Re: [PATCH v11 0/7] KVM: PPC: Driver to manage pages of secure guest

[Bharata B Rao <bharata@linux.ibm.com>]

On Mon, Nov 25, 2019 at 08:36:24AM +0530, Bharata B Rao wrote:
> Hi,
> 
> This is the next version of the patchset that adds required support
> in the KVM hypervisor to run secure guests on PEF-enabled POWER platforms.
> 

Here is a fix for the issue Hugh identified with the usage of ksm_madvise()
in this patchset. It applies on top of this patchset.
----

From 8a4d769bf4c61f921c79ce68923be3c403bd5862 Mon Sep 17 00:00:00 2001
From: Bharata B Rao <bharata@linux.ibm.com>
Date: Thu, 28 Nov 2019 09:31:54 +0530
Subject: [PATCH 1/1] KVM: PPC: Book3S HV: Take write mmap_sem when calling
 ksm_madvise

In order to prevent the device private pages (that correspond to
pages of secure guest) from participating in KSM merging, H_SVM_PAGE_IN
calls ksm_madvise() under read version of mmap_sem. However ksm_madvise()
needs to be under write lock, fix this.

Signed-off-by: Bharata B Rao <bharata@linux.ibm.com>
---
 arch/powerpc/kvm/book3s_hv_uvmem.c | 29 ++++++++++++++++++++---------
 1 file changed, 20 insertions(+), 9 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_hv_uvmem.c b/arch/powerpc/kvm/book3s_hv_uvmem.c
index f24ac3cfb34c..2de264fc3156 100644
--- a/arch/powerpc/kvm/book3s_hv_uvmem.c
+++ b/arch/powerpc/kvm/book3s_hv_uvmem.c
@@ -46,11 +46,10 @@
  *
  * Locking order
  *
- * 1. srcu_read_lock(&kvm->srcu) - Protects KVM memslots
- * 2. down_read(&kvm->mm->mmap_sem) - find_vma, migrate_vma_pages and helpers
- * 3. mutex_lock(&kvm->arch.uvmem_lock) - protects read/writes to uvmem slots
- *					  thus acting as sync-points
- *					  for page-in/out
+ * 1. kvm->srcu - Protects KVM memslots
+ * 2. kvm->mm->mmap_sem - find_vma, migrate_vma_pages and helpers, ksm_madvise
+ * 3. kvm->arch.uvmem_lock - protects read/writes to uvmem slots thus acting
+ *			     as sync-points for page-in/out
  */
 
 /*
@@ -344,7 +343,7 @@ static struct page *kvmppc_uvmem_get_page(unsigned long gpa, struct kvm *kvm)
 static int
 kvmppc_svm_page_in(struct vm_area_struct *vma, unsigned long start,
 		   unsigned long end, unsigned long gpa, struct kvm *kvm,
-		   unsigned long page_shift)
+		   unsigned long page_shift, bool *downgrade)
 {
 	unsigned long src_pfn, dst_pfn = 0;
 	struct migrate_vma mig;
@@ -360,8 +359,15 @@ kvmppc_svm_page_in(struct vm_area_struct *vma, unsigned long start,
 	mig.src = &src_pfn;
 	mig.dst = &dst_pfn;
 
+	/*
+	 * We come here with mmap_sem write lock held just for
+	 * ksm_madvise(), otherwise we only need read mmap_sem.
+	 * Hence downgrade to read lock once ksm_madvise() is done.
+	 */
 	ret = ksm_madvise(vma, vma->vm_start, vma->vm_end,
 			  MADV_UNMERGEABLE, &vma->vm_flags);
+	downgrade_write(&kvm->mm->mmap_sem);
+	*downgrade = true;
 	if (ret)
 		return ret;
 
@@ -456,6 +462,7 @@ unsigned long
 kvmppc_h_svm_page_in(struct kvm *kvm, unsigned long gpa,
 		     unsigned long flags, unsigned long page_shift)
 {
+	bool downgrade = false;
 	unsigned long start, end;
 	struct vm_area_struct *vma;
 	int srcu_idx;
@@ -476,7 +483,7 @@ kvmppc_h_svm_page_in(struct kvm *kvm, unsigned long gpa,
 
 	ret = H_PARAMETER;
 	srcu_idx = srcu_read_lock(&kvm->srcu);
-	down_read(&kvm->mm->mmap_sem);
+	down_write(&kvm->mm->mmap_sem);
 
 	start = gfn_to_hva(kvm, gfn);
 	if (kvm_is_error_hva(start))
@@ -492,12 +499,16 @@ kvmppc_h_svm_page_in(struct kvm *kvm, unsigned long gpa,
 	if (!vma || vma->vm_start > start || vma->vm_end < end)
 		goto out_unlock;
 
-	if (!kvmppc_svm_page_in(vma, start, end, gpa, kvm, page_shift))
+	if (!kvmppc_svm_page_in(vma, start, end, gpa, kvm, page_shift,
+				&downgrade))
 		ret = H_SUCCESS;
 out_unlock:
 	mutex_unlock(&kvm->arch.uvmem_lock);
 out:
-	up_read(&kvm->mm->mmap_sem);
+	if (downgrade)
+		up_read(&kvm->mm->mmap_sem);
+	else
+		up_write(&kvm->mm->mmap_sem);
 	srcu_read_unlock(&kvm->srcu, srcu_idx);
 	return ret;
 }
-- 
2.21.0