[PATCH v2 4/5] KVM: x86: Use __try_cmpxchg_user() to emulate atomic accesses

Sean Christopherson
2022-02-02T00:50:10Z

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: 42e35f8072c3 ("KVM/X86: Use kvm_vcpu_map in emulator_cmpxchg_emulated")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/x86.c | 35 ++++++++++++++---------------------
1 file changed, 14 insertions(+), 21 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 74b53a16f38a..c9cac3100f77 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7155,15 +7155,8 @@ 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,
@@ -7172,12 +7165,11 @@ 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)))
@@ -7201,31 +7193,32 @@ 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);
--
2.35.0.rc2.247.g8bbb082509-goog