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.

BUG=b/242694075
TEST=http://sponge2/5746147e-f595-4b7e-a4d3-709a5d307e2b
RELEASE_NOTE=Fixed CVE-2022-1158 in Linux Kernel
SOURCE=UPSTREAM(f122dfe4476890d60b8c679128cd2259ec96a24c)

cos-patch: security-high
Fixes: bd53cb35a3e9 ("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>
Change-Id: I8eab2801d2a6c322f5530f2e14214d5623d85010
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/36294
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Main-Branch-Verified: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Meena Shanmugam <meenashanmugam@google.com>

arch/x86/kvm/paging_tmpl.h

diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index 1a1d2b5..d1a5e5d 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -129,45 +129,6 @@ static inline int FNAME(is_present_gpte)(unsigned long pte)
 #endif
 }
 
-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)
-{
-	int r = -EFAULT;
-
-	if (!user_access_begin(ptep_user, sizeof(pt_element_t)))
-		return -EFAULT;
-
-#ifdef CMPXCHG
-	asm volatile("1:" LOCK_PREFIX CMPXCHG " %[new], %[ptr]\n"
-		     "mov $0, %[r]\n"
-		     "setnz %b[r]\n"
-		     "2:"
-		     _ASM_EXTABLE_UA(1b, 2b)
-		     : [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"
-		     "movl $0, %[r]\n"
-		     "jz 2f\n"
-		     "incl %[r]\n"
-		     "2:"
-		     _ASM_EXTABLE_UA(1b, 2b)
-		     : [ptr] "+m" (*ptep_user),
-		       [old] "+A" (orig_pte),
-		       [r] "+rm" (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)
@@ -266,7 +227,7 @@ 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;