Re: [PATCH v2 20/25] x86: Fix x86_cpu_new() error API violations

[Igor Mammedov <imammedo@redhat.com>]

On Fri, 26 Jun 2020 14:54:38 +0200
Markus Armbruster <armbru@redhat.com> wrote:

> Igor Mammedov <imammedo@redhat.com> writes:
> 
> > On Wed, 24 Jun 2020 10:37:32 +0200
> > Markus Armbruster <armbru@redhat.com> wrote:
> >  
> >> The Error ** argument must be NULL, &error_abort, &error_fatal, or a
> >> pointer to a variable containing NULL.  Passing an argument of the
> >> latter kind twice without clearing it in between is wrong: if the
> >> first call sets an error, it no longer points to NULL for the second
> >> call.
> >> 
> >> x86_cpu_new() is wrong that way: it passes &local_err to
> >> object_property_set_uint() without checking it, and then to
> >> qdev_realize().  Harmless, because the former can't actually fail
> >> here.
> >> 
> >> Fix by checking for failure right away.  While there, replace
> >> qdev_realize(); object_unref() by qdev_realize_and_unref().
> >> 
> >> Cc: Paolo Bonzini <pbonzini@redhat.com>
> >> Cc: Richard Henderson <rth@twiddle.net>
> >> Cc: Eduardo Habkost <ehabkost@redhat.com>
> >> Signed-off-by: Markus Armbruster <armbru@redhat.com>
> >> ---
> >>  hw/i386/x86.c | 12 +++---------
> >>  1 file changed, 3 insertions(+), 9 deletions(-)
> >> 
> >> diff --git a/hw/i386/x86.c b/hw/i386/x86.c
> >> index 34229b45c7..3a7029e6db 100644
> >> --- a/hw/i386/x86.c
> >> +++ b/hw/i386/x86.c
> >> @@ -118,16 +118,10 @@ uint32_t x86_cpu_apic_id_from_index(X86MachineState *x86ms,
> >>  
> >>  void x86_cpu_new(X86MachineState *x86ms, int64_t apic_id, Error **errp)
> >>  {
> >> -    Object *cpu = NULL;
> >> -    Error *local_err = NULL;
> >> +    Object *cpu = object_new(MACHINE(x86ms)->cpu_type);
> >>  
> >> -    cpu = object_new(MACHINE(x86ms)->cpu_type);
> >> -
> >> -    object_property_set_uint(cpu, apic_id, "apic-id", &local_err);
> >> -    qdev_realize(DEVICE(cpu), NULL, &local_err);
> >> -
> >> -    object_unref(cpu);
> >> -    error_propagate(errp, local_err);
> >> +    object_property_set_uint(cpu, apic_id, "apic-id", &error_abort);  
> > it may fail here if user specified wrong cpu flags, but there is nothing we can do to fix it.  
> 
> Really?
> 
> object_property_set_uint() fails when property "apic-id" doesn't exist,
> has no ->set() method, or its ->set() fails.
> 
> Property "apic-id" is defined in x86_cpu_properties[] as
> 
>     DEFINE_PROP_UINT32("apic-id", X86CPU, apic_id, UNASSIGNED_APIC_ID),
> 
> This means "apic-id" exists, and its ->set() is set_uint32().  That
> leaves only set_uint32() as possible source of failure.
> 
> It fails when
> 
> * the device is already realized: programming error
> 
> * the value to be stored is not an integer: object_property_set_uint()
>   makes it one, can't fail
> 
> * the value is not representable as uint32_t: @api_id is declared as
>   int64_t, but:
> 
>   - pc_hot_add_cpu() passes x86_cpu_apic_id_from_index(), which is
>     uint32_t, converted to int64_t.  Can't fail.
> 
>   - x86_cpus_init() passes possible_cpus->cpus[i].arch_id, which is
>     uint64_t.  Is this the "if user specified wrong cpu flags" case?

looking more on it, object_property_set_uint() can't really fail

>   Aside: should the integer types be cleaned up?

apic_id is x86 specific subset of .arch_id.
The later is used by other targets which may need larger than 32bit integer
(if I recall correctly virt-arm uses 64bit id). 


> To assess the bug's impact, we need to know when the other call in this
> error pileup fails.  If we can make both fail, we have a crash bug.
> Else, we have a harmless API violation.
> 
> Any ideas on how to make the qdev_realize() fail?
qemu CLI case
  QEMU -cpu qemu64,enforce,topoext

legacy hotplug case:
  QEMU -smp 1,maxcpus=2
  (monitor) cpu-add 1
  (monitor) cpu-add 1  <= fail