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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-changelog] [xen stable-4.8] x86: limit linear page table use to a single level
commit c4f969d25463586103a70f2bc36624e2287b880c
Author: Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Thu Oct 12 15:17:20 2017 +0200
Commit: Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Oct 12 15:17:20 2017 +0200
x86: limit linear page table use to a single level
That's the only way that they're meant to be used. Without such a
restriction arbitrarily long chains of same-level page tables can be
built, tearing down of which may then cause arbitrarily deep recursion,
causing a stack overflow. To facilitate this restriction, a counter is
being introduced to track both the number of same-level entries in a
page table as well as the number of uses of a page table in another
same-level one (counting into positive and negative direction
respectively, utilizing the fact that both counts can't be non-zero at
the same time).
Note that the added accounting introduces a restriction on the number
of times a page can be used in other same-level page tables - more than
32k of such uses are no longer possible.
Note also that some put_page_and_type[_preemptible]() calls are
replaced with open-coded equivalents. This seemed preferrable to
adding "parent_table" to the matrix of functions.
Note further that cross-domain same-level page table references are no
longer permitted (they probably never should have been).
This is XSA-240.
Reported-by: Jann Horn <jannh@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>
master commit: 6987fc7558bdbab8119eabf026e3cdad1053f0e5
master date: 2017-10-12 14:44:34 +0200
---
xen/arch/x86/domain.c | 1 +
xen/arch/x86/mm.c | 171 ++++++++++++++++++++++++++++++++++++++-----
xen/include/asm-x86/domain.h | 2 +
xen/include/asm-x86/mm.h | 25 +++++--
4 files changed, 175 insertions(+), 24 deletions(-)
diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index a725b43..5265b04 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -1245,6 +1245,7 @@ int arch_set_info_guest(
rc = -ERESTART;
/* Fallthrough */
case -ERESTART:
+ v->arch.old_guest_ptpg = NULL;
v->arch.old_guest_table =
pagetable_get_page(v->arch.guest_table);
v->arch.guest_table = pagetable_null();
diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 03c70d0..1f1ea2e 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -733,6 +733,61 @@ static void put_data_page(
put_page(page);
}
+static bool inc_linear_entries(struct page_info *pg)
+{
+ typeof(pg->linear_pt_count) nc = read_atomic(&pg->linear_pt_count), oc;
+
+ do {
+ /*
+ * The check below checks for the "linear use" count being non-zero
+ * as well as overflow. Signed integer overflow is undefined behavior
+ * according to the C spec. However, as long as linear_pt_count is
+ * smaller in size than 'int', the arithmetic operation of the
+ * increment below won't overflow; rather the result will be truncated
+ * when stored. Ensure that this is always true.
+ */
+ BUILD_BUG_ON(sizeof(nc) >= sizeof(int));
+ oc = nc++;
+ if ( nc <= 0 )
+ return false;
+ nc = cmpxchg(&pg->linear_pt_count, oc, nc);
+ } while ( oc != nc );
+
+ return true;
+}
+
+static void dec_linear_entries(struct page_info *pg)
+{
+ typeof(pg->linear_pt_count) oc;
+
+ oc = arch_fetch_and_add(&pg->linear_pt_count, -1);
+ ASSERT(oc > 0);
+}
+
+static bool inc_linear_uses(struct page_info *pg)
+{
+ typeof(pg->linear_pt_count) nc = read_atomic(&pg->linear_pt_count), oc;
+
+ do {
+ /* See the respective comment in inc_linear_entries(). */
+ BUILD_BUG_ON(sizeof(nc) >= sizeof(int));
+ oc = nc--;
+ if ( nc >= 0 )
+ return false;
+ nc = cmpxchg(&pg->linear_pt_count, oc, nc);
+ } while ( oc != nc );
+
