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

[Sean Christopherson <seanjc@google.com>]

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
>