linux-kernel - [PATCH 5.4] KVM: x86/mmu: do compare-and-exchange of gPTE via the user address

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Date:   Mon,  4 Apr 2022 09:41:41 -0400
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     stable@...r.kernel.org, Qiuhao Li <qiuhao@...ec.org>,
        Gaoning Pan <pgn@....edu.cn>, Yongkang Jia <kangel@....edu.cn>,
        syzbot+6cde2282daa792c49ab8@...kaller.appspotmail.com,
        Tadeusz Struk <tadeusz.struk@...aro.org>,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: [PATCH 5.4] KVM: x86/mmu: do compare-and-exchange of gPTE via the user address

commit 2a8859f373b0a86f0ece8ec8312607eacf12485d upstream.

FNAME(cmpxchg_gpte) is an inefficient mess.  It is at least decent if it
can go through get_user_pages_fast(), but if it cannot then it tries to
use memremap(); that is not just terribly slow, it is also wrong because
it assumes that the VM_PFNMAP VMA is contiguous.

The right way to do it would be to do the same thing as
hva_to_pfn_remapped() does since commit add6a0cd1c5b ("KVM: MMU: try to
fix up page faults before giving up", 2016-07-05), using follow_pte()
and fixup_user_fault() to determine the correct address to use for
memremap().  To do this, one could for example extract hva_to_pfn()
for use outside virt/kvm/kvm_main.c.  But really there is no reason to
do that either, because there is already a perfectly valid address to
do the cmpxchg() on, only it is a userspace address.  That means doing
user_access_begin()/user_access_end() and writing the code in assembly
to handle any exception correctly.  Worse, the guest PTE can be 8-byte
even on i686 so there is the extra complication of using cmpxchg8b to
account for.  But at least it is an efficient mess.

Reported-by: Qiuhao Li <qiuhao@...ec.org>
Reported-by: Gaoning Pan <pgn@....edu.cn>
Reported-by: Yongkang Jia <kangel@....edu.cn>
Reported-by: syzbot+6cde2282daa792c49ab8@...kaller.appspotmail.com
Debugged-by: Tadeusz Struk <tadeusz.struk@...aro.org>
Tested-by: Maxim Levitsky <mlevitsk@...hat.com>
Cc: stable@...r.kernel.org
Fixes: bd53cb35a3e9 ("X86/KVM: Handle PFNs outside of kernel reach when touching GPTEs")
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/paging_tmpl.h | 77 ++++++++++++++++++--------------------
 1 file changed, 37 insertions(+), 40 deletions(-)

diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index 97b21e7fd013..13b5c424adb2 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -34,9 +34,8 @@
 	#define PT_HAVE_ACCESSED_DIRTY(mmu) true
 	#ifdef CONFIG_X86_64
 	#define PT_MAX_FULL_LEVELS 4
-	#define CMPXCHG cmpxchg
+	#define CMPXCHG "cmpxchgq"
 	#else
-	#define CMPXCHG cmpxchg64
 	#define PT_MAX_FULL_LEVELS 2
 	#endif
 #elif PTTYPE == 32
@@ -52,7 +51,7 @@
 	#define PT_GUEST_DIRTY_SHIFT PT_DIRTY_SHIFT
 	#define PT_GUEST_ACCESSED_SHIFT PT_ACCESSED_SHIFT
 	#define PT_HAVE_ACCESSED_DIRTY(mmu) true
-	#define CMPXCHG cmpxchg
+	#define CMPXCHG "cmpxchgl"
 #elif PTTYPE == PTTYPE_EPT
 	#define pt_element_t u64
 	#define guest_walker guest_walkerEPT
@@ -65,8 +64,10 @@
 	#define PT_GUEST_DIRTY_SHIFT 9
 	#define PT_GUEST_ACCESSED_SHIFT 8
 	#define PT_HAVE_ACCESSED_DIRTY(mmu) ((mmu)->ept_ad)
-	#define CMPXCHG cmpxchg64
 	#define PT_MAX_FULL_LEVELS 4
+	#ifdef CONFIG_X86_64
+	#define CMPXCHG "cmpxchgq"
+	#endif
 #else
 	#error Invalid PTTYPE value
 #endif
@@ -132,43 +133,39 @@ 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 npages;
-	pt_element_t ret;
-	pt_element_t *table;
-	struct page *page;
-
-	npages = get_user_pages_fast((unsigned long)ptep_user, 1, FOLL_WRITE, &page);
-	if (likely(npages == 1)) {
-		table = kmap_atomic(page);
-		ret = CMPXCHG(&table[index], orig_pte, new_pte);
-		kunmap_atomic(table);
-
-		kvm_release_page_dirty(page);
-	} else {
-		struct vm_area_struct *vma;
-		unsigned long vaddr = (unsigned long)ptep_user & PAGE_MASK;
-		unsigned long pfn;
-		unsigned long paddr;
-
-		down_read(&current->mm->mmap_sem);
-		vma = find_vma_intersection(current->mm, vaddr, vaddr + PAGE_SIZE);
-		if (!vma || !(vma->vm_flags & VM_PFNMAP)) {
-			up_read(&current->mm->mmap_sem);
-			return -EFAULT;
-		}
-		pfn = ((vaddr - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
-		paddr = pfn << PAGE_SHIFT;
-		table = memremap(paddr, PAGE_SIZE, MEMREMAP_WB);
-		if (!table) {
-			up_read(&current->mm->mmap_sem);
-			return -EFAULT;
-		}
-		ret = CMPXCHG(&table[index], orig_pte, new_pte);
-		memunmap(table);
-		up_read(&current->mm->mmap_sem);
-	}
+	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
 
-	return (ret != orig_pte);
+	user_access_end();
+	return r;
 }
 
 static bool FNAME(prefetch_invalid_gpte)(struct kvm_vcpu *vcpu,
-- 
2.31.1