David Vrabel
2013-Aug-29 12:10 UTC
[PATCHv2] xen/p2m: check MFN is in range before using the m2p table
From: David Vrabel <david.vrabel@citrix.com>
On hosts with more than 168 GB of memory, a 32-bit guest may attempt
to grant map an MFN that is not cannot lookup in its mapping of the
m2p table. There is an m2p lookup as part of m2p_add_override() and
m2p_remove_override(). The lookup falls off the end of the mapped
portion of the m2p and (because the mapping is at the highest virtual
address) wraps around and the lookup causes a fault on what appears to
be a user space address.
do_page_fault() (thinking it''s a fault to a userspace address), tries
to lock mm->mmap_sem. If the gntdev device is used for the grant map,
m2p_add_override() is called from from gnttab_mmap() with mm->mmap_sem
already locked. do_page_fault() then deadlocks.
The deadlock would most commonly occur when a 64-bit guest is started
and xenconsoled attempts to grant map its console ring.
Introduce mfn_to_pfn_no_overrides() which checks the MFN is within the
mapped portion of the m2p table before accessing the table and use
this in m2p_add_override(), m2p_remove_override(), and mfn_to_pfn()
(which already had the correct range check).
All faults caused by accessing the non-existant parts of the m2p are
thus within the kernel address space and exception_fixup() is called
without trying to lock mm->mmap_sem.
This means that for MFNs that are outside the mapped range of the m2p
then mfn_to_pfn() will always look in the m2p overrides. This is
correct because it must be a foreign MFN (and the PFN in the m2p in
this case is only relevant for the other domain).
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@citrix.com>
Cc: Jan Beulich <JBeulich@suse.com>
--
v2: in mfn_to_pfn() look in m2p_overrides if the MFN is is out of
range as it''s probably foreign.
---
arch/x86/include/asm/xen/page.h | 32 +++++++++++++++++++-------------
arch/x86/xen/p2m.c | 10 ++++------
2 files changed, 23 insertions(+), 19 deletions(-)
diff --git a/arch/x86/include/asm/xen/page.h b/arch/x86/include/asm/xen/page.h
index 6aef9fb..979d7a2 100644
--- a/arch/x86/include/asm/xen/page.h
+++ b/arch/x86/include/asm/xen/page.h
@@ -79,30 +79,35 @@ static inline int phys_to_machine_mapping_valid(unsigned
long pfn)
return get_phys_to_machine(pfn) != INVALID_P2M_ENTRY;
}
-static inline unsigned long mfn_to_pfn(unsigned long mfn)
+static inline unsigned long mfn_to_pfn_no_overrides(unsigned long mfn)
{
unsigned long pfn;
- int ret = 0;
+ int ret;
- if (xen_feature(XENFEAT_auto_translated_physmap))
- return mfn;
+ if (unlikely(mfn >= machine_to_phys_nr))
+ return ~0;
- if (unlikely(mfn >= machine_to_phys_nr)) {
- pfn = ~0;
- goto try_override;
- }
- pfn = 0;
/*
* The array access can fail (e.g., device space beyond end of RAM).
* In such cases it doesn''t matter what we return (we return
garbage),
* but we must handle the fault without crashing!
*/
ret = __get_user(pfn, &machine_to_phys_mapping[mfn]);
-try_override:
- /* ret might be < 0 if there are no entries in the m2p for mfn */
if (ret < 0)
- pfn = ~0;
- else if (get_phys_to_machine(pfn) != mfn)
+ return ~0;
+
+ return pfn;
+}
+
+static inline unsigned long mfn_to_pfn(unsigned long mfn)
+{
+ unsigned long pfn;
+
+ if (xen_feature(XENFEAT_auto_translated_physmap))
+ return mfn;
+
+ pfn = mfn_to_pfn_no_overrides(mfn);
+ if (get_phys_to_machine(pfn) != mfn) {
/*
* If this appears to be a foreign mfn (because the pfn
* doesn''t map back to the mfn), then check the local override
@@ -111,6 +116,7 @@ try_override:
* m2p_find_override_pfn returns ~0 if it doesn''t find anything.
