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    Subject[PATCH 5.15 151/201] KVM: x86: Use __try_cmpxchg_user() to emulate atomic accesses
    Date
    From: Sean Christopherson <seanjc@google.com>

    [ Upstream commit 1c2361f667f3648855ceae25f1332c18413fdb9f ]

    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>
    Message-Id: <20220202004945.2540433-5-seanjc@google.com>
    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    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 4525d0b25a43..f9802ceed60a 100644
    --- a/arch/x86/kvm/x86.c
    +++ b/arch/x86/kvm/x86.c
    @@ -6894,15 +6894,8 @@ 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,
    @@ -6911,12 +6904,11 @@ 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)))
    @@ -6940,31 +6932,32 @@ 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);
    --
    2.35.1


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