Patch "KVM: arm64: Don't retire aborted MMIO instruction" has been added to the 6.6-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    KVM: arm64: Don't retire aborted MMIO instruction

to the 6.6-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     kvm-arm64-don-t-retire-aborted-mmio-instruction.patch
and it can be found in the queue-6.6 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit e47b4a47b2f51b393a4cbd8b55210a915a74f5a4
Author: Oliver Upton <oliver.upton@xxxxxxxxx>
Date:   Fri Oct 25 20:31:03 2024 +0000

    KVM: arm64: Don't retire aborted MMIO instruction
    
    [ Upstream commit e735a5da64420a86be370b216c269b5dd8e830e2 ]
    
    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.
    
    Cc: stable@xxxxxxxxxxxxxxx
    Fixes: da345174ceca ("KVM: arm/arm64: Allow user injection of external data aborts")
    Reported-by: Alexander Potapenko <glider@xxxxxxxxxx>
    Reviewed-by: Marc Zyngier <maz@xxxxxxxxxx>
    Link: https://lore.kernel.org/r/20241025203106.3529261-2-oliver.upton@xxxxxxxxx
    Signed-off-by: Oliver Upton <oliver.upton@xxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/arch/arm64/kvm/mmio.c b/arch/arm64/kvm/mmio.c
index 886ef30e12196..2aa503ff742ee 100644
--- a/arch/arm64/kvm/mmio.c
+++ b/arch/arm64/kvm/mmio.c
@@ -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;