[PATCH] KVM: x86: Fix rmap allocation for very large memslots

[Maciej S. Szmigiero]

From: "Maciej S. Szmigiero" <maciej.szmigiero@xxxxxxxxxx>

Commit 7661809d493b ("mm: don't allow oversized kvmalloc() calls") has
forbidden using kvmalloc() to make allocations larger than INT_MAX (2 GiB).

Unfortunately, adding a memslot exceeding 1 TiB in size will result in rmap
code trying to make an allocation exceeding this limit.
Besides failing this allocation, such operation will also trigger a
WARN_ON_ONCE() added by the aforementioned commit.

Since we probably still want to use kernel slab for small rmap allocations
let's only redirect such oversized allocations to vmalloc.

A possible alternative would be to add some kind of a __GFP_LARGE flag to
skip the INT_MAX check behind kvmalloc(), however this will impact the
common kernel memory allocation code, not just KVM.

Fixes: a7c3e901a4 ("mm: introduce kv[mz]alloc helpers")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@xxxxxxxxxx>
---
 arch/x86/kvm/x86.c | 26 +++++++++++++++++++-------
 1 file changed, 19 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 8033eca6f3a1..c64bac8614c7 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11806,24 +11806,36 @@ void kvm_arch_free_memslot(struct kvm *kvm, struct kvm_memory_slot *slot)
 
 int memslot_rmap_alloc(struct kvm_memory_slot *slot, unsigned long npages)
 {
-	const int sz = sizeof(*slot->arch.rmap[0]);
+	const size_t sz = sizeof(*slot->arch.rmap[0]);
 	int i;
 
 	for (i = 0; i < KVM_NR_PAGE_SIZES; ++i) {
 		int level = i + 1;
-		int lpages = __kvm_mmu_slot_lpages(slot, npages, level);
+		size_t lpages = __kvm_mmu_slot_lpages(slot, npages, level);
+		size_t rmap_size;
 
 		if (slot->arch.rmap[i])
 			continue;
 
-		slot->arch.rmap[i] = kvcalloc(lpages, sz, GFP_KERNEL_ACCOUNT);
-		if (!slot->arch.rmap[i]) {
-			memslot_rmap_free(slot);
-			return -ENOMEM;
-		}
+		if (unlikely(check_mul_overflow(lpages, sz, &rmap_size)))
+			goto ret_fail;
+
+		/* kvzalloc() only allows sizes up to INT_MAX */
+		if (unlikely(rmap_size > INT_MAX))
+			slot->arch.rmap[i] = __vmalloc(rmap_size,
+						       GFP_KERNEL_ACCOUNT | __GFP_ZERO);
+		else
+			slot->arch.rmap[i] = kvzalloc(rmap_size, GFP_KERNEL_ACCOUNT);
+
+		if (!slot->arch.rmap[i])
+			goto ret_fail;
 	}
 
 	return 0;
+
+ret_fail:
+	memslot_rmap_free(slot);
+	return -ENOMEM;
 }
 
 static int kvm_alloc_memslot_metadata(struct kvm *kvm,