+ return true;
+}
+
+static void dec_linear_uses(struct page_info *pg)
+{
+ typeof(pg->linear_pt_count) oc;
+
+ oc = arch_fetch_and_add(&pg->linear_pt_count, 1);
+ ASSERT(oc < 0);
+}
+
/*
* We allow root tables to map each other (a.k.a. linear page tables). It
* needs some special care with reference counts and access permissions:
@@ -762,15 +817,35 @@ get_##level##_linear_pagetable(
\
\
if ( (pfn = level##e_get_pfn(pde)) != pde_pfn ) \
{ \
+ struct page_info *ptpg = mfn_to_page(pde_pfn); \
+ \
+ /* Make sure the page table belongs to the correct domain. */ \
+ if ( unlikely(page_get_owner(ptpg) != d) ) \
+ return 0; \
+ \
/* Make sure the mapped frame belongs to the correct domain. */ \
if ( unlikely(!get_page_from_pagenr(pfn, d)) ) \
return 0; \
\
/* \
- * Ensure that the mapped frame is an already-validated page table. \
+ * Ensure that the mapped frame is an already-validated page table \
+ * and is not itself having linear entries, as well as that the \
+ * containing page table is not iself in use as a linear page table \
+ * elsewhere. \
* If so, atomically increment the count (checking for overflow). \
*/ \
page = mfn_to_page(pfn); \
+ if ( !inc_linear_entries(ptpg) ) \
+ { \
+ put_page(page); \
+ return 0; \
+ } \
+ if ( !inc_linear_uses(page) ) \
+ { \
+ dec_linear_entries(ptpg); \
+ put_page(page); \
+ return 0; \
+ } \
y = page->u.inuse.type_info; \
do { \
x = y; \
@@ -778,6 +853,8 @@ get_##level##_linear_pagetable(
\
unlikely((x & (PGT_type_mask|PGT_validated)) != \
(PGT_##level##_page_table|PGT_validated)) ) \
{ \
+ dec_linear_uses(page); \
+ dec_linear_entries(ptpg); \
put_page(page); \
return 0; \
} \
@@ -1202,6 +1279,9 @@ get_page_from_l4e(
l3e_remove_flags((pl3e), _PAGE_USER|_PAGE_RW|_PAGE_ACCESSED); \
} while ( 0 )
+static int _put_page_type(struct page_info *page, bool preemptible,
+ struct page_info *ptpg);
+
void put_page_from_l1e(l1_pgentry_t l1e, struct domain *l1e_owner)
{
unsigned long pfn = l1e_get_pfn(l1e);
@@ -1271,17 +1351,22 @@ static int put_page_from_l2e(l2_pgentry_t l2e, unsigned
long pfn)
if ( l2e_get_flags(l2e) & _PAGE_PSE )
put_superpage(l2e_get_pfn(l2e));
else
- put_page_and_type(l2e_get_page(l2e));
+ {
+ struct page_info *pg = l2e_get_page(l2e);
+ int rc = _put_page_type(pg, false, mfn_to_page(pfn));
+
+ ASSERT(!rc);
+ put_page(pg);
+ }
return 0;
}
-static int __put_page_type(struct page_info *, int preemptible);
-
static int put_page_from_l3e(l3_pgentry_t l3e, unsigned long pfn,
int partial, bool_t defer)
{
struct page_info *pg;
+ int rc;
if ( !(l3e_get_flags(l3e) & _PAGE_PRESENT) || (l3e_get_pfn(l3e) == pfn) )
return 1;
@@ -1304,21 +1389,28 @@ static int put_page_from_l3e(l3_pgentry_t l3e, unsigned
long pfn,
if ( unlikely(partial > 0) )
{
ASSERT(!defer);
- return __put_page_type(pg, 1);
+ return _put_page_type(pg, true, mfn_to_page(pfn));
}
if ( defer )
{
+ current->arch.old_guest_ptpg = mfn_to_page(pfn);
current->arch.old_guest_table = pg;
return 0;
}
- return put_page_and_type_preemptible(pg);
+ rc = _put_page_type(pg, true, mfn_to_page(pfn));
+ if ( likely(!rc) )
+ put_page(pg);
+
+ return rc;
}
static int put_page_from_l4e(l4_pgentry_t l4e, unsigned long pfn,
int partial, bool_t defer)
{
+ int rc = 1;
+
if ( (l4e_get_flags(l4e) & _PAGE_PRESENT) &&
(l4e_get_pfn(l4e) != pfn) )
{
@@ -1327,18 +1419,22 @@ static int put_page_from_l4e(l4_pgentry_t l4e, unsigned
long pfn,
if ( unlikely(partial > 0) )
{
ASSERT(!defer);
- return __put_page_type(pg, 1);
+ return _put_page_type(pg, true, mfn_to_page(pfn));
}
if ( defer )
{
+ current->arch.old_guest_ptpg = mfn_to_page(pfn);
current->arch.old_guest_table = pg;
return 0;
}
- return put_page_and_type_preemptible(pg);
+ rc = _put_page_type(pg, true, mfn_to_page(pfn));
+ if ( likely(!rc) )
+ put_page(pg);
}
