linux-kernel - [PATCH 2/2] KVM: do not assume PTE is writable after follow

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Message-Id: <20210205103259.42866-3-pbonzini@redhat.com>
Date:   Fri,  5 Feb 2021 05:32:59 -0500
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     jgg@...pe.ca, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        dan.j.williams@...el.com
Subject: [PATCH 2/2] KVM: do not assume PTE is writable after follow_pfn

In order to convert an HVA to a PFN, KVM usually tries to use
the get_user_pages family of functions.  This however is not
possible for VM_IO vmas; in that case, KVM instead uses follow_pfn.

In doing this however KVM loses the information on whether the
PFN is writable.  That is usually not a problem because the main
use of VM_IO vmas with KVM is for BARs in PCI device assignment,
however it is a bug.  To fix it, use follow_pte and check pte_write
while under the protection of the PTE lock.  The information can
be used to fail hva_to_pfn_remapped or passed back to the
caller via *writable.

Usage of follow_pfn was introduced in commit add6a0cd1c5b ("KVM: MMU: try to fix
up page faults before giving up", 2016-07-05); however, even older version
have the same issue, all the way back to commit 2e2e3738af33 ("KVM:
Handle vma regions with no backing page", 2008-07-20), as they also did
not check whether the PFN was writable.

Fixes: 2e2e3738af33 ("KVM: Handle vma regions with no backing page")
Reported-by: David Stevens <stevensd@...gle.com>
Cc: 3pvd@...gle.com
Cc: Jann Horn <jannh@...gle.com>
Cc: Jason Gunthorpe <jgg@...pe.ca>
Cc: stable@...r.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 virt/kvm/kvm_main.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index f34a85b93c2d..ee4ac2618ec5 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1907,9 +1907,11 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
 			       kvm_pfn_t *p_pfn)
 {
 	unsigned long pfn;
+	pte_t *ptep;
+	spinlock_t *ptl;
 	int r;

-	r = follow_pfn(vma, addr, &pfn);
+	r = follow_pte(vma->vm_mm, addr, &ptep, &ptl);
 	if (r) {
 		/*
 		 * get_user_pages fails for VM_IO and VM_PFNMAP vmas and does
@@ -1924,14 +1926,19 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
 		if (r)
 			return r;

-		r = follow_pfn(vma, addr, &pfn);
+		r = follow_pte(vma->vm_mm, addr, &ptep, &ptl);
 		if (r)
 			return r;
+	}

+	if (write_fault && !pte_write(*ptep)) {
+		pfn = KVM_PFN_ERR_RO_FAULT;
+		goto out;
 	}

 	if (writable)
-		*writable = true;
+		*writable = pte_write(*ptep);
+	pfn = pte_pfn(*ptep);

 	/*
 	 * Get a reference here because callers of *hva_to_pfn* and
@@ -1946,6 +1953,8 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
 	 */ 
 	kvm_get_pfn(pfn);

+out:
+	pte_unmap_unlock(ptep, ptl);
 	*p_pfn = pfn;
 	return 0;
 }
-- 
2.26.2