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From 67620c1ccb13f7b58645f48248ba1f408b021fdc Mon Sep 17 00:00:00 2001
From: George Dunlap <george.dunlap@citrix.com>
Date: Fri, 18 Jan 2019 15:00:34 +0000
Subject: [PATCH] steal_page: Get rid of bogus struct page states
The original rules for `struct page` required the following invariants
at all times:
- refcount > 0 implies owner != NULL
- PGC_allocated implies refcount > 0
steal_page, in a misguided attempt to protect against unknown races,
violates both of these rules, thus introducing other races:
- Temporarily, the count_info has the refcount go to 0 while
PGC_allocated is set
- It explicitly returns the page PGC_allocated set, but owner == NULL
and page not on the page_list.
The second one meant that page_get_owner_and_reference() could return
NULL even after having successfully grabbed a reference on the page,
leading the caller to leak the reference (since "couldn't get ref" and
"got ref but no owner" look the same).
Furthermore, rather than grabbing a page reference to ensure that the
owner doesn't change under its feet, it appears to rely on holding
d->page_alloc lock to prevent this.
Unfortunately, this is ineffective: page->owner remains non-NULL for
some time after the count has been set to 0; meaning that it would be
entirely possible for the page to be freed and re-allocated to a
different domain between the page_get_owner() check and the count_info
check.
Modify steal_page to instead follow the appropriate access discipline,
taking the page through series of states similar to being freed and
then re-allocated with MEMF_no_owner:
- Grab an extra reference to make sure we don't race with anyone else
freeing the page
- Drop both references and PGC_allocated atomically, so that (if
successful), anyone else trying to grab a reference will fail
- Attempt to reset Xen's mappings
- Reset the rest of the state.
Then, modify the two callers appropriately:
- Leave count_info alone (it's already been cleared)
- Call free_domheap_page() directly if appropriate
- Call assign_pages() rather than open-coding a partial assign
With all callers to assign_pages() now passing in pages with the
type_info field clear, tighten the respective assertion there.
This is XSA-287.
Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
---
xen/arch/x86/mm.c | 84 ++++++++++++++++++++++++++++------------
xen/common/grant_table.c | 20 +++++-----
xen/common/memory.c | 19 +++++----
xen/common/page_alloc.c | 2 +-
4 files changed, 83 insertions(+), 42 deletions(-)
diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 6509035a5c..d8ff58c901 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -3966,70 +3966,106 @@ int donate_page(
return -EINVAL;
}
+/*
+ * Steal page will attempt to remove `page` from domain `d`. Upon
+ * return, `page` will be in a state similar to the state of a page
+ * returned from alloc_domheap_page() with MEMF_no_owner set:
+ * - refcount 0
+ * - type count cleared
+ * - owner NULL
+ * - page caching attributes cleaned up
+ * - removed from the domain's page_list
+ *
+ * If MEMF_no_refcount is not set, the domain's tot_pages will be
+ * adjusted. If this results in the page count falling to 0,
+ * put_domain() will be called.
+ *
+ * The caller should either call free_domheap_page() to free the
+ * page, or assign_pages() to put it back on some domain's page list.
+ */
int steal_page(
struct domain *d, struct page_info *page, unsigned int memflags)
{
unsigned long x, y;
bool drop_dom_ref = false;
- const struct domain *owner = dom_xen;
+ const struct domain *owner;
+ int rc;
if ( paging_mode_external(d) )
return -EOPNOTSUPP;
- spin_lock(&d->page_alloc_lock);
-
- if ( is_xen_heap_page(page) || ((owner = page_get_owner(page)) != d) )
+ /* Grab a reference to make sure the page doesn't change under our feet */
+ rc = -EINVAL;
+ if ( !(owner = page_get_owner_and_reference(page)) )
goto fail;
+ if ( owner != d || is_xen_heap_page(page) )
+ goto fail_put;
+
/*
- * We require there is just one reference (PGC_allocated). We temporarily
- * drop this reference now so that we can safely swizzle the owner.