- return 1;
+
+ return rc;
}
static int alloc_l1_table(struct page_info *page)
@@ -1536,6 +1632,7 @@ static int alloc_l3_table(struct page_info *page)
{
page->nr_validated_ptes = i;
page->partial_pte = 0;
+ current->arch.old_guest_ptpg = NULL;
current->arch.old_guest_table = page;
}
while ( i-- > 0 )
@@ -1628,6 +1725,7 @@ static int alloc_l4_table(struct page_info *page)
{
if ( current->arch.old_guest_table )
page->nr_validated_ptes++;
+ current->arch.old_guest_ptpg = NULL;
current->arch.old_guest_table = page;
}
}
@@ -2370,14 +2468,20 @@ int free_page_type(struct page_info *page, unsigned
long type,
}
-static int __put_final_page_type(
- struct page_info *page, unsigned long type, int preemptible)
+static int _put_final_page_type(struct page_info *page, unsigned long type,
+ bool preemptible, struct page_info *ptpg)
{
int rc = free_page_type(page, type, preemptible);
/* No need for atomic update of type_info here: noone else updates it. */
if ( rc == 0 )
{
+ if ( ptpg && PGT_type_equal(type, ptpg->u.inuse.type_info) )
+ {
+ dec_linear_uses(page);
+ dec_linear_entries(ptpg);
+ }
+ ASSERT(!page->linear_pt_count || page_get_owner(page)->is_dying);
/*
* Record TLB information for flush later. We do not stamp page tables
* when running in shadow mode:
@@ -2413,8 +2517,8 @@ static int __put_final_page_type(
}
-static int __put_page_type(struct page_info *page,
- int preemptible)
+static int _put_page_type(struct page_info *page, bool preemptible,
+ struct page_info *ptpg)
{
unsigned long nx, x, y = page->u.inuse.type_info;
int rc = 0;
@@ -2441,12 +2545,28 @@ static int __put_page_type(struct page_info *page,
x, nx)) != x) )
continue;
/* We cleared the 'valid bit' so we do the clean up. */
- rc = __put_final_page_type(page, x, preemptible);
+ rc = _put_final_page_type(page, x, preemptible, ptpg);
+ ptpg = NULL;
if ( x & PGT_partial )
put_page(page);
break;
}
+ if ( ptpg && PGT_type_equal(x, ptpg->u.inuse.type_info) )
+ {
+ /*
+ * page_set_tlbflush_timestamp() accesses the same union
+ * linear_pt_count lives in. Unvalidated page table pages,
+ * however, should occur during domain destruction only
+ * anyway. Updating of linear_pt_count luckily is not
+ * necessary anymore for a dying domain.
+ */
+ ASSERT(page_get_owner(page)->is_dying);
+ ASSERT(page->linear_pt_count < 0);
+ ASSERT(ptpg->linear_pt_count > 0);
+ ptpg = NULL;
+ }
+
/*
* Record TLB information for flush later. We do not stamp page
* tables when running in shadow mode:
@@ -2466,6 +2586,13 @@ static int __put_page_type(struct page_info *page,
return -EINTR;
}
+ if ( ptpg && PGT_type_equal(x, ptpg->u.inuse.type_info) )
+ {
+ ASSERT(!rc);
+ dec_linear_uses(page);
+ dec_linear_entries(ptpg);
+ }
+
return rc;
}
@@ -2600,6 +2727,7 @@ static int __get_page_type(struct page_info *page,
unsigned long type,
page->nr_validated_ptes = 0;
page->partial_pte = 0;
}
+ page->linear_pt_count = 0;
rc = alloc_page_type(page, type, preemptible);
}
@@ -2614,7 +2742,7 @@ static int __get_page_type(struct page_info *page,
unsigned long type,
void put_page_type(struct page_info *page)
{
- int rc = __put_page_type(page, 0);
+ int rc = _put_page_type(page, false, NULL);
ASSERT(rc == 0);
(void)rc;
}
@@ -2630,7 +2758,7 @@ int get_page_type(struct page_info *page, unsigned long
type)
int put_page_type_preemptible(struct page_info *page)
{
- return __put_page_type(page, 1);
+ return _put_page_type(page, true, NULL);
}
int get_page_type_preemptible(struct page_info *page, unsigned long type)
@@ -2836,11 +2964,14 @@ int put_old_guest_table(struct vcpu *v)
if ( !v->arch.old_guest_table )
return 0;
- switch ( rc = put_page_and_type_preemptible(v->arch.old_guest_table) )
+ switch ( rc = _put_page_type(v->arch.old_guest_table, true,
+ v->arch.old_guest_ptpg) )
{
