[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-changelog] [xen stable-4.8] x86/mm: Rework get_page_and_type_from_mfn conditional
commit 6699295816aa32c3d057425f04f5ba485fc6e6ea Author: George Dunlap <george.dunlap@xxxxxxxxxx> AuthorDate: Mon Nov 4 15:25:18 2019 +0100 Commit: Jan Beulich <jbeulich@xxxxxxxx> CommitDate: Mon Nov 4 15:25:18 2019 +0100 x86/mm: Rework get_page_and_type_from_mfn conditional Make it easier to read by declaring the conditions in which we will retain the ref, rather than the conditions under which we release it. The only way (page == current->arch.old_guest_table) can be true is if preemptible is true; so remove this from the query itself, and add an ASSERT() to that effect on the opposite path. No functional change intended. NB that alloc_lN_table() mishandle the "linear pt failure" situation described in the comment; this will be addressed in a future patch. This is part of XSA-299. Reported-by: George Dunlap <george.dunlap@xxxxxxxxxx> Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx> master commit: 2aab06d742e13d7a9d248f1fc7f0ec62b295ada1 master date: 2019-10-31 16:13:23 +0100 --- xen/arch/x86/mm.c | 39 +++++++++++++++++++++++++++++++++++++-- 1 file changed, 37 insertions(+), 2 deletions(-) diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c index 1576a083df..16aef2f652 100644 --- a/xen/arch/x86/mm.c +++ b/xen/arch/x86/mm.c @@ -784,8 +784,43 @@ static int get_page_and_type_from_pagenr(unsigned long page_nr, rc = __get_page_type(page, type, preemptible); - if ( unlikely(rc) && !partial_ref && - (!preemptible || page != current->arch.old_guest_table) ) + /* + * Retain the refcount if: + * - page is fully validated (rc == 0) + * - page is not validated (rc < 0) but: + * - We came in with a reference (partial_ref) + * - page is partially validated but there's been an error + * (page == current->arch.old_guest_table) + * + * The partial_ref-on-error clause is worth an explanation. There + * are two scenarios where partial_ref might be true coming in: + * - mfn has been partially demoted as type `type`; i.e. has + * PGT_partial set + * - mfn has been partially demoted as L(type+1) (i.e., a linear + * page; e.g. we're being called from get_page_from_l2e with + * type == PGT_l1_table, but the mfn is PGT_l2_table) + * + * If there's an error, in the first case, _get_page_type will + * either return -ERESTART, in which case we want to retain the + * ref (as the caller will consider it retained), or -EINVAL, in + * which case old_guest_table will be set; in both cases, we need + * to retain the ref. + * + * In the second case, if there's an error, _get_page_type() can + * *only* return -EINVAL, and *never* set old_guest_table. In + * that case we also want to retain the reference, to allow the + * page to continue to be torn down (i.e., PGT_partial cleared) + * safely. + * + * Also note that we shouldn't be able to leave with the reference + * count retained unless we succeeded, or the operation was + * preemptible. + */ + if ( likely(!rc) || partial_ref ) + /* nothing */; + else if ( page == current->arch.old_guest_table ) + ASSERT(preemptible); + else put_page(page); return rc; -- generated by git-patchbot for /home/xen/git/xen.git#stable-4.8 _______________________________________________ Xen-changelog mailing list Xen-changelog@xxxxxxxxxxxxxxxxxxxx https://lists.xenproject.org/xen-changelog
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