nvram: Exit QEMU if NVRAM cannot contain all -prom-env data

nvram: Exit QEMU if NVRAM cannot contain all -prom-env data

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 case 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 " ; printf "%0.sx" {1..1024}; \
  done) # this requires ~128 Kib
malloc(): corrupted top size
Aborted (core dumped)

This happens because we don't check if all the prom-env data fits in
the NVRAM and chrp_nvram_set_var() happily memcpy() it passed the
buffer.

This crash affects basically all ppc/ppc64 machine types that use -prom-env:
- pseries (all versions)
- g3beige
- mac99

and also sparc/sparc64 machine types:
- LX
- SPARCClassic
- SPARCbook
- SS-10
- SS-20
- SS-4
- SS-5
- SS-600MP
- Voyager
- sun4u
- sun4v

Add a max_len argument to chrp_nvram_create_system_partition() so that
it can check the available size before writing to memory.

Since NVRAM is populated at machine init, it seems reasonable to consider
this error as fatal. So, instead of reporting an error when we detect that
the NVRAM is too small and adapt all machine types to handle it, we simply
exit QEMU in all cases. This is still better than crashing. If someone
wants another behavior, I guess this can be reworked later.

Tested with:

$ yes q | \
  (for arch in ppc ppc64 sparc sparc64; do \
       echo == $arch ==; \
       qemu=${arch}-softmmu/qemu-system-$arch; \
       for mach in $($qemu -M help | awk '! /^Supported/ { print $1 }'); do \
           echo $mach; \
           $qemu -M $mach -monitor stdio -nodefaults -nographic \
           $(for ((x=0;x<128;x++)); do \
                 echo -n " -prom-env " ; printf "%0.sx" {1..1024}; \
             done) >/dev/null; \
        done; echo; \
   done)

Without the patch, affected machine types cause QEMU to report some
memory corruption and crash:

malloc(): corrupted top size

free(): invalid size

*** stack smashing detected ***: terminated

With the patch, QEMU prints the following message and exits:

NVRAM is too small. Try to pass less data to -prom-env

It seems that the conditions for the crash have always existed, but it
affects pseries, the machine type I care for, since commit 61f20b9dc5b7
only.

Fixes: 61f20b9dc5b7 ("spapr_nvram: Pre-initialize the NVRAM to support the -prom-env parameter")
RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1867739
Reported-by: John Snow <jsnow@redhat.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <159736033937.350502.12402444542194031035.stgit@bahia.lan>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>