case -EINTR:
case -ERESTART:
return -ERESTART;
+ case 0:
+ put_page(v->arch.old_guest_table);
}
v->arch.old_guest_table = NULL;
@@ -2997,6 +3128,7 @@ int new_guest_cr3(unsigned long mfn)
rc = -ERESTART;
/* fallthrough */
case -ERESTART:
+ curr->arch.old_guest_ptpg = NULL;
curr->arch.old_guest_table = page;
break;
default:
@@ -3264,7 +3396,10 @@ long do_mmuext_op(
if ( type == PGT_l1_page_table )
put_page_and_type(page);
else
+ {
+ curr->arch.old_guest_ptpg = NULL;
curr->arch.old_guest_table = page;
+ }
}
}
@@ -3297,6 +3432,7 @@ long do_mmuext_op(
{
case -EINTR:
case -ERESTART:
+ curr->arch.old_guest_ptpg = NULL;
curr->arch.old_guest_table = page;
rc = 0;
break;
@@ -3375,6 +3511,7 @@ long do_mmuext_op(
rc = -ERESTART;
/* fallthrough */
case -ERESTART:
+ curr->arch.old_guest_ptpg = NULL;
curr->arch.old_guest_table = page;
break;
default:
diff --git a/xen/include/asm-x86/domain.h b/xen/include/asm-x86/domain.h
index f6a40eb..60bb8c9 100644
--- a/xen/include/asm-x86/domain.h
+++ b/xen/include/asm-x86/domain.h
@@ -531,6 +531,8 @@ struct arch_vcpu
pagetable_t guest_table_user; /* (MFN) x86/64 user-space pagetable */
pagetable_t guest_table; /* (MFN) guest notion of cr3 */
struct page_info *old_guest_table; /* partially destructed pagetable */
+ struct page_info *old_guest_ptpg; /* containing page table of the */
+ /* former, if any */
/* guest_table holds a ref to the page, and also a type-count unless
* shadow refcounts are in use */
pagetable_t shadow_table[4]; /* (MFN) shadow(s) of guest */
diff --git a/xen/include/asm-x86/mm.h b/xen/include/asm-x86/mm.h
index 6687dbc..63590a7 100644
--- a/xen/include/asm-x86/mm.h
+++ b/xen/include/asm-x86/mm.h
@@ -125,11 +125,11 @@ struct page_info
u32 tlbflush_timestamp;
/*
- * When PGT_partial is true then this field is valid and indicates
- * that PTEs in the range [0, @nr_validated_ptes) have been validated.
- * An extra page reference must be acquired (or not dropped) whenever
- * PGT_partial gets set, and it must be dropped when the flag gets
- * cleared. This is so that a get() leaving a page in partially
+ * When PGT_partial is true then the first two fields are valid and
+ * indicate that PTEs in the range [0, @nr_validated_ptes) have been
+ * validated. An extra page reference must be acquired (or not dropped)
+ * whenever PGT_partial gets set, and it must be dropped when the flag
+ * gets cleared. This is so that a get() leaving a page in partially
* validated state (where the caller would drop the reference acquired
* due to the getting of the type [apparently] failing [-ERESTART])
* would not accidentally result in a page left with zero general
@@ -153,10 +153,18 @@ struct page_info
* put_page_from_lNe() (due to the apparent failure), and hence it
* must be dropped when the put operation is resumed (and completes),
* but it must not be acquired if picking up the page for validation.
+ *
+ * The 3rd field, @linear_pt_count, indicates
+ * - by a positive value, how many same-level page table entries a page
+ * table has,
+ * - by a negative value, in how many same-level page tables a page is
+ * in use.
*/
struct {
- u16 nr_validated_ptes;
- s8 partial_pte;
+ u16 nr_validated_ptes:PAGETABLE_ORDER + 1;
+ u16 :16 - PAGETABLE_ORDER - 1 - 2;
+ s16 partial_pte:2;
+ s16 linear_pt_count;
};
/*
@@ -207,6 +215,9 @@ struct page_info
#define PGT_count_width PG_shift(9)
#define PGT_count_mask ((1UL<<PGT_count_width)-1)
+/* Are the 'type mask' bits identical? */
+#define PGT_type_equal(x, y) (!(((x) ^ (y)) & PGT_type_mask))
+
/* Cleared when the owning guest 'frees' this page. */
#define _PGC_allocated PG_shift(1)
#define PGC_allocated PG_mask(1, 1)
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.8
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