[PATCH][next] KVM: x86: Fix allocation sizeof argument

[PATCH][next] KVM: x86: Fix allocation sizeof argument

[Colin King]

From: Colin Ian King <colin.king@canonical.com>

The allocation for *gfn_track should be for a slot->npages lot of
short integers, however the current allocation is using sizeof(*gfn_track)
and that is the size of a pointer, which is too large. Fix this by
using sizeof(**gfn_track) instead.

Addresses-Coverity: ("Wrong sizeof argument")
Fixes: 35b330bba6a7 ("KVM: x86: only allocate gfn_track when necessary")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
---
 arch/x86/kvm/mmu/page_track.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu/page_track.c b/arch/x86/kvm/mmu/page_track.c
index bb5d60bd4dbf..5b785a5f7dc9 100644
--- a/arch/x86/kvm/mmu/page_track.c
+++ b/arch/x86/kvm/mmu/page_track.c
@@ -92,7 +92,7 @@ int kvm_page_track_enable_mmu_write_tracking(struct kvm *kvm)
 		slots = __kvm_memslots(kvm, i);
 		kvm_for_each_memslot(slot, slots) {
 			gfn_track = slot->arch.gfn_track + KVM_PAGE_TRACK_WRITE;
-			*gfn_track = kvcalloc(slot->npages, sizeof(*gfn_track),
+			*gfn_track = kvcalloc(slot->npages, sizeof(**gfn_track),
 					      GFP_KERNEL_ACCOUNT);
 			if (*gfn_track == NULL) {
 				mutex_unlock(&kvm->slots_arch_lock);
-- 
2.32.0

Re: [PATCH][next] KVM: x86: Fix allocation sizeof argument

[Sean Christopherson]

On Fri, Oct 01, 2021, Colin King wrote:
> From: Colin Ian King <colin.king@canonical.com>
> 
> The allocation for *gfn_track should be for a slot->npages lot of
> short integers, however the current allocation is using sizeof(*gfn_track)
> and that is the size of a pointer, which is too large. Fix this by
> using sizeof(**gfn_track) instead.
> 
> Addresses-Coverity: ("Wrong sizeof argument")
> Fixes: 35b330bba6a7 ("KVM: x86: only allocate gfn_track when necessary")
> Signed-off-by: Colin Ian King <colin.king@canonical.com>
> ---
>  arch/x86/kvm/mmu/page_track.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu/page_track.c b/arch/x86/kvm/mmu/page_track.c
> index bb5d60bd4dbf..5b785a5f7dc9 100644
> --- a/arch/x86/kvm/mmu/page_track.c
> +++ b/arch/x86/kvm/mmu/page_track.c
> @@ -92,7 +92,7 @@ int kvm_page_track_enable_mmu_write_tracking(struct kvm *kvm)
>  		slots = __kvm_memslots(kvm, i);
>  		kvm_for_each_memslot(slot, slots) {
>  			gfn_track = slot->arch.gfn_track + KVM_PAGE_TRACK_WRITE;
> -			*gfn_track = kvcalloc(slot->npages, sizeof(*gfn_track),
> +			*gfn_track = kvcalloc(slot->npages, sizeof(**gfn_track),
>  					      GFP_KERNEL_ACCOUNT);

Eww (not your patch, the original code).  IMO the double indirection is completely
unnecessary, e.g. I find this far easier to follow

diff --git a/arch/x86/kvm/mmu/page_track.c b/arch/x86/kvm/mmu/page_track.c
index bb5d60bd4dbf..8cae41b831dd 100644
--- a/arch/x86/kvm/mmu/page_track.c
+++ b/arch/x86/kvm/mmu/page_track.c
@@ -75,7 +75,7 @@ int kvm_page_track_enable_mmu_write_tracking(struct kvm *kvm)
 {
        struct kvm_memslots *slots;
        struct kvm_memory_slot *slot;
-       unsigned short **gfn_track;
+       unsigned short *gfn_track;
        int i;
 
        if (write_tracking_enabled(kvm))
@@ -91,13 +91,13 @@ int kvm_page_track_enable_mmu_write_tracking(struct kvm *kvm)
        for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
                slots = __kvm_memslots(kvm, i);
                kvm_for_each_memslot(slot, slots) {
-                       gfn_track = slot->arch.gfn_track + KVM_PAGE_TRACK_WRITE;
-                       *gfn_track = kvcalloc(slot->npages, sizeof(*gfn_track),
-                                             GFP_KERNEL_ACCOUNT);
-                       if (*gfn_track == NULL) {
+                       gfn_track = kvcalloc(slot->npages, sizeof(*gfn_track),
+                                            GFP_KERNEL_ACCOUNT);
+                       if (gfn_track == NULL) {
                                mutex_unlock(&kvm->slots_arch_lock);
                                return -ENOMEM;
                        }
+                       slot->arch.gfn_track[KVM_PAGE_TRACK_WRITE] = gfn_track;
                }
        }
 


>  			if (*gfn_track == NULL) {
>  				mutex_unlock(&kvm->slots_arch_lock);

Hrm, this fails to free the gfn_track allocations for previous memslots.  The
on-demand rmaps code has the exact same bug (it frees rmaps for previous lpages
in the _current_ slot, but does not free previous slots).

