FAILED: patch "[PATCH] KVM: arm64: Don't retire aborted MMIO instruction" failed to apply to 5.10-stable tree

The patch below does not apply to the 5.10-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.10.y
git checkout FETCH_HEAD
git cherry-pick -x e735a5da64420a86be370b216c269b5dd8e830e2
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2024120209-cranberry-font-7473@gregkh' --subject-prefix 'PATCH 5.10.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From e735a5da64420a86be370b216c269b5dd8e830e2 Mon Sep 17 00:00:00 2001
From: Oliver Upton <[email protected]>
Date: Fri, 25 Oct 2024 20:31:03 +0000
Subject: [PATCH] KVM: arm64: Don't retire aborted MMIO instruction

Returning an abort to the guest for an unsupported MMIO access is a documented feature of the KVM UAPI. Nevertheless, it's clear that this plumbing has seen limited testing, since userspace can trivially cause a WARN in the MMIO return:

  WARNING: CPU: 0 PID: 30558 at arch/arm64/include/asm/kvm_emulate.h:536 kvm_handle_mmio_return+0x46c/0x5c4 arch/arm64/include/asm/kvm_emulate.h:536
  Call trace:
   kvm_handle_mmio_return+0x46c/0x5c4 arch/arm64/include/asm/kvm_emulate.h:536
   kvm_arch_vcpu_ioctl_run+0x98/0x15b4 arch/arm64/kvm/arm.c:1133
   kvm_vcpu_ioctl+0x75c/0xa78 virt/kvm/kvm_main.c:4487
   __do_sys_ioctl fs/ioctl.c:51 [inline]
   __se_sys_ioctl fs/ioctl.c:893 [inline]
   __arm64_sys_ioctl+0x14c/0x1c8 fs/ioctl.c:893
   __invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
   invoke_syscall+0x98/0x2b8 arch/arm64/kernel/syscall.c:49
   el0_svc_common+0x1e0/0x23c arch/arm64/kernel/syscall.c:132
   do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:151
   el0_svc+0x38/0x68 arch/arm64/kernel/entry-common.c:712
   el0t_64_sync_handler+0x90/0xfc arch/arm64/kernel/entry-common.c:730
   el0t_64_sync+0x190/0x194 arch/arm64/kernel/entry.S:598

The splat is complaining that KVM is advancing PC while an exception is pending, i.e. that KVM is retiring the MMIO instruction despite a pending synchronous external abort. Womp womp.

Fix the glaring UAPI bug by skipping over all the MMIO emulation in case there is a pending synchronous exception. Note that while userspace is capable of pending an asynchronous exception (SError, IRQ, or FIQ), it is still safe to retire the MMIO instruction in this case as (1) they are by definition asynchronous, and (2) KVM relies on hardware support for pending/delivering these exceptions instead of the software state machine for advancing PC.

index cd6b7b83e2c3..ab365e839874 100644
@@ -72,6 +72,31 @@ unsigned long kvm_mmio_read_buf(const void *buf, unsigned int len)
 	return data;
 }
 
+static bool kvm_pending_sync_exception(struct kvm_vcpu *vcpu)
+{
+	if (!vcpu_get_flag(vcpu, PENDING_EXCEPTION))
+		return false;
+
+	if (vcpu_el1_is_32bit(vcpu)) {
+		switch (vcpu_get_flag(vcpu, EXCEPT_MASK)) {
+		case unpack_vcpu_flag(EXCEPT_AA32_UND):
+		case unpack_vcpu_flag(EXCEPT_AA32_IABT):
+		case unpack_vcpu_flag(EXCEPT_AA32_DABT):
+			return true;
+		default:
+			return false;
+		}
+	} else {
+		switch (vcpu_get_flag(vcpu, EXCEPT_MASK)) {
+		case unpack_vcpu_flag(EXCEPT_AA64_EL1_SYNC):
+		case unpack_vcpu_flag(EXCEPT_AA64_EL2_SYNC):
+			return true;
+		default:
+			return false;
+		}
+	}
+}
+
 /**
  * kvm_handle_mmio_return -- Handle MMIO loads after user space emulation
  *			     or in-kernel IO emulation

@@ -84,8 +109,11 @@ int kvm_handle_mmio_return(struct kvm_vcpu *vcpu)
 	unsigned int len;
 	int mask;
 
-	/* Detect an already handled MMIO return */
-	if (unlikely(!vcpu->mmio_needed))
+	/*
+	 * Detect if the MMIO return was already handled or if userspace aborted
+	 * the MMIO access.
+	 */
+	if (unlikely(!vcpu->mmio_needed || kvm_pending_sync_exception(vcpu)))
 		return 1;
 
 	vcpu->mmio_needed = 0;