[PATCH 2/3] KVM: guest_memfd: Fix possible signed integer overflow

[Ackerley Tng via B4 Relay <devnull+ackerleytng.google.com@kernel.org>]

From: Sean Christopherson <seanjc@google.com>

The caller, kvm_set_memory_region(), checks for an overflow in an unsigned
u64 guest_memfd_offset. When guest_memfd_offset is passed to kvm_gmem_bind,
it is cast into a signed 64-bit integer.

Hence, a large 64-bit offset could result in a negative loff_t, which could
result in the overflow checks failing.

Make kvm_gmem_bind() take u64 instead of loff_t to consistently deal with
unsigned values to avoid this issue.

Fixes: a7800aa80ea4d ("KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory")
Signed-off-by: Sean Christopherson <seanjc@google.com>
[Use size_t for size instead of u64]
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
 virt/kvm/guest_memfd.c | 7 +++----
 virt/kvm/kvm_mm.h      | 2 +-
 2 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 07d8db344872b..d203135969d13 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -640,9 +640,9 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
 }
 
 int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
-		  unsigned int fd, loff_t offset)
+		  unsigned int fd, u64 offset)
 {
-	loff_t size = slot->npages << PAGE_SHIFT;
+	size_t size = slot->npages << PAGE_SHIFT;
 	unsigned long start, end;
 	struct gmem_file *f;
 	struct inode *inode;
@@ -664,8 +664,7 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
 
 	inode = file_inode(file);
 
-	if (offset < 0 || !PAGE_ALIGNED(offset) ||
-	    offset + size > i_size_read(inode))
+	if (!PAGE_ALIGNED(offset) || offset + size > i_size_read(inode))
 		goto err;
 
 	filemap_invalidate_lock(inode->i_mapping);
diff --git a/virt/kvm/kvm_mm.h b/virt/kvm/kvm_mm.h
index 9fcc5d5b7f8d0..23813d74ce709 100644
--- a/virt/kvm/kvm_mm.h
+++ b/virt/kvm/kvm_mm.h
@@ -72,7 +72,7 @@ int kvm_gmem_init(struct module *module);
 void kvm_gmem_exit(void);
 int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args);
 int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
-		  unsigned int fd, loff_t offset);
+		  unsigned int fd, u64 offset);
 void kvm_gmem_unbind(struct kvm_memory_slot *slot);
 #else
 static inline int kvm_gmem_init(struct module *module)

-- 
2.54.0.794.g4f17f83d09-goog