KVM: MMU: kvm_pv_mmu_op should not take mmap_sem

KVM: MMU: kvm_pv_mmu_op should not take mmap_sem

[Marcelo Tosatti]

kvm_pv_mmu_op should not take mmap_sem. All gfn_to_page() callers down
in the MMU processing will take it if necessary, so as it is it can
deadlock.

Apparently a leftover from the days before slots_lock.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>


diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 078a7f1..2ad6f54 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -2173,8 +2173,6 @@ int kvm_pv_mmu_op(struct kvm_vcpu *vcpu, unsigned long bytes,
 	int r;
 	struct kvm_pv_mmu_op_buffer buffer;
 
-	down_read(&current->mm->mmap_sem);
-
 	buffer.ptr = buffer.buf;
 	buffer.len = min_t(unsigned long, bytes, sizeof buffer.buf);
 	buffer.processed = 0;
@@ -2194,7 +2192,6 @@ int kvm_pv_mmu_op(struct kvm_vcpu *vcpu, unsigned long bytes,
 	r = 1;
 out:
 	*ret = buffer.processed;
-	up_read(&current->mm->mmap_sem);
 	return r;
 }
 

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Re: KVM: MMU: kvm_pv_mmu_op should not take mmap_sem

[Avi Kivity]

Marcelo Tosatti wrote:
> kvm_pv_mmu_op should not take mmap_sem. All gfn_to_page() callers down
> in the MMU processing will take it if necessary, so as it is it can
> deadlock.
>
> Apparently a leftover from the days before slots_lock.
>   

Applied, thanks.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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