[PATCH] powerpc/kvm: Fix PR KVM with KUAP/MEM_KEYS enabled

The changes to add KUAP support with the hash MMU broke booting of KVM
PR guests. The symptom is no visible progress of the guest, or possibly
just "SLOF" being printed to the qemu console.

Host code is still executing, but breaking into xmon might show a stack
trace such as:

  __might_fault+0x84/0xe0 (unreliable)
  kvm_read_guest+0x1c8/0x2f0 [kvm]
  kvmppc_ld+0x1b8/0x2d0 [kvm]
  kvmppc_load_last_inst+0x50/0xa0 [kvm]
  kvmppc_exit_pr_progint+0x178/0x220 [kvm_pr]
  kvmppc_handle_exit_pr+0x31c/0xe30 [kvm_pr]
  after_sprg3_load+0x80/0x90 [kvm_pr]
  kvmppc_vcpu_run_pr+0x104/0x260 [kvm_pr]
  kvmppc_vcpu_run+0x34/0x48 [kvm]
  kvm_arch_vcpu_ioctl_run+0x340/0x450 [kvm]
  kvm_vcpu_ioctl+0x2ac/0x8c0 [kvm]
  sys_ioctl+0x320/0x1060
  system_call_exception+0x160/0x270
  system_call_common+0xf0/0x27c

Bisect points to commit b2ff33a10c8b ("powerpc/book3s64/hash/kuap:
Enable kuap on hash"), but that's just the commit that enabled KUAP with
hash and made the bug visible.

The root cause seems to be that KVM PR is creating kernel mappings that
don't use the correct key, since we switched to using key 3.

We have a helper for adding the right key value, however it's designed
to take a pteflags variable, which the KVM code doesn't have. But we can
make it work by passing 0 for the pteflags, and tell it explicitly that
it should use the kernel key.

With that changed guests boot successfully.

Fixes: d94b827e89dc ("powerpc/book3s64/kuap: Use Key 3 for kernel mapping with hash translation")
Cc: stable@vger.kernel.org # v5.11+
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 arch/powerpc/kvm/book3s_64_mmu_host.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/powerpc/kvm/book3s_64_mmu_host.c b/arch/powerpc/kvm/book3s_64_mmu_host.c
index e452158a18d7..5ac66be1cb3c 100644
--- a/arch/powerpc/kvm/book3s_64_mmu_host.c
+++ b/arch/powerpc/kvm/book3s_64_mmu_host.c

@@ -12,6 +12,7 @@
 #include <asm/kvm_ppc.h>
 #include <asm/kvm_book3s.h>
 #include <asm/book3s/64/mmu-hash.h>
+#include <asm/book3s/64/pkeys.h>
 #include <asm/machdep.h>
 #include <asm/mmu_context.h>
 #include <asm/hw_irq.h>

@@ -133,6 +134,7 @@ int kvmppc_mmu_map_page(struct kvm_vcpu *vcpu, struct kvmppc_pte *orig_pte,
 	else
 		kvmppc_mmu_flush_icache(pfn);
 
+	rflags |= pte_to_hpte_pkey_bits(0, HPTE_USE_KERNEL_KEY);
 	rflags = (rflags & ~HPTE_R_WIMG) | orig_pte->wimg;
 
 	/*

-- 
2.25.1

On Mon, 19 Apr 2021 22:01:39 +1000, Michael Ellerman wrote:

The changes to add KUAP support with the hash MMU broke booting of KVM
PR guests. The symptom is no visible progress of the guest, or possibly
just "SLOF" being printed to the qemu console.

Host code is still executing, but breaking into xmon might show a stack
trace such as:

[...]

Applied to powerpc/next.

[1/1] powerpc/kvm: Fix PR KVM with KUAP/MEM_KEYS enabled
      https://git.kernel.org/powerpc/c/e4e8bc1df691ba5ba749d1e2b67acf9827e51a35

cheers