Re: [RFC v2 4/5] tpm: Increase TPM_BUFSIZE to 8kB for chunking support

[Jarkko Sakkinen <jarkko@kernel.org>]

On Wed, May 13, 2026 at 04:36:05PM +0530, Arun Menon wrote:
> On Sat, May 09, 2026 at 06:07:11PM +0300, Jarkko Sakkinen wrote:
> > On Sat, May 09, 2026 at 05:54:25PM +0300, Jarkko Sakkinen wrote:
> > > On Tue, Mar 24, 2026 at 11:42:43PM +0530, Arun Menon wrote:
> > > > The size of the command is checked against TPM_BUFSIZE early on before
> > > > even sending it to the backend. We therefore need to increase the
> > > > TPM_BUFSIZE to allow support for larger commands.
> > > > 
> > > > For now, 8KB seems sufficient for ML-KEM and ML-DSA algorithms and it is
> > > > also order-1 safe.
> > > > 
> > > > Signed-off-by: Arun Menon <armenon@redhat.com>
> > > > ---
> > > >  drivers/char/tpm/tpm.h | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/drivers/char/tpm/tpm.h b/drivers/char/tpm/tpm.h
> > > > index 87d68ddf270a7..26c3765fbd732 100644
> > > > --- a/drivers/char/tpm/tpm.h
> > > > +++ b/drivers/char/tpm/tpm.h
> > > > @@ -33,7 +33,7 @@
> > > >  #endif
> > > >  
> > > >  #define TPM_MINOR		224	/* officially assigned */
> > > > -#define TPM_BUFSIZE		4096
> > > > +#define TPM_BUFSIZE		8192
> > > >  #define TPM_NUM_DEVICES		65536
> > > >  #define TPM_RETRY		50
> > > >  
> > > > -- 
> > > > 2.53.0
> > > > 
> > > 
> > > Shouldn't this prepend previous patch?
> > 
> > Also did you remark that tpm_buf would also need changes as it is fixed
> > to PAGE_SIZE?
> 
> TPM_BUFSIZE can be increased, in its new location include/linux/tpm.h as
> per the patch : https://lore.kernel.org/linux-integrity/20260125192526.782202-12-jarkko@kernel.org/
> and I think that alone will take care of the check if (size > TPM_BUFSIZE)
> in tpm_common_write() in drivers/char/tpm/tpm-dev-common.c.
> 
> However I was not able to apply the mbox file cleanly on the existing
> branches for-next-tpm and for-next-keys. I could apply them cleanly on
> the old branch (next). Please guide.
> 
> I would only change the TPM_BUFSIZE set in 
> [PATCH v9 11/11] tpm-buf: Implement managed allocations to 8192.

So.. why can't you just rebase them and resolve possible merge
conflicts? If you use git-am, you'll like want to use '-3' flag.

> 
> > 
> > I've made a patch that essentially makes tpm_buf size variable as caller
> > does kzalloc:
> > 
> > https://lore.kernel.org/linux-integrity/20260125192526.782202-12-jarkko@kernel.org/
> > 
> > I'd see this as pretty good long-term solution.
> 
> Indeed. 
> 
> > 
> > BR, Jarkko
> > 
> 
> 
> Regards,
> Arun Menon
> 

BR, Jarkko