diff options
Diffstat (limited to 'system/xen/xsa/xsa299-4.12-0006-x86-mm-Have-alloc_l-23-_table-clear-partial_flags-wh.patch')
| -rw-r--r-- | system/xen/xsa/xsa299-4.12-0006-x86-mm-Have-alloc_l-23-_table-clear-partial_flags-wh.patch | 111 |
1 files changed, 111 insertions, 0 deletions
diff --git a/system/xen/xsa/xsa299-4.12-0006-x86-mm-Have-alloc_l-23-_table-clear-partial_flags-wh.patch b/system/xen/xsa/xsa299-4.12-0006-x86-mm-Have-alloc_l-23-_table-clear-partial_flags-wh.patch new file mode 100644 index 0000000000000..72ee3eac9e4e5 --- /dev/null +++ b/system/xen/xsa/xsa299-4.12-0006-x86-mm-Have-alloc_l-23-_table-clear-partial_flags-wh.patch @@ -0,0 +1,111 @@ +From 4ad70553611a7a4e4494d5a3b51b5cc295a488e0 Mon Sep 17 00:00:00 2001 +From: George Dunlap <george.dunlap@citrix.com> +Date: Thu, 10 Oct 2019 17:57:49 +0100 +Subject: [PATCH 06/11] x86/mm: Have alloc_l[23]_table clear partial_flags when + preempting + +In order to allow recursive pagetable promotions and demotions to be +interrupted, Xen must keep track of the state of the sub-pages +promoted or demoted. This is stored in two elements in the page +struct: nr_entries_validated and partial_flags. + +The rule is that entries [0, nr_entries_validated) should always be +validated and hold a general reference count. If partial_flags is +zero, then [nr_entries_validated] is not validated and no reference +count is held. If PTF_partial_set is set, then [nr_entries_validated] +is partially validated. + +At the moment, a distinction is made between promotion and demotion +with regard to whether the entry itself "holds" a general reference +count: when entry promotion is interrupted (i.e., returns -ERESTART), +the entry is not considered to hold a reference; when entry demotion +is interrupted, the entry is still considered to hold a general +reference. + +PTF_partial_general_ref is used to distinguish between these cases. +If clear, it's a partial promotion => no general reference count held +by the entry; if set, it's partial demotion, so a general reference +count held. Because promotions and demotions can be interleaved, this +value is passed to get_page_and_type_from_mfn and put_page_from_l*e, +to be able to properly handle reference counts. + +Unfortunately, when alloc_l[23]_table check hypercall_preempt_check() +and return -ERESTART, they set nr_entries_validated, but don't clear +partial_flags. + +If we were picking up from a previously-interrupted promotion, that +means that PTF_partial_set would be set even though +[nr_entries_validated] was not partially validated. This means that +if the page in this state were de-validated, put_page_type() would +erroneously be called on that entry. + +Perhaps worse, if we were racing with a de-validation, then we might +leave both PTF_partial_set and PTF_partial_general_ref; and when +de-validation picked up again, both the type and the general ref would +be erroneously dropped from [nr_entries_validated]. + +In a sense, the real issue here is code duplication. Rather than +duplicate the interruption code, set rc to -EINTR and fall through to +the code which already handles that case correctly. + +Given the logic at this point, it should be impossible for +partial_flags to be non-zero; add an ASSERT() to catch any changes. + +This is part of XSA-299. + +Reported-by: George Dunlap <george.dunlap@citrix.com> +Signed-off-by: George Dunlap <george.dunlap@citrix.com> +Reviewed-by: Jan Beulich <jbeulich@suse.com> +--- + xen/arch/x86/mm.c | 25 ++++++------------------- + 1 file changed, 6 insertions(+), 19 deletions(-) + +diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c +index 0a4d39a2c3..bbd29a68f4 100644 +--- a/xen/arch/x86/mm.c ++++ b/xen/arch/x86/mm.c +@@ -1554,21 +1554,13 @@ static int alloc_l2_table(struct page_info *page, unsigned long type) + for ( i = page->nr_validated_ptes; i < L2_PAGETABLE_ENTRIES; + i++, partial_flags = 0 ) + { +- l2_pgentry_t l2e; ++ l2_pgentry_t l2e = pl2e[i]; + + if ( i > page->nr_validated_ptes && hypercall_preempt_check() ) +- { +- page->nr_validated_ptes = i; +- rc = -ERESTART; +- break; +- } +- +- if ( !is_guest_l2_slot(d, type, i) ) ++ rc = -EINTR; ++ else if ( !is_guest_l2_slot(d, type, i) ) + continue; +- +- l2e = pl2e[i]; +- +- if ( !(l2e_get_flags(l2e) & _PAGE_PRESENT) ) ++ else if ( !(l2e_get_flags(l2e) & _PAGE_PRESENT) ) + { + if ( !pv_l1tf_check_l2e(d, l2e) ) + continue; +@@ -1640,13 +1632,8 @@ static int alloc_l3_table(struct page_info *page) + l3_pgentry_t l3e = pl3e[i]; + + if ( i > page->nr_validated_ptes && hypercall_preempt_check() ) +- { +- page->nr_validated_ptes = i; +- rc = -ERESTART; +- break; +- } +- +- if ( is_pv_32bit_domain(d) && (i == 3) ) ++ rc = -EINTR; ++ else if ( is_pv_32bit_domain(d) && (i == 3) ) + { + if ( !(l3e_get_flags(l3e) & _PAGE_PRESENT) || + (l3e_get_flags(l3e) & l3_disallow_mask(d)) ) +-- +2.23.0 + |