+ * We require there are exactly two references -- the one we just
+ * took, and PGC_allocated. We temporarily drop both these
+ * references so that the page becomes effectively non-"live" for
+ * the domain.
*/
y = page->count_info;
do {
x = y;
- if ( (x & (PGC_count_mask|PGC_allocated)) != (1 | PGC_allocated) )
- goto fail;
- y = cmpxchg(&page->count_info, x, x & ~PGC_count_mask);
+ if ( (x & (PGC_count_mask|PGC_allocated)) != (2 | PGC_allocated) )
+ goto fail_put;
+ y = cmpxchg(&page->count_info, x, x & ~(PGC_count_mask|PGC_allocated));
} while ( y != x );
/*
- * With the sole reference dropped temporarily, no-one can update type
- * information. Type count also needs to be zero in this case, but e.g.
- * PGT_seg_desc_page may still have PGT_validated set, which we need to
- * clear before transferring ownership (as validation criteria vary
- * depending on domain type).
+ * NB this is safe even if the page ends up being given back to
+ * the domain, because the count is zero: subsequent mappings will
+ * cause the cache attributes to be re-instated inside
+ * get_page_from_l1e().
+ */
+ if ( (rc = cleanup_page_cacheattr(page)) )
+ {
+ /*
+ * Couldn't fixup Xen's mappings; put things the way we found
+ * it and return an error
+ */
+ page->count_info |= PGC_allocated | 1;
+ goto fail;
+ }
+
+ /*
+ * With the reference count now zero, nobody can grab references
+ * to do anything else with the page. Return the page to a state
+ * that it might be upon return from alloc_domheap_pages with
+ * MEMF_no_owner set.
*/
+ spin_lock(&d->page_alloc_lock);
+
BUG_ON(page->u.inuse.type_info & (PGT_count_mask | PGT_locked |
PGT_pinned));
page->u.inuse.type_info = 0;
-
- /* Swizzle the owner then reinstate the PGC_allocated reference. */
page_set_owner(page, NULL);
- y = page->count_info;
- do {
- x = y;
- BUG_ON((x & (PGC_count_mask|PGC_allocated)) != PGC_allocated);
- } while ( (y = cmpxchg(&page->count_info, x, x | 1)) != x );
+ page_list_del(page, &d->page_list);
/* Unlink from original owner. */
if ( !(memflags & MEMF_no_refcount) && !domain_adjust_tot_pages(d, -1) )
drop_dom_ref = true;
- page_list_del(page, &d->page_list);
spin_unlock(&d->page_alloc_lock);
+
if ( unlikely(drop_dom_ref) )
put_domain(d);
+
return 0;
+ fail_put:
+ put_page(page);
fail:
- spin_unlock(&d->page_alloc_lock);
gdprintk(XENLOG_WARNING, "Bad steal mfn %" PRI_mfn
" from d%d (owner d%d) caf=%08lx taf=%" PRtype_info "\n",
mfn_x(page_to_mfn(page)), d->domain_id,
owner ? owner->domain_id : DOMID_INVALID,
page->count_info, page->u.inuse.type_info);
- return -EINVAL;
+ return rc;
}
static int __do_update_va_mapping(
diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index c0585d33f4..656fad1b42 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -2179,7 +2179,7 @@ gnttab_transfer(
rcu_unlock_domain(e);
put_gfn_and_copyback:
put_gfn(d, gop.mfn);
- page->count_info &= ~(PGC_count_mask|PGC_allocated);
+ /* The count_info has already been cleaned */
free_domheap_page(page);
goto copyback;
}
@@ -2202,10 +2202,9 @@ gnttab_transfer(
copy_domain_page(page_to_mfn(new_page), mfn);
