diff mbox series

[v2,4.19,1/3] KVM: do not assume PTE is writable after follow_pfn

Message ID 20210727082924.2336367-1-ovidiu.panait@windriver.com
State Superseded
Headers show
Series [v2,4.19,1/3] KVM: do not assume PTE is writable after follow_pfn | expand

Commit Message

Ovidiu Panait July 27, 2021, 8:29 a.m. UTC 
From: Paolo Bonzini <pbonzini@redhat.com>

commit bd2fae8da794b55bf2ac02632da3a151b10e664c upstream.

In order to convert an HVA to a PFN, KVM usually tries to use
the get_user_pages family of functinso.  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@google.com>
Cc: 3pvd@google.com
Cc: Jann Horn <jannh@google.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[OP: backport to 4.19, adjust follow_pte() -> follow_pte_pmd()]
Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
---
 virt/kvm/kvm_main.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

Comments

Paolo Bonzini July 27, 2021, 11:01 a.m. UTC | #1 
On 27/07/21 10:29, Ovidiu Panait wrote:
> From: Sean Christopherson <seanjc@google.com>
> 
> commit a9545779ee9e9e103648f6f2552e73cfe808d0f4 upstream
> 
> Use kvm_pfn_t, a.k.a. u64, for the local 'pfn' variable when retrieving
> a so called "remapped" hva/pfn pair.  In theory, the hva could resolve to
> a pfn in high memory on a 32-bit kernel.
> 
> This bug was inadvertantly exposed by commit bd2fae8da794 ("KVM: do not
> assume PTE is writable after follow_pfn"), which added an error PFN value
> to the mix, causing gcc to comlain about overflowing the unsigned long.
> 
>    arch/x86/kvm/../../../virt/kvm/kvm_main.c: In function ‘hva_to_pfn_remapped’:
>    include/linux/kvm_host.h:89:30: error: conversion from ‘long long unsigned int’
>                                    to ‘long unsigned int’ changes value from
>                                    ‘9218868437227405314’ to ‘2’ [-Werror=overflow]
>     89 | #define KVM_PFN_ERR_RO_FAULT (KVM_PFN_ERR_MASK + 2)
>        |                              ^
> virt/kvm/kvm_main.c:1935:9: note: in expansion of macro ‘KVM_PFN_ERR_RO_FAULT’
> 
> Cc: stable@vger.kernel.org
> Fixes: add6a0cd1c5b ("KVM: MMU: try to fix up page faults before giving up")
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Message-Id: <20210208201940.1258328-1-seanjc@google.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
> ---
>   virt/kvm/kvm_main.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 3559eba5f502..a3d82113ae1c 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1501,7 +1501,7 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
>   			       bool write_fault, bool *writable,
>   			       kvm_pfn_t *p_pfn)
>   {
> -	unsigned long pfn;
> +	kvm_pfn_t pfn;
>   	pte_t *ptep;
>   	spinlock_t *ptl;
>   	int r;
> 

Acked-by: Paolo Bonzini <pbonzini@redhat.com>

and the other two are the same as v1.

Paolo
diff mbox series

Patch

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 1ecb27b3421a..6aeac96bf147 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1495,9 +1495,11 @@  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_pmd(vma->vm_mm, addr, NULL, NULL, &ptep, NULL, &ptl);
 	if (r) {
 		/*
 		 * get_user_pages fails for VM_IO and VM_PFNMAP vmas and does
@@ -1512,14 +1514,19 @@  static int hva_to_pfn_remapped(struct vm_area_struct *vma,
 		if (r)
 			return r;
 
-		r = follow_pfn(vma, addr, &pfn);
+		r = follow_pte_pmd(vma->vm_mm, addr, NULL, NULL, &ptep, NULL, &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
@@ -1534,6 +1541,8 @@  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;
 }