*/
pfn = m2p_find_override_pfn(mfn, ~0);
+ }
/*
* pfn is ~0 if there are no entries in the m2p for mfn or if the
diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
index 95fb2aa..1563444 100644
--- a/arch/x86/xen/p2m.c
+++ b/arch/x86/xen/p2m.c
@@ -878,7 +878,6 @@ int m2p_add_override(unsigned long mfn, struct page *page,
unsigned long uninitialized_var(address);
unsigned level;
pte_t *ptep = NULL;
- int ret = 0;
pfn = page_to_pfn(page);
if (!PageHighMem(page)) {
@@ -925,8 +924,8 @@ int m2p_add_override(unsigned long mfn, struct page *page,
* frontend pages while they are being shared with the backend,
* because mfn_to_pfn (that ends up being called by GUPF) will
* return the backend pfn rather than the frontend pfn. */
- ret = __get_user(pfn, &machine_to_phys_mapping[mfn]);
- if (ret == 0 && get_phys_to_machine(pfn) == mfn)
+ pfn = mfn_to_pfn_no_overrides(mfn);
+ if (get_phys_to_machine(pfn) == mfn)
set_phys_to_machine(pfn, FOREIGN_FRAME(mfn));
return 0;
@@ -941,7 +940,6 @@ int m2p_remove_override(struct page *page,
unsigned long uninitialized_var(address);
unsigned level;
pte_t *ptep = NULL;
- int ret = 0;
pfn = page_to_pfn(page);
mfn = get_phys_to_machine(pfn);
@@ -1019,8 +1017,8 @@ int m2p_remove_override(struct page *page,
* the original pfn causes mfn_to_pfn(mfn) to return the frontend
* pfn again. */
mfn &= ~FOREIGN_FRAME_BIT;
- ret = __get_user(pfn, &machine_to_phys_mapping[mfn]);
- if (ret == 0 && get_phys_to_machine(pfn) == FOREIGN_FRAME(mfn)
&&
+ pfn = mfn_to_pfn_no_overrides(mfn);
+ if (get_phys_to_machine(pfn) == FOREIGN_FRAME(mfn) &&
m2p_find_override(mfn) == NULL)
set_phys_to_machine(pfn, mfn);
--
1.7.2.5
Stefano Stabellini
2013-Aug-29 15:36 UTC
Re: [PATCHv2] xen/p2m: check MFN is in range before using the m2p table
On Thu, 29 Aug 2013, David Vrabel wrote:> From: David Vrabel <david.vrabel@citrix.com> > > On hosts with more than 168 GB of memory, a 32-bit guest may attempt > to grant map an MFN that is not cannot lookup in its mapping of the > m2p table. There is an m2p lookup as part of m2p_add_override() and > m2p_remove_override(). The lookup falls off the end of the mapped > portion of the m2p and (because the mapping is at the highest virtual > address) wraps around and the lookup causes a fault on what appears to > be a user space address. > > do_page_fault() (thinking it''s a fault to a userspace address), tries > to lock mm->mmap_sem. If the gntdev device is used for the grant map, > m2p_add_override() is called from from gnttab_mmap() with mm->mmap_sem > already locked. do_page_fault() then deadlocks. > > The deadlock would most commonly occur when a 64-bit guest is started > and xenconsoled attempts to grant map its console ring. > > Introduce mfn_to_pfn_no_overrides() which checks the MFN is within the > mapped portion of the m2p table before accessing the table and use > this in m2p_add_override(), m2p_remove_override(), and mfn_to_pfn() > (which already had the correct range check). > > All faults caused by accessing the non-existant parts of the m2p are > thus within the kernel address space and exception_fixup() is called > without trying to lock mm->mmap_sem. > > This means that for MFNs that are outside the mapped range of the m2p > then mfn_to_pfn() will always look in the m2p overrides. This is > correct because it must be a foreign MFN (and the PFN in the m2p in > this case is only relevant for the other domain). > > Signed-off-by: David Vrabel <david.vrabel@citrix.com> > Cc: Stefano Stabellini <stefano.stabellini@citrix.com> > Cc: Jan Beulich <JBeulich@suse.com> > -- > v2: in mfn_to_pfn() look in m2p_overrides if the MFN is is out of > range as it''s probably foreign. > --- > arch/x86/include/asm/xen/page.h | 32 +++++++++++++++++++------------- > arch/x86/xen/p2m.c | 10 ++++------ > 2 files changed, 23 insertions(+), 19 deletions(-) > > diff --git a/arch/x86/include/asm/xen/page.h b/arch/x86/include/asm/xen/page.h > index 6aef9fb..979d7a2 100644 > --- a/arch/x86/include/asm/xen/page.h > +++ b/arch/x86/include/asm/xen/page.h > @@ -79,30 +79,35 @@ static inline int phys_to_machine_mapping_valid(unsigned long pfn) > return get_phys_to_machine(pfn) != INVALID_P2M_ENTRY; > } > > -static inline unsigned long mfn_to_pfn(unsigned long mfn) > +static inline unsigned long mfn_to_pfn_no_overrides(unsigned long mfn) > { > unsigned long pfn; > - int ret = 0; > + int ret; > > - if (xen_feature(XENFEAT_auto_translated_physmap)) > - return mfn; > + if (unlikely(mfn >= machine_to_phys_nr)) > + return ~0; > > - if (unlikely(mfn >= machine_to_phys_nr)) { > - pfn = ~0; > - goto try_override; > - }I think we need to keep an XENFEAT_auto_translated_physmap test at the beginning of mfn_to_pfn_no_overrides: this function is exported in page.h, anybody could start using it in common code paths. Other than that, I think it''s OK.> - pfn = 0; > /* > * The array access can fail (e.g., device space beyond end of RAM). > * In such cases it doesn''t matter what we return (we return garbage), > * but we must handle the fault without crashing! > */ > ret = __get_user(pfn, &machine_to_phys_mapping[mfn]); > -try_override: > - /* ret might be < 0 if there are no entries in the m2p for mfn */ > if (ret < 0) > - pfn = ~0; > - else if (get_phys_to_machine(pfn) != mfn) > + return ~0; > + > + return pfn; > +} > + > +static inline unsigned long mfn_to_pfn(unsigned long mfn) > +{ > + unsigned long pfn; > + > + if (xen_feature(XENFEAT_auto_translated_physmap)) > + return mfn; > + > + pfn = mfn_to_pfn_no_overrides(mfn); > + if (get_phys_to_machine(pfn) != mfn) { > /* > * If this appears to be a foreign mfn (because the pfn > * doesn''t map back to the mfn), then check the local override > @@ -111,6 +116,7 @@ try_override: > * m2p_find_override_pfn returns ~0 if it doesn''t find anything. > */ > pfn = m2p_find_override_pfn(mfn, ~0); > + } > > /* > * pfn is ~0 if there are no entries in the m2p for mfn or if the > diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c > index 95fb2aa..1563444 100644 > --- a/arch/x86/xen/p2m.c > +++ b/arch/x86/xen/p2m.c > @@ -878,7 +878,6 @@ int m2p_add_override(unsigned long mfn, struct page *page, > unsigned long uninitialized_var(address); > unsigned level; > pte_t *ptep = NULL; > - int ret = 0; > > pfn = page_to_pfn(page); > if (!PageHighMem(page)) { > @@ -925,8 +924,8 @@ int m2p_add_override(unsigned long mfn, struct page *page, > * frontend pages while they are being shared with the backend, > * because mfn_to_pfn (that ends up being called by GUPF) will > * return the backend pfn rather than the frontend pfn. */ > - ret = __get_user(pfn, &machine_to_phys_mapping[mfn]); > - if (ret == 0 && get_phys_to_machine(pfn) == mfn) > + pfn = mfn_to_pfn_no_overrides(mfn); > + if (get_phys_to_machine(pfn) == mfn) > set_phys_to_machine(pfn, FOREIGN_FRAME(mfn)); > > return 0; > @@ -941,7 +940,6 @@ int m2p_remove_override(struct page *page, > unsigned long uninitialized_var(address); > unsigned level; > pte_t *ptep = NULL; > - int ret = 0; > > pfn = page_to_pfn(page); > mfn = get_phys_to_machine(pfn); > @@ -1019,8 +1017,8 @@ int m2p_remove_override(struct page *page, > * the original pfn causes mfn_to_pfn(mfn) to return the frontend > * pfn again. */ > mfn &= ~FOREIGN_FRAME_BIT; > - ret = __get_user(pfn, &machine_to_phys_mapping[mfn]); > - if (ret == 0 && get_phys_to_machine(pfn) == FOREIGN_FRAME(mfn) && > + pfn = mfn_to_pfn_no_overrides(mfn); > + if (get_phys_to_machine(pfn) == FOREIGN_FRAME(mfn) && > m2p_find_override(mfn) == NULL) > set_phys_to_machine(pfn, mfn); > > -- > 1.7.2.5 >