[Xen-changelog] [xen stable-4.12] x86/mm: L1TF checks don't leave a partial entry

[patchbot@xxxxxxx]

commit 09513ab8bd1270c22dd776858e2c937433d39173
Author:     George Dunlap <george.dunlap@xxxxxxxxxx>
AuthorDate: Thu Oct 31 16:49:43 2019 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Oct 31 16:49:43 2019 +0100

    x86/mm: L1TF checks don't leave a partial entry
    
    On detection of a potential L1TF issue, most validation code returns
    -ERESTART to allow the switch to shadow mode to happen and cause the
    original operation to be restarted.
    
    However, in the validation code, the return value -ERESTART has been
    repurposed to indicate 1) the function has partially completed
    something which needs to be undone, and 2) calling put_page_type()
    should cleanly undo it.  This causes problems in several places.
    
    For L1 tables, on receiving an -ERESTART return from alloc_l1_table(),
    alloc_page_type() will set PGT_partial on the page.  If for some
    reason the original operation never restarts, then on domain
    destruction, relinquish_memory() will call free_page_type() on the
    page.
    
    Unfortunately, alloc_ and free_l1_table() aren't set up to deal with
    PGT_partial.  When returning a failure, alloc_l1_table() always
    de-validates whatever it's validated so far, and free_l1_table()
    always devalidates the whole page.  This means that if
    relinquish_memory() calls free_page_type() on an L1 that didn't
    complete due to an L1TF, it will call put_page_from_l1e() on "page
    entries" that have never been validated.
    
    For L2+ tables, setting rc to ERESTART causes the rest of the
    alloc_lN_table() function to *think* that the entry in question will
    have PGT_partial set.  This will cause it to set partial_pte = 1.  If
    relinqush_memory() then calls free_page_type() on one of those pages,
    then free_lN_table() will call put_page_from_lNe() on the entry when
    it shouldn't.
    
    Rather than indicating -ERESTART, indicate -EINTR.  This is the code
    to indicate that nothing has changed from when you started the call
    (which is effectively how alloc_l1_table() handles errors).
    
    mod_lN_entry() shouldn't have any of these types of problems, so leave
    potential changes there for a clean-up patch later.
    
    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: 3165ffef09e89d38f84d26051f606d2c1421aea3
    master date: 2019-10-31 16:11:12 +0100
---
 xen/arch/x86/mm.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 3557cd1178..a1b55c10ff 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -1409,7 +1409,7 @@ static int alloc_l1_table(struct page_info *page)
     {
         if ( !(l1e_get_flags(pl1e[i]) & _PAGE_PRESENT) )
         {
-            ret = pv_l1tf_check_l1e(d, pl1e[i]) ? -ERESTART : 0;
+            ret = pv_l1tf_check_l1e(d, pl1e[i]) ? -EINTR : 0;
             if ( ret )
                 goto out;
         }
@@ -1517,7 +1517,7 @@ static int alloc_l2_table(struct page_info *page, 
unsigned long type)
         {
             if ( !pv_l1tf_check_l2e(d, l2e) )
                 continue;
-            rc = -ERESTART;
+            rc = -EINTR;
         }
         else
             rc = get_page_from_l2e(l2e, pfn, d, partial);
@@ -1603,7 +1603,7 @@ static int alloc_l3_table(struct page_info *page)
         {
             if ( !pv_l1tf_check_l3e(d, l3e) )
                 continue;
-            rc = -ERESTART;
+            rc = -EINTR;
         }
         else
             rc = get_page_from_l3e(l3e, pfn, d, partial);
@@ -1783,7 +1783,7 @@ static int alloc_l4_table(struct page_info *page)
         {
             if ( !pv_l1tf_check_l4e(d, l4e) )
                 continue;
-            rc = -ERESTART;
+            rc = -EINTR;
         }
         else
             rc = get_page_from_l4e(l4e, pfn, d, partial);
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.12

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