[Xen-devel] question: is it a CVE in relinquish_memory()[xen/arch/x86/domain.c]

["Xuquan (Quan Xu)" <xuquan8@xxxxxxxxxx>]

Hi,
Based on CVE-2015-7814 and commit 1ef01396fdff, ' arm: handle races between 
relinquish_memory and free_domheap_pages'..
relinquish_memory() [xen/arch/arm/domain.c, arm code], 
when couldn't get a reference -- someone is freeing this page and has already 
committed to doing so, so no more to do here, continue.


But in relinquish_memory()[xen/arch/x86/domain.c, __x86__ code], when couldn't 
get a reference -- someone is freeing this page,
Why adding this page to d->arch.relmem_list again. 
Is it a CVE to double free page, then hit the ''" alloc_heap_pages() : 
BUG_ON(pg[i].count_info != PGC_state_free)"" in creating guests later..




========

[1] CVE-2015-7814
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2015-7814

[2] commit 1ef01396fdff

commit 1ef01396fdff88b1c3331a09ca5c69619b90f4ea
Author: Ian Campbell <ian.campbell@xxxxxxxxxx>
Date:   Thu Oct 29 13:34:17 2015 +0100

    arm: handle races between relinquish_memory and free_domheap_pages

    Primarily this means XENMEM_decrease_reservation from a toolstack
    domain.

    Unlike x86 we have no requirement right now to queue such pages onto
    a separate list, if we hit this race then the other code has already
    fully accepted responsibility for freeing this page and therefore
    there is no more for relinquish_memory to do.

    This is CVE-2015-7814 / XSA-147.

    Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
    Reviewed-by: Julien Grall <julien.grall@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
index 683e769..880d0a6 100644
--- a/xen/arch/arm/domain.c
+++ b/xen/arch/arm/domain.c
@@ -772,8 +772,15 @@ static int relinquish_memory(struct domain *d, struct 
page_list_head *list)
     {
         /* Grab a reference to the page so it won't disappear from under us. */
         if ( unlikely(!get_page(page, d)) )
-            /* Couldn't get a reference -- someone is freeing this page. */
-            BUG();
+            /*
+             * Couldn't get a reference -- someone is freeing this page and
+             * has already committed to doing so, so no more to do here.
+             *
+             * Note that the page must be left on the list, a list_del
+             * here will clash with the list_del done by the other
+             * party in the race and corrupt the list head.
+             */
+            continue;

         if ( test_and_clear_bit(_PGC_allocated, &page->count_info) )
             put_page(page);
~


Quan

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