[Xen-changelog] [xen stable-4.12] x86/mm: Don't re-set PGT_pinned on a partially de-validated page

[patchbot@xxxxxxx]

commit f5af2b91fe43f2c96ca3fd2249fa19cba4a32657
Author:     George Dunlap <george.dunlap@xxxxxxxxxx>
AuthorDate: Thu Oct 31 16:50:19 2019 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Oct 31 16:50:19 2019 +0100

    x86/mm: Don't re-set PGT_pinned on a partially de-validated page
    
    When unpinning pagetables, if an operation is interrupted,
    relinquish_memory() re-sets PGT_pinned so that the un-pin will
    pickedup again when the hypercall restarts.
    
    This is appropriate when put_page_and_type_preemptible() returns
    -EINTR, which indicates that the page is back in its initial state
    (i.e., completely validated).  However, for -ERESTART, this leads to a
    state where a page has both PGT_pinned and PGT_partial set.
    
    This happens to work at the moment, although it's not really a
    "canonical" state; but in subsequent patches, where we need to make a
    distinction in handling between PGT_validated and PGT_partial pages,
    this causes issues.
    
    Move to a "canonical" state by:
    - Only re-setting PGT_pinned on -EINTR
    - Re-dropping the refcount held by PGT_pinned on -ERESTART
    
    In the latter case, the PGT_partial bit will be cleared further down
    with the rest of the other PGT_partial pages.
    
    While here, clean up some trainling whitespace.
    
    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: bf656e02d8e7f49b484e2587aef4f18deda6e2ab
    master date: 2019-10-31 16:11:46 +0100
---
 xen/arch/x86/domain.c | 31 ++++++++++++++++++++++++++++---
 1 file changed, 28 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index 5a088dde1b..75e439e296 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -114,7 +114,7 @@ static void play_dead(void)
      * this case, heap corruption or #PF can occur (when heap debugging is
      * enabled). For example, even printk() can involve tasklet scheduling,
      * which touches per-cpu vars.
-     * 
+     *
      * Consider very carefully when adding code to *dead_idle. Most hypervisor
      * subsystems are unsafe to call.
      */
@@ -1912,9 +1912,34 @@ static int relinquish_memory(
             break;
         case -ERESTART:
         case -EINTR:
+            /*
+             * -EINTR means PGT_validated has been re-set; re-set
+             * PGT_pinned again so that it gets picked up next time
+             * around.
+             *
+             * -ERESTART, OTOH, means PGT_partial is set instead.  Put
+             * it back on the list, but don't set PGT_pinned; the
+             * section below will finish off de-validation.  But we do
+             * need to drop the general ref associated with
+             * PGT_pinned, since put_page_and_type_preemptible()
+             * didn't do it.
+             *
+             * NB we can do an ASSERT for PGT_validated, since we
+             * "own" the type ref; but theoretically, the PGT_partial
+             * could be cleared by someone else.
+             */
+            if ( ret == -EINTR )
+            {
+                ASSERT(page->u.inuse.type_info & PGT_validated);
+                set_bit(_PGT_pinned, &page->u.inuse.type_info);
+            }
+            else
+                put_page(page);
+
             ret = -ERESTART;
+
+            /* Put the page back on the list and drop the ref we grabbed above 
*/
             page_list_add(page, list);
-            set_bit(_PGT_pinned, &page->u.inuse.type_info);
             put_page(page);
             goto out;
         default:
@@ -2164,7 +2189,7 @@ void vcpu_kick(struct vcpu *v)
      * pending flag. These values may fluctuate (after all, we hold no
      * locks) but the key insight is that each change will cause
      * evtchn_upcall_pending to be polled.
-     * 
+     *
      * NB2. We save the running flag across the unblock to avoid a needless
      * IPI for domains that we IPI'd to unblock.
      */
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.12

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