d2be5b3ccb6afd1dd.camel@kernel.org>
Date:   Mon, 06 Nov 2023 00:01:30 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     James Bottomley <James.Bottomley@...senPartnership.com>,
        Jerry Snitselaar <jsnitsel@...hat.com>
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 Sun, 2023-11-05 at 23:59 +0200, Jarkko Sakkinen wrote:
> On Thu, 2023-10-26 at 13:55 -0400, James Bottomley wrote:
> > 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.
> 
> 
> Yeah, this was my thinking too. So in HMAC case you anyway would not
> need to check it because crypto is destined to fail anyway.
> 
> Returning boolean here does no harm so I thought that this is overally
> good compromise.

Or actually maybe we should go just with void, as it does have even
then "return value", as it emits to klog, right?

BR, Jarkko