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To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org,
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From: Mark Johnston <markj@FreeBSD.org>
Subject: git: 304a533fd2ec - stable/12 - vm_fault: Shoot down multiply mapped
COW source page mappings
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The branch stable/12 has been updated by markj:
URL: https://cgit.FreeBSD.org/src/commit/?id=304a533fd2ec6d61805eb7c991b3e9ce502fc730
commit 304a533fd2ec6d61805eb7c991b3e9ce502fc730
Author: Mark Johnston <markj@FreeBSD.org>
AuthorDate: 2021-04-06 18:56:37 +0000
Commit: Mark Johnston <markj@