Commit 1c2361f6 authored by Sean Christopherson's avatar Sean Christopherson Committed by Paolo Bonzini
Browse files

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: 42e35f80 ("KVM/X86: Use kvm_vcpu_map in emulator_cmpxchg_emulated")
Cc: stable@vger.kernel.org
Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
Message-Id: <20220202004945.2540433-5-seanjc@google.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent f122dfe4

arch/x86/kvm/x86.c

+14 −21
Original line number Diff line number Diff line
@@ -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);