And having two separate flows (and flags) for rmaps vs. gfn_track is pointless,
and means we have to maintain two near-identical copies of non-obvious code.

Paolo, is it too late to just drop the original deae4a10f166 ("KVM: x86: only
allocate gfn_track when necessary")?

> -- 
> 2.32.0
> 

Re: [PATCH][next] KVM: x86: Fix allocation sizeof argument

[Paolo Bonzini]

On 05/10/21 17:41, Sean Christopherson wrote:
>>   			if (*gfn_track == NULL) {
>>   				mutex_unlock(&kvm->slots_arch_lock);
> Hrm, this fails to free the gfn_track allocations for previous memslots.  The
> on-demand rmaps code has the exact same bug (it frees rmaps for previous lpages
> in the_current_  slot, but does not free previous slots).

That's not a huge deal because the syscall is failing.  So as long as 
it's not leaked forever, it's okay.  The problem is the 
WARN_ON(slot->arch.rmap[i]), or the missing check in 
kvm_page_track_enable_mmu_write_tracking, but that's easily fixed.  I'd 
even remove the call to memslot_rmaps_free.

> And having two separate flows (and flags) for rmaps vs. gfn_track is pointless,
> and means we have to maintain two near-identical copies of non-obvious code.

I was thinking the separate flow (not so much the flag) is needed 
because, if KVMGT is enabled, gfn_track is allocated unconditionally. 
rmaps are added on top of that if shadow paging is enabled; but 
kvm_page_track_create_memslot will have already created the counter, 
including the one for KVM_PAGE_TRACK_WRITE.

But looking at the code again, I guess you could call 
kvm_page_track_enable_mmu_write_tracking inside alloc_all_memslots_rmaps 
(with a little bit of renaming), and with that the flag would go away.

I'll take a look tomorrow, but I'd rather avoid reverting the patch.

Thanks,

Paolo

> Paolo, is it too late to just drop the original deae4a10f166 ("KVM: x86: only
> allocate gfn_track when necessary")?
> 

Re: [PATCH][next] KVM: x86: Fix allocation sizeof argument

[Sean Christopherson]

On Tue, Oct 05, 2021, Paolo Bonzini wrote:
> On 05/10/21 17:41, Sean Christopherson wrote:
> > >   			if (*gfn_track == NULL) {
> > >   				mutex_unlock(&kvm->slots_arch_lock);
> > Hrm, this fails to free the gfn_track allocations for previous memslots.  The
> > on-demand rmaps code has the exact same bug (it frees rmaps for previous lpages
> > in the_current_  slot, but does not free previous slots).
> 
> That's not a huge deal because the syscall is failing.  So as long as it's
> not leaked forever, it's okay.  The problem is the
> WARN_ON(slot->arch.rmap[i]), or the missing check in
> kvm_page_track_enable_mmu_write_tracking, but that's easily fixed.  I'd even
> remove the call to memslot_rmaps_free.

It can be leaked forever though, e.g. if userspace invokes KVM_RUN over and over
on -ENOMEM.  That would trigger the WARN_ON(slot->arch.rmap[i]) and leak the
previous allocation.  I think it would be safe to change that WARN_ON to a
check-and-continue, i.e. to preserve the previous allocation

> > And having two separate flows (and flags) for rmaps vs. gfn_track is pointless,
> > and means we have to maintain two near-identical copies of non-obvious code.
> 
> I was thinking the separate flow (not so much the flag) is needed because,
> if KVMGT is enabled, gfn_track is allocated unconditionally. rmaps are added
> on top of that if shadow paging is enabled; but
> kvm_page_track_create_memslot will have already created the counter,
> including the one for KVM_PAGE_TRACK_WRITE.
> 
> But looking at the code again, I guess you could call
> kvm_page_track_enable_mmu_write_tracking inside alloc_all_memslots_rmaps
> (with a little bit of renaming), and with that the flag would go away.

Yes, and reuse the control flow, which is what I really care about since that's
the part that both features get wrong.
 
> I'll take a look tomorrow, but I'd rather avoid reverting the patch.

I can poke at it too if you don't have time.  I wasn't suggesting a full revert,
rather a "drop and pretend it never got applied", with a plan to apply a new
version instead of fixing up the current code.

Re: [PATCH][next] KVM: x86: Fix allocation sizeof argument

[Paolo Bonzini]

On 05/10/21 19:55, Sean Christopherson wrote:
>   I wasn't suggesting a full revert,
> rather a "drop and pretend it never got applied", with a plan to apply a new
> version instead of fixing up the current code.

Considering that there are issues in the rmaps as well, I'd rather fix 
both the right way.

