Messages in this thread Patch in this message | | | Subject | Re: [PATCH 1/4] kvm: cpuid: adjust the returned nent field of kvm_cpuid2 for KVM_GET_SUPPORTED_CPUID and KVM_GET_EMULATED_CPUID | From | Emanuele Giuseppe Esposito <> | Date | Wed, 31 Mar 2021 12:07:14 +0200 |
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On 31/03/2021 09:56, Vitaly Kuznetsov wrote: > Emanuele Giuseppe Esposito <eesposit@redhat.com> writes: > >> On 31/03/2021 05:01, Sean Christopherson wrote: >>> On Tue, Mar 30, 2021, Emanuele Giuseppe Esposito wrote: >>>> Calling the kvm KVM_GET_[SUPPORTED/EMULATED]_CPUID ioctl requires >>>> a nent field inside the kvm_cpuid2 struct to be big enough to contain >>>> all entries that will be set by kvm. >>>> Therefore if the nent field is too high, kvm will adjust it to the >>>> right value. If too low, -E2BIG is returned. >>>> >>>> However, when filling the entries do_cpuid_func() requires an >>>> additional entry, so if the right nent is known in advance, >>>> giving the exact number of entries won't work because it has to be increased >>>> by one. >>>> >>>> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com> >>>> --- >>>> arch/x86/kvm/cpuid.c | 6 ++++++ >>>> 1 file changed, 6 insertions(+) >>>> >>>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c >>>> index 6bd2f8b830e4..5412b48b9103 100644 >>>> --- a/arch/x86/kvm/cpuid.c >>>> +++ b/arch/x86/kvm/cpuid.c >>>> @@ -975,6 +975,12 @@ int kvm_dev_ioctl_get_cpuid(struct kvm_cpuid2 *cpuid, >>>> >>>> if (cpuid->nent < 1) >>>> return -E2BIG; >>>> + >>>> + /* if there are X entries, we need to allocate at least X+1 >>>> + * entries but return the actual number of entries >>>> + */ >>>> + cpuid->nent++; >>> >>> I don't see how this can be correct. >>> >>> If this bonus entry really is needed, then won't that be reflected in array.nent? >>> I.e won't KVM overrun the userspace buffer? >>> >>> If it's not reflected in array.nent, that would imply there's an off-by-one check >>> somewhere, or KVM is creating an entry that it doesn't copy to userspace. The >>> former seems unlikely as there are literally only two checks against maxnent, >>> and they both look correct (famous last words...). >>> >>> KVM does decrement array->nent in one specific case (CPUID.0xD.2..64), i.e. a >>> false positive is theoretically possible, but that carries a WARN and requires a >>> kernel or CPU bug as well. And fudging nent for that case would still break >>> normal use cases due to the overrun problem. >>> >>> What am I missing? >> >> (Maybe I should have put this series as RFC) >> >> The problem I see and noticed while doing the KVM_GET_EMULATED_CPUID >> selftest is the following: assume there are 3 kvm emulated entries, and >> the user sets cpuid->nent = 3. This should work because kvm sets 3 >> array->entries[], and copies them to user space. >> >> However, when the 3rd entry is populated inside kvm (array->entries[2]), >> array->nent is increased once more (do_host_cpuid and >> __do_cpuid_func_emulated). At that point, the loop in >> kvm_dev_ioctl_get_cpuid and get_cpuid_func can potentially iterate once >> more, going into the >> >> if (array->nent >= array->maxnent) >> return -E2BIG; >> >> in __do_cpuid_func_emulated and do_host_cpuid, returning the error. I >> agree that we need that check there because the following code tries to >> access the array entry at array->nent index, but from what I understand >> that access can be potentially useless because it might just jump to the >> default entry in the switch statement and not set the entry, leaving >> array->nent to 3. > > The problem seems to be exclusive to __do_cpuid_func_emulated(), > do_host_cpuid() always does > > entry = &array->entries[array->nent++]; > > Something like (completely untested and stupid): > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c > index 6bd2f8b830e4..54dcabd3abec 100644 > --- a/arch/x86/kvm/cpuid.c > +++ b/arch/x86/kvm/cpuid.c > @@ -565,14 +565,22 @@ static struct kvm_cpuid_entry2 *do_host_cpuid(struct kvm_cpuid_array *array, > return entry; > } > > +static bool cpuid_func_emulated(u32 func) > +{ > + return (func == 0) || (func == 1) || (func == 7); > +} > + > static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func) > { > struct kvm_cpuid_entry2 *entry; > > + if (!cpuid_func_emulated()) > + return 0; > + > if (array->nent >= array->maxnent) > return -E2BIG; > > - entry = &array->entries[array->nent]; > + entry = &array->entries[array->nent++]; > entry->function = func; > entry->index = 0; > entry->flags = 0; > @@ -580,18 +588,14 @@ static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func) > switch (func) { > case 0: > entry->eax = 7; > - ++array->nent; > break; > case 1: > entry->ecx = F(MOVBE); > - ++array->nent; > break; > case 7: > entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX; > entry->eax = 0; > entry->ecx = F(RDPID); > - ++array->nent; > - default: > break; > } > > should do the job, right? > >
Yes, it would work better. Alternatively:
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c index ba7437308d28..452b0acd6e9d 100644 --- a/arch/x86/kvm/cpuid.c +++ b/arch/x86/kvm/cpuid.c @@ -567,34 +567,37 @@ static struct kvm_cpuid_entry2 *do_host_cpuid(struct kvm_cpuid_array *array,
static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func) { - struct kvm_cpuid_entry2 *entry; - - if (array->nent >= array->maxnent) - return -E2BIG; + struct kvm_cpuid_entry2 entry; + bool changed = true;
- entry = &array->entries[array->nent]; - entry->function = func; - entry->index = 0; - entry->flags = 0; + entry.function = func; + entry.index = 0; + entry.flags = 0;
switch (func) { case 0: - entry->eax = 7; - ++array->nent; + entry.eax = 7; break; case 1: - entry->ecx = F(MOVBE); - ++array->nent; + entry.ecx = F(MOVBE); break; case 7: - entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX; - entry->eax = 0; - entry->ecx = F(RDPID); - ++array->nent; + entry.flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX; + entry.eax = 0; + entry.ecx = F(RDPID); + break; default: + changed = false; break; }
+ if (changed) { + if (array->nent >= array->maxnent) + return -E2BIG; + + memcpy(&array->entries[array->nent++], &entry, sizeof(entry)); + } + return 0; }
pros: avoids hard-coding another function that would check what the switch already does. it will be more flexible if another func has to be added. cons: there is a memcpy for each entry. What do you think?
Emanuele
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