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KVM: Make locked operations truly atomic

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Once upon a time, locked operations were emulated while holding the mmu mutex.
Since mmu pages were write protected, it was safe to emulate the writes in
a non-atomic manner, since there could be no other writer, either in the
guest or in the kernel.

These days emulation takes place without holding the mmu spinlock, so the
write could be preempted by an unshadowing event, which exposes the page
to writes by the guest.  This may cause corruption of guest page tables.

Fix by using an atomic cmpxchg for these operations.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
This commit is contained in:
Avi Kivity 2010-03-15 13:59:54 +02:00
parent 72016f3a42
commit daea3e73cb
1 changed files with 48 additions and 21 deletions
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69
arch/x86/kvm/x86.c
View file

@ -3291,41 +3291,68 @@ int emulator_write_emulated(unsigned long addr,
}
EXPORT_SYMBOL_GPL(emulator_write_emulated);
#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), *(u *)(new)) == *(u64 *)(old))
#endif
static int emulator_cmpxchg_emulated(unsigned long addr,
const void *old,
const void *new,
unsigned int bytes,
struct kvm_vcpu *vcpu)
{
printk_once(KERN_WARNING "kvm: emulating exchange as write\n");
#ifndef CONFIG_X86_64
gpa_t gpa;
struct page *page;
char *kaddr;
bool exchanged;
/* guests cmpxchg8b have to be emulated atomically */
if (bytes == 8) {
gpa_t gpa;
struct page *page;
char *kaddr;
u64 val;
if (bytes > 8 || (bytes & (bytes - 1)))
goto emul_write;
gpa = kvm_mmu_gva_to_gpa_write(vcpu, addr, NULL);
gpa = kvm_mmu_gva_to_gpa_write(vcpu, addr, NULL);
if (gpa == UNMAPPED_GVA ||
(gpa & PAGE_MASK) == APIC_DEFAULT_PHYS_BASE)
goto emul_write;
if (gpa == UNMAPPED_GVA ||
(gpa & PAGE_MASK) == APIC_DEFAULT_PHYS_BASE)
goto emul_write;
if (((gpa + bytes - 1) & PAGE_MASK) != (gpa & PAGE_MASK))
goto emul_write;
if (((gpa + bytes - 1) & PAGE_MASK) != (gpa & PAGE_MASK))
goto emul_write;
val = *(u64 *)new;
page = gfn_to_page(vcpu->kvm, gpa >> PAGE_SHIFT);
page = gfn_to_page(vcpu->kvm, gpa >> PAGE_SHIFT);
kaddr = kmap_atomic(page, KM_USER0);
set_64bit((u64 *)(kaddr + offset_in_page(gpa)), val);
kunmap_atomic(kaddr, KM_USER0);
kvm_release_page_dirty(page);
kaddr = kmap_atomic(page, KM_USER0);
kaddr += offset_in_page(gpa);
switch (bytes) {
case 1:
exchanged = CMPXCHG_TYPE(u8, kaddr, old, new);
break;
case 2:
exchanged = CMPXCHG_TYPE(u16, kaddr, old, new);
break;
case 4:
exchanged = CMPXCHG_TYPE(u32, kaddr, old, new);
break;
case 8:
exchanged = CMPXCHG64(kaddr, old, new);
break;
default:
BUG();
}
kunmap_atomic(kaddr, KM_USER0);
kvm_release_page_dirty(page);
if (!exchanged)
return X86EMUL_CMPXCHG_FAILED;
emul_write:
#endif
printk_once(KERN_WARNING "kvm: emulating exchange as write\n");
return emulator_write_emulated(addr, new, bytes, vcpu);
}