Paolo

Re: [PATCH][next] KVM: x86: Fix allocation sizeof argument

[David Stevens]

On Wed, Oct 6, 2021 at 12:41 AM Sean Christopherson <seanjc@google.com> wrote:
>
> On Fri, Oct 01, 2021, Colin King wrote:
> > From: Colin Ian King <colin.king@canonical.com>
> >
> > The allocation for *gfn_track should be for a slot->npages lot of
> > short integers, however the current allocation is using sizeof(*gfn_track)
> > and that is the size of a pointer, which is too large. Fix this by
> > using sizeof(**gfn_track) instead.
> >
> > Addresses-Coverity: ("Wrong sizeof argument")
> > Fixes: 35b330bba6a7 ("KVM: x86: only allocate gfn_track when necessary")
> > Signed-off-by: Colin Ian King <colin.king@canonical.com>
> > ---
> >  arch/x86/kvm/mmu/page_track.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/mmu/page_track.c b/arch/x86/kvm/mmu/page_track.c
> > index bb5d60bd4dbf..5b785a5f7dc9 100644
> > --- a/arch/x86/kvm/mmu/page_track.c
> > +++ b/arch/x86/kvm/mmu/page_track.c
> > @@ -92,7 +92,7 @@ int kvm_page_track_enable_mmu_write_tracking(struct kvm *kvm)
> >               slots = __kvm_memslots(kvm, i);
> >               kvm_for_each_memslot(slot, slots) {
> >                       gfn_track = slot->arch.gfn_track + KVM_PAGE_TRACK_WRITE;
> > -                     *gfn_track = kvcalloc(slot->npages, sizeof(*gfn_track),
> > +                     *gfn_track = kvcalloc(slot->npages, sizeof(**gfn_track),
> >                                             GFP_KERNEL_ACCOUNT);
>
> Eww (not your patch, the original code).  IMO the double indirection is completely
> unnecessary, e.g. I find this far easier to follow
>
> diff --git a/arch/x86/kvm/mmu/page_track.c b/arch/x86/kvm/mmu/page_track.c
> index bb5d60bd4dbf..8cae41b831dd 100644
> --- a/arch/x86/kvm/mmu/page_track.c
> +++ b/arch/x86/kvm/mmu/page_track.c
> @@ -75,7 +75,7 @@ int kvm_page_track_enable_mmu_write_tracking(struct kvm *kvm)
>  {
>         struct kvm_memslots *slots;
>         struct kvm_memory_slot *slot;
> -       unsigned short **gfn_track;
> +       unsigned short *gfn_track;
>         int i;
>
>         if (write_tracking_enabled(kvm))
> @@ -91,13 +91,13 @@ int kvm_page_track_enable_mmu_write_tracking(struct kvm *kvm)
>         for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
>                 slots = __kvm_memslots(kvm, i);
>                 kvm_for_each_memslot(slot, slots) {
> -                       gfn_track = slot->arch.gfn_track + KVM_PAGE_TRACK_WRITE;
> -                       *gfn_track = kvcalloc(slot->npages, sizeof(*gfn_track),
> -                                             GFP_KERNEL_ACCOUNT);
> -                       if (*gfn_track == NULL) {
> +                       gfn_track = kvcalloc(slot->npages, sizeof(*gfn_track),
> +                                            GFP_KERNEL_ACCOUNT);
> +                       if (gfn_track == NULL) {
>                                 mutex_unlock(&kvm->slots_arch_lock);
>                                 return -ENOMEM;
>                         }
> +                       slot->arch.gfn_track[KVM_PAGE_TRACK_WRITE] = gfn_track;
>                 }
>         }
>
>
>
> >                       if (*gfn_track == NULL) {
> >                               mutex_unlock(&kvm->slots_arch_lock);
>
> Hrm, this fails to free the gfn_track allocations for previous memslots.  The
> on-demand rmaps code has the exact same bug (it frees rmaps for previous lpages
> in the _current_ slot, but does not free previous slots).
>
> And having two separate flows (and flags) for rmaps vs. gfn_track is pointless,
> and means we have to maintain two near-identical copies of non-obvious code.

I agree that's better than my patch. I can put together a new patch
once it's decided whether or not my patch should be dropped.

-David

Re: [PATCH][next] KVM: x86: Fix allocation sizeof argument

[Sean Christopherson]

On Wed, Oct 06, 2021, David Stevens wrote:
> On Wed, Oct 6, 2021 at 12:41 AM Sean Christopherson <seanjc@google.com> wrote:
> > Hrm, this fails to free the gfn_track allocations for previous memslots.  The
> > on-demand rmaps code has the exact same bug (it frees rmaps for previous lpages
> > in the _current_ slot, but does not free previous slots).
> >
> > And having two separate flows (and flags) for rmaps vs. gfn_track is pointless,
> > and means we have to maintain two near-identical copies of non-obvious code.
> 
> I agree that's better than my patch. I can put together a new patch
> once it's decided whether or not my patch should be dropped.

All yours, unless Paolo wants to fight you for it :-)  I'm totally ok doing
cleanup/fixes on top.