- page->count_info &= ~(PGC_count_mask|PGC_allocated);
+ /* The count_info has already been cleared */
free_domheap_page(page);
page = new_page;
- page->count_info = PGC_allocated | 1;
mfn = page_to_mfn(page);
}
@@ -2245,12 +2244,17 @@ gnttab_transfer(
*/
spin_unlock(&e->page_alloc_lock);
okay = gnttab_prepare_for_transfer(e, d, gop.ref);
- spin_lock(&e->page_alloc_lock);
- if ( unlikely(!okay) || unlikely(e->is_dying) )
+ if ( unlikely(!okay || assign_pages(e, page, 0, MEMF_no_refcount)) )
{
- bool_t drop_dom_ref = !domain_adjust_tot_pages(e, -1);
+ bool drop_dom_ref;
+ /*
+ * Need to grab this again to safely free our "reserved"
+ * page in the page total
+ */
+ spin_lock(&e->page_alloc_lock);
+ drop_dom_ref = !domain_adjust_tot_pages(e, -1);
spin_unlock(&e->page_alloc_lock);
if ( okay /* i.e. e->is_dying due to the surrounding if() */ )
@@ -2263,10 +2267,6 @@ gnttab_transfer(
goto unlock_and_copyback;
}
- page_list_add_tail(page, &e->page_list);
- page_set_owner(page, e);
-
- spin_unlock(&e->page_alloc_lock);
put_gfn(d, gop.mfn);
TRACE_1D(TRC_MEM_PAGE_GRANT_TRANSFER, e->domain_id);
diff --git a/xen/common/memory.c b/xen/common/memory.c
index 4fb7962c79..f71163221f 100644
--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -675,20 +675,22 @@ static long memory_exchange(XEN_GUEST_HANDLE_PARAM(xen_memory_exchange_t) arg)
* Success! Beyond this point we cannot fail for this chunk.
*/
- /* Destroy final reference to each input page. */
+ /*
+ * These pages have already had owner and reference cleared.
+ * Do the final two steps: Remove from the physmap, and free
+ * them.
+ */
while ( (page = page_list_remove_head(&in_chunk_list)) )
{
unsigned long gfn;
- if ( !test_and_clear_bit(_PGC_allocated, &page->count_info) )
- BUG();
mfn = page_to_mfn(page);
gfn = mfn_to_gmfn(d, mfn_x(mfn));
/* Pages were unshared above */
BUG_ON(SHARED_M2P(gfn));
if ( guest_physmap_remove_page(d, _gfn(gfn), mfn, 0) )
domain_crash(d);
- put_page(page);
+ free_domheap_page(page);
}
/* Assign each output page to the domain. */
@@ -761,13 +763,16 @@ static long memory_exchange(XEN_GUEST_HANDLE_PARAM(xen_memory_exchange_t) arg)
* chunks succeeded.
*/
fail:
- /* Reassign any input pages we managed to steal. */
+ /*
+ * Reassign any input pages we managed to steal. NB that if the assign
+ * fails again, we're on the hook for freeing the page, since we've already
+ * cleared PGC_allocated.
+ */
while ( (page = page_list_remove_head(&in_chunk_list)) )
if ( assign_pages(d, page, 0, MEMF_no_refcount) )
{
BUG_ON(!d->is_dying);
- if ( test_and_clear_bit(_PGC_allocated, &page->count_info) )
- put_page(page);
+ free_domheap_page(page);
}
dying:
diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index 482f0988f7..52da7762e3 100644
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -2221,7 +2221,7 @@ int assign_pages(
for ( i = 0; i < (1 << order); i++ )
{
ASSERT(page_get_owner(&pg[i]) == NULL);
- ASSERT((pg[i].count_info & ~(PGC_allocated | 1)) == 0);
+ ASSERT(!pg[i].count_info);
page_set_owner(&pg[i], d);
smp_wmb(); /* Domain pointer must be visible before updating refcnt. */
pg[i].count_info = PGC_allocated | 1;
--
2.20.1