[PATCH 14/22] KVM: arm64: Grant write permission when DBM is set at S1

[Oliver Upton <oupton@kernel.org>]

The DBM bit effectively behaves as a permission bit when using direct
permissions, especially so in the context of accesses that do not update
the dirty state (like AT).

Treat AP[1] == 0 for the purposes of computing permissions, relying on
the walker to abort if it fails to set the dirty state where required.

Signed-off-by: Oliver Upton <oupton@kernel.org>
---
 arch/arm64/kvm/at.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kvm/at.c b/arch/arm64/kvm/at.c
index bfba31b270a9..faffb69c2eef 100644
--- a/arch/arm64/kvm/at.c
+++ b/arch/arm64/kvm/at.c
@@ -1077,11 +1077,23 @@ static void compute_s1_direct_permissions(struct kvm_vcpu *vcpu,
 					  struct s1_walk_info *wi,
 					  struct s1_walk_result *wr)
 {
+	u8 ap = FIELD_GET(PTE_USER | PTE_RDONLY, wr->desc);
 	bool wxn;
 
+	/*
+	 * Descriptors with the DBM bit set while hardware dirty state are
+	 * considered writable, even though certain accesses (like AT instructions)
+	 * don't actually update the dirty state.
+	 *
+	 * Assume that walk_s1() made the necessary descriptor updates for the
+	 * access and just treat DBM as writable here.
+	 */
+	if (wi->hd && (wr->desc & PTE_DBM))
+		ap &= ~BIT(1);
+
 	/* Non-hierarchical part of AArch64.S1DirectBasePermissions() */
 	if (wi->regime != TR_EL2) {
-		switch (FIELD_GET(PTE_USER | PTE_RDONLY, wr->desc)) {
+		switch (ap) {
 		case 0b00:
 			wr->pr = wr->pw = true;
 			wr->ur = wr->uw = false;
@@ -1105,7 +1117,7 @@ static void compute_s1_direct_permissions(struct kvm_vcpu *vcpu,
 	} else {
 		wr->ur = wr->uw = wr->ux = false;
 
-		if (!(wr->desc & PTE_RDONLY)) {
+		if (!(ap & BIT(1))) {
 			wr->pr = wr->pw = true;
 		} else {
 			wr->pr = true;
-- 
2.47.3