* [PATCH] x86/pv: Fill all Xen slots in init_guest_l4_table()
@ 2017-08-29 10:24 Andrew Cooper
2017-08-29 13:47 ` Jan Beulich
0 siblings, 1 reply; 2+ messages in thread
From: Andrew Cooper @ 2017-08-29 10:24 UTC (permalink / raw)
To: Xen-devel; +Cc: Andrew Cooper, Wei Liu, Jan Beulich
There is a bug when using highmem-start= where some L4 directmap slots are not
audited in alloc_l4_table(), and not overwritten by init_guest_l4_table().
As highmem_start is only available in debug builds of the hypervisor, this
does not constitute a security issue.
Ensure that init_guest_l4_table() writes to all of the Xen slots.
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Wei Liu <wei.liu2@citrix.com>
highmem-start= has actually bitrotten and is unusable. This was tested by
hacking up a similar situation.
---
xen/arch/x86/mm.c | 20 +++++++++++++++++---
1 file changed, 17 insertions(+), 3 deletions(-)
diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index b6d6ae3..791e526 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -1596,6 +1596,10 @@ static int alloc_l3_table(struct page_info *page)
return rc > 0 ? 0 : rc;
}
+/*
+ * This function must write all ROOT_PAGETABLE_PV_XEN_SLOTS, to clobber any
+ * values a guest may have left there from alloc_l4_table().
+ */
void init_guest_l4_table(l4_pgentry_t l4tab[], const struct domain *d,
bool zap_ro_mpt)
{
@@ -1604,9 +1608,19 @@ void init_guest_l4_table(l4_pgentry_t l4tab[], const struct domain *d,
&idle_pg_table[ROOT_PAGETABLE_FIRST_XEN_SLOT],
root_pgt_pv_xen_slots * sizeof(l4_pgentry_t));
#ifndef NDEBUG
- if ( l4e_get_intpte(split_l4e) )
- l4tab[ROOT_PAGETABLE_FIRST_XEN_SLOT + root_pgt_pv_xen_slots] =
- split_l4e;
+ if ( unlikely(root_pgt_pv_xen_slots < ROOT_PAGETABLE_PV_XEN_SLOTS) )
+ {
+ l4_pgentry_t *next = &l4tab[ROOT_PAGETABLE_FIRST_XEN_SLOT +
+ root_pgt_pv_xen_slots];
+
+ if ( l4e_get_intpte(split_l4e) )
+ *next++ = split_l4e;
+
+ memset(next, 0,
+ _p(&l4tab[ROOT_PAGETABLE_LAST_XEN_SLOT + 1]) - _p(next));
+ }
+#else
+ BUILD_BUG_ON(root_pgt_pv_xen_slots != ROOT_PAGETABLE_PV_XEN_SLOTS);
#endif
l4tab[l4_table_offset(LINEAR_PT_VIRT_START)] =
l4e_from_pfn(domain_page_map_to_mfn(l4tab), __PAGE_HYPERVISOR_RW);
--
2.1.4
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^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH] x86/pv: Fill all Xen slots in init_guest_l4_table()
2017-08-29 10:24 [PATCH] x86/pv: Fill all Xen slots in init_guest_l4_table() Andrew Cooper
@ 2017-08-29 13:47 ` Jan Beulich
0 siblings, 0 replies; 2+ messages in thread
From: Jan Beulich @ 2017-08-29 13:47 UTC (permalink / raw)
To: Andrew Cooper; +Cc: Wei Liu, Xen-devel
>>> On 29.08.17 at 12:24, <andrew.cooper3@citrix.com> wrote:
> There is a bug when using highmem-start= where some L4 directmap slots are not
> audited in alloc_l4_table(), and not overwritten by init_guest_l4_table().
>
> As highmem_start is only available in debug builds of the hypervisor, this
> does not constitute a security issue.
>
> Ensure that init_guest_l4_table() writes to all of the Xen slots.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
albeit ...
> @@ -1604,9 +1608,19 @@ void init_guest_l4_table(l4_pgentry_t l4tab[], const struct domain *d,
> &idle_pg_table[ROOT_PAGETABLE_FIRST_XEN_SLOT],
> root_pgt_pv_xen_slots * sizeof(l4_pgentry_t));
> #ifndef NDEBUG
> - if ( l4e_get_intpte(split_l4e) )
> - l4tab[ROOT_PAGETABLE_FIRST_XEN_SLOT + root_pgt_pv_xen_slots] =
> - split_l4e;
> + if ( unlikely(root_pgt_pv_xen_slots < ROOT_PAGETABLE_PV_XEN_SLOTS) )
> + {
> + l4_pgentry_t *next = &l4tab[ROOT_PAGETABLE_FIRST_XEN_SLOT +
> + root_pgt_pv_xen_slots];
> +
> + if ( l4e_get_intpte(split_l4e) )
> + *next++ = split_l4e;
> +
> + memset(next, 0,
> + _p(&l4tab[ROOT_PAGETABLE_LAST_XEN_SLOT + 1]) - _p(next));
... these disguised casts. But all alternatives I can think of make the
code more difficult to read.
Jan
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