[PATCH v4 15/23] KVM: Pre-check uaccesses in KVM's APIs to read/write guest memory

[Sean Christopherson <seanjc@google.com>]

Add a check on whether or not doing a user access is ok in KVM's APIs for
accessing guest memory, *before* doing memslot lookup/validation, so that
buggy KVM accesses to guest memory are more likely to fail noisily.
Because KVM installs dummy/empty memslots early in VM destruction, bugs in
the VM teardown path will fail on the memslot checks without getting to the
WARNs buried in kvm_copy_{to,from}_user() and friends.

Reviewed-by: James Houghton <jthoughton@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 virt/kvm/kvm_main.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 2eda12a52bf4..c6af11cea296 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3300,6 +3300,12 @@ static int next_segment(unsigned long len, int offset)
 		return len;
 }
 
+static bool kvm_is_guest_access_ok(struct kvm *kvm, unsigned long offset,
+				   unsigned long len, unsigned long size)
+{
+	return kvm_can_do_uaccess(kvm) && !WARN_ON_ONCE(offset + len > size);
+}
+
 /* Copy @len bytes from guest memory at '(@gfn * PAGE_SIZE) + @offset' to @data */
 static int __kvm_read_guest_page(struct kvm *kvm, struct kvm_memory_slot *slot,
 				 gfn_t gfn, void *data, int offset, int len)
@@ -3307,7 +3313,7 @@ static int __kvm_read_guest_page(struct kvm *kvm, struct kvm_memory_slot *slot,
 	int r;
 	unsigned long addr;
 
-	if (WARN_ON_ONCE(offset + len > PAGE_SIZE))
+	if (!kvm_is_guest_access_ok(kvm, offset, len, PAGE_SIZE))
 		return -EFAULT;
 
 	addr = gfn_to_hva_memslot_prot(slot, gfn, NULL);
@@ -3383,7 +3389,7 @@ static int __kvm_read_guest_atomic(struct kvm *kvm, struct kvm_memory_slot *slot
 	int r;
 	unsigned long addr;
 
-	if (WARN_ON_ONCE(offset + len > PAGE_SIZE))
+	if (!kvm_is_guest_access_ok(kvm, offset, len, PAGE_SIZE))
 		return -EFAULT;
 
 	addr = gfn_to_hva_memslot_prot(slot, gfn, NULL);
@@ -3416,7 +3422,7 @@ static int __kvm_write_guest_page(struct kvm *kvm,
 	int r;
 	unsigned long addr;
 
-	if (WARN_ON_ONCE(offset + len > PAGE_SIZE))
+	if (!kvm_is_guest_access_ok(kvm, offset, len, PAGE_SIZE))
 		return -EFAULT;
 
 	addr = gfn_to_hva_memslot(memslot, gfn);
@@ -3546,7 +3552,7 @@ int kvm_write_guest_offset_cached(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
 	int r;
 	gpa_t gpa = ghc->gpa + offset;
 
-	if (WARN_ON_ONCE(len + offset > ghc->len))
+	if (!kvm_is_guest_access_ok(kvm, offset, len, ghc->len))
 		return -EINVAL;
 
 	if (slots->generation != ghc->generation) {
@@ -3584,7 +3590,7 @@ int kvm_read_guest_offset_cached(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
 	int r;
 	gpa_t gpa = ghc->gpa + offset;
 
-	if (WARN_ON_ONCE(len + offset > ghc->len))
+	if (!kvm_is_guest_access_ok(kvm, offset, len, ghc->len))
 		return -EINVAL;
 
 	if (slots->generation != ghc->generation) {
-- 
2.56.0.385.gac671c819a-goog