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Message-Id: <20220404154913.482520-1-pbonzini@redhat.com>
Date: Mon, 4 Apr 2022 11:49:13 -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 v2] 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 PT64_ROOT_MAX_LEVEL
- #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(¤t->mm->mmap_sem);
- vma = find_vma_intersection(current->mm, vaddr, vaddr + PAGE_SIZE);
- if (!vma || !(vma->vm_flags & VM_PFNMAP)) {
- up_read(¤t->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(¤t->mm->mmap_sem);
- return -EFAULT;
- }
- ret = CMPXCHG(&table[index], orig_pte, new_pte);
- memunmap(table);
- up_read(¤t->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
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