KVM: x86: Use __try_cmpxchg_user() to update guest PTE A/D bits

Use the recently introduced __try_cmpxchg_user() to update guest PTE A/D
bits instead of mapping the PTE into kernel address space.  The VM_PFNMAP
path is broken as it assumes that vm_pgoff is the base pfn of the mapped
VMA range, which is conceptually wrong as vm_pgoff is the offset relative
to the file and has nothing to do with the pfn.  The horrific hack worked
for the original use case (backing guest memory with /dev/mem), but leads
to accessing "random" pfns for pretty much any other VM_PFNMAP case.

Fixes: f335d3af0395 ("X86/KVM: Handle PFNs outside of kernel reach when touching GPTEs")
Debugged-by: Tadeusz Struk <tadeusz.struk@linaro.org>
Tested-by: Tadeusz Struk <tadeusz.struk@linaro.org>
Reported-by: syzbot+6cde2282daa792c49ab8@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220202004945.2540433-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index 1dad8f3f2bb007157187f9bd71426d6a9ae154d0..7d4377f1ef2a87b87f6279b0350df2116ebee0e6 100644 (file)
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -144,42 +144,6 @@ static bool FNAME(is_rsvd_bits_set)(struct kvm_mmu *mmu, u64 gpte, int level)
               FNAME(is_bad_mt_xwr)(&mmu->guest_rsvd_check, gpte);
 }
 
-static int FNAME(cmpxchg_gpte)(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
-                              pt_element_t __user *ptep_user, unsigned index,
-                              pt_element_t orig_pte, pt_element_t new_pte)
-{
-       signed char r;
-
-       if (!user_access_begin(ptep_user, sizeof(pt_element_t)))
-               return -EFAULT;
-
-#ifdef CMPXCHG
-       asm volatile("1:" LOCK_PREFIX CMPXCHG " %[new], %[ptr]\n"
-                    "setnz %b[r]\n"
-                    "2:"
-                    _ASM_EXTABLE_TYPE_REG(1b, 2b, EX_TYPE_EFAULT_REG, %k[r])
-                    : [ptr] "+m" (*ptep_user),
-                      [old] "+a" (orig_pte),
-                      [r] "=q" (r)
-                    : [new] "r" (new_pte)
-                    : "memory");
-#else
-       asm volatile("1:" LOCK_PREFIX "cmpxchg8b %[ptr]\n"
-                    "setnz %b[r]\n"
-                    "2:"
-                    _ASM_EXTABLE_TYPE_REG(1b, 2b, EX_TYPE_EFAULT_REG, %k[r])
-                    : [ptr] "+m" (*ptep_user),
-                      [old] "+A" (orig_pte),
-                      [r] "=q" (r)
-                    : [new_lo] "b" ((u32)new_pte),
-                      [new_hi] "c" ((u32)(new_pte >> 32))
-                    : "memory");
-#endif
-
-       user_access_end();
-       return r;
-}
-
 static bool FNAME(prefetch_invalid_gpte)(struct kvm_vcpu *vcpu,
                                  struct kvm_mmu_page *sp, u64 *spte,
                                  u64 gpte)
@@ -278,7 +242,7 @@ static int FNAME(update_accessed_dirty_bits)(struct kvm_vcpu *vcpu,
                if (unlikely(!walker->pte_writable[level - 1]))
                        continue;
 
-               ret = FNAME(cmpxchg_gpte)(vcpu, mmu, ptep_user, index, orig_pte, pte);
+               ret = __try_cmpxchg_user(ptep_user, &orig_pte, pte, fault);
                if (ret)
                        return ret;