KVM: x86: Use __try_cmpxchg_user() to emulate atomic accesses

Use the recently introduce __try_cmpxchg_user() to emulate atomic guest
accesses via the associated userspace address instead of mapping the
backing pfn into kernel address space.  Using kvm_vcpu_map() is unsafe as
it does not coordinate with KVM's mmu_notifier to ensure the hva=>pfn
translation isn't changed/unmapped in the memremap() path, i.e. when
there's no struct page and thus no elevated refcount.

Fixes: 27c238573dcc ("KVM/X86: Use kvm_vcpu_map in emulator_cmpxchg_emulated")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220202004945.2540433-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index fb8153d584c80018dbf29888e71b05e5765b8e33..e5b0dd24b200aa5a62b4265c77a1c841476b7bad 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7257,15 +7257,8 @@ static int emulator_write_emulated(struct x86_emulate_ctxt *ctxt,
                                   exception, &write_emultor);
 }
 
-#define CMPXCHG_TYPE(t, ptr, old, new) \
-       (cmpxchg((t *)(ptr), *(t *)(old), *(t *)(new)) == *(t *)(old))
-
-#ifdef CONFIG_X86_64
-#  define CMPXCHG64(ptr, old, new) CMPXCHG_TYPE(u64, ptr, old, new)
-#else
-#  define CMPXCHG64(ptr, old, new) \
-       (cmpxchg64((u64 *)(ptr), *(u64 *)(old), *(u64 *)(new)) == *(u64 *)(old))
-#endif
+#define emulator_try_cmpxchg_user(t, ptr, old, new) \
+       (__try_cmpxchg_user((t __user *)(ptr), (t *)(old), *(t *)(new), efault ## t))
 
 static int emulator_cmpxchg_emulated(struct x86_emulate_ctxt *ctxt,
                                     unsigned long addr,
@@ -7274,12 +7267,11 @@ static int emulator_cmpxchg_emulated(struct x86_emulate_ctxt *ctxt,
                                     unsigned int bytes,
                                     struct x86_exception *exception)
 {
-       struct kvm_host_map map;
        struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
        u64 page_line_mask;
+       unsigned long hva;
        gpa_t gpa;
-       char *kaddr;
-       bool exchanged;
+       int r;
 
        /* guests cmpxchg8b have to be emulated atomically */
        if (bytes > 8 || (bytes & (bytes - 1)))
@@ -7303,31 +7295,32 @@ static int emulator_cmpxchg_emulated(struct x86_emulate_ctxt *ctxt,
        if (((gpa + bytes - 1) & page_line_mask) != (gpa & page_line_mask))
                goto emul_write;
 
-       if (kvm_vcpu_map(vcpu, gpa_to_gfn(gpa), &map))
+       hva = kvm_vcpu_gfn_to_hva(vcpu, gpa_to_gfn(gpa));
+       if (kvm_is_error_hva(addr))
                goto emul_write;
 
-       kaddr = map.hva + offset_in_page(gpa);
+       hva += offset_in_page(gpa);
 
        switch (bytes) {
        case 1:
-               exchanged = CMPXCHG_TYPE(u8, kaddr, old, new);
+               r = emulator_try_cmpxchg_user(u8, hva, old, new);
                break;
        case 2:
-               exchanged = CMPXCHG_TYPE(u16, kaddr, old, new);
+               r = emulator_try_cmpxchg_user(u16, hva, old, new);
                break;
        case 4:
-               exchanged = CMPXCHG_TYPE(u32, kaddr, old, new);
+               r = emulator_try_cmpxchg_user(u32, hva, old, new);
                break;
        case 8:
-               exchanged = CMPXCHG64(kaddr, old, new);
+               r = emulator_try_cmpxchg_user(u64, hva, old, new);
                break;
        default:
                BUG();
        }
 
-       kvm_vcpu_unmap(vcpu, &map, true);
-
-       if (!exchanged)
+       if (r < 0)
+               goto emul_write;
+       if (r)
                return X86EMUL_CMPXCHG_FAILED;
 
        kvm_page_track_write(vcpu, gpa, new, bytes);