Re: [PATCH v2 2/2] spapr/nvram: Error out if NVRAM cannot contain all -prom-env data

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Aug 12, 2020 at 09:06:54PM +0200, Greg Kurz wrote:
> On Wed, 12 Aug 2020 19:29:26 +0200
> Laurent Vivier <laurent@vivier.eu> wrote:
> 
> > Le 12/08/2020 à 19:08, Greg Kurz a écrit :
> > > Since commit 61f20b9dc5b7 ("spapr_nvram: Pre-initialize the NVRAM to
> > > support the -prom-env parameter"), pseries machines can pre-initialize
> > > the "system" partition in the NVRAM with the data passed to all -prom-env
> > > parameters on the QEMU command line.
> > > 
> > > In this cases it is assumed that all the data fits in 64 KiB, but the user
> > > can easily pass more and crash QEMU:
> > > 
> > > $ qemu-system-ppc64 -M pseries $(for ((x=0;x<128;x++)); do \
> > >   echo -n " -prom-env "$(for ((y=0;y<1024;y++)); do echo -n x ; done) ; \
> > >   done) # this requires ~128 Kib
> > > malloc(): corrupted top size
> > > Aborted (core dumped)
> > > 
> > > Call chrp_nvram_create_system_partition() first, with its recently added
> > > parameter dry_run set to true, in order to know the required size and fail
> > > gracefully if it's too small.
> > 
> > Why do you need the dry_run parameter?
> > Can't you fail on the normal case?
> > 
> 
> Not sure what the "normal case" stands for... but basically, only
> chrp_nvram_create_system_partition() knows the exact size of the
> partition (ie. size of the header + size of all prom-env strings
> including the terminal nul + padding to the upper 16-byte aligment).
> 
> Another solution could be to pass the buffer size and errp to
> chrp_nvram_create_system_partition() and chrp_nvram_set_var(),
> and let chrp_nvram_set_var() check it won't memcpy() past the
> buffer. But this is more code and since this is also used by
> other machine types, I chose to go for the dry_run parameter.

Hm, it does feel like a more natural interface to me, though, rather
than always having to call it twice.  Basically just add a "max_size"
parameter.

> Should I improve the changelog to make this clearer or are
> you thinking to something else ?
> 
> > Thanks,
> > Laurent
> > 
> > > 
> > > Reported-by: John Snow <jsnow@redhat.com>
> > > Signed-off-by: Greg Kurz <groug@kaod.org>
> > > ---
> > >  hw/nvram/spapr_nvram.c |   15 +++++++++++++++
> > >  1 file changed, 15 insertions(+)
> > > 
> > > diff --git a/hw/nvram/spapr_nvram.c b/hw/nvram/spapr_nvram.c
> > > index 992b818d34e7..c29d797ae1f0 100644
> > > --- a/hw/nvram/spapr_nvram.c
> > > +++ b/hw/nvram/spapr_nvram.c
> > > @@ -145,6 +145,7 @@ static void rtas_nvram_store(PowerPCCPU *cpu, SpaprMachineState *spapr,
> > >  
> > >  static void spapr_nvram_realize(SpaprVioDevice *dev, Error **errp)
> > >  {
> > > +    ERRP_GUARD();
> > >      SpaprNvram *nvram = VIO_SPAPR_NVRAM(dev);
> > >      int ret;
> > >  
> > > @@ -187,6 +188,20 @@ static void spapr_nvram_realize(SpaprVioDevice *dev, Error **errp)
> > >              return;
> > >          }
> > >      } else if (nb_prom_envs > 0) {
> > > +        int len = chrp_nvram_create_system_partition(nvram->buf,
> > > +                                                     MIN_NVRAM_SIZE / 4,
> > > +                                                     true);
> > > +
> > > +        /* Check the partition is large enough for all the -prom-env data */
> > > +        if (nvram->size < len) {
> > > +            error_setg(errp, "-prom-env data requires %d bytes but spapr-nvram "
> > > +                       "is only %d bytes in size", len, nvram->size);
> > > +            error_append_hint(errp,
> > > +                              "Try to pass %d less bytes to -prom-env.\n",
> > > +                              len - nvram->size);
> > > +            return;
> > > +        }
> > > +
> > >          /* Create a system partition to pass the -prom-env variables */
> > >          chrp_nvram_create_system_partition(nvram->buf, MIN_NVRAM_SIZE / 4,
> > >                                             false);
> > > 
> > > 
> > 
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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