[PATCH for 5.4] Fix unsynchronized access to sev members through svm_register_enc_region

[Peter Gonda <pgonda@xxxxxxxxxx>]

commit 19a23da53932bc8011220bd8c410cb76012de004 upstream.

Grab kvm->lock before pinning memory when registering an encrypted
region; sev_pin_memory() relies on kvm->lock being held to ensure
correctness when checking and updating the number of pinned pages.

Add a lockdep assertion to help prevent future regressions.

Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Joerg Roedel <joro@xxxxxxxxxx>
Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
Cc: Brijesh Singh <brijesh.singh@xxxxxxx>
Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: kvm@xxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Fixes: 1e80fdc09d12 ("KVM: SVM: Pin guest memory when SEV is active")
Signed-off-by: Peter Gonda <pgonda@xxxxxxxxxx>

V2
 - Fix up patch description
 - Correct file paths svm.c -> sev.c
 - Add unlock of kvm->lock on sev_pin_memory error

V1
 - https://lore.kernel.org/kvm/20210126185431.1824530-1-pgonda@xxxxxxxxxx/

Message-Id: <20210127161524.2832400-1-pgonda@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 arch/x86/kvm/svm.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 2b506904be02..93c89f1ffc5d 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1830,6 +1830,8 @@ static struct page **sev_pin_memory(struct kvm *kvm, unsigned long uaddr,
 	struct page **pages;
 	unsigned long first, last;
 
+	lockdep_assert_held(&kvm->lock);
+
 	if (ulen == 0 || uaddr + ulen < uaddr)
 		return NULL;
 
@@ -7086,12 +7088,21 @@ static int svm_register_enc_region(struct kvm *kvm,
 	if (!region)
 		return -ENOMEM;
 
+	mutex_lock(&kvm->lock);
 	region->pages = sev_pin_memory(kvm, range->addr, range->size, &region->npages, 1);
 	if (!region->pages) {
 		ret = -ENOMEM;
+		mutex_unlock(&kvm->lock);
 		goto e_free;
 	}
 
+	region->uaddr = range->addr;
+	region->size = range->size;
+
+	mutex_lock(&kvm->lock);
+	list_add_tail(&region->list, &sev->regions_list);
+	mutex_unlock(&kvm->lock);
+
 	/*
 	 * The guest may change the memory encryption attribute from C=0 -> C=1
 	 * or vice versa for this memory range. Lets make sure caches are
@@ -7100,13 +7111,6 @@ static int svm_register_enc_region(struct kvm *kvm,
 	 */
 	sev_clflush_pages(region->pages, region->npages);
 
-	region->uaddr = range->addr;
-	region->size = range->size;
-
-	mutex_lock(&kvm->lock);
-	list_add_tail(&region->list, &sev->regions_list);
-	mutex_unlock(&kvm->lock);
-
 	return ret;
 
 e_free:
-- 
2.30.0.478.g8a0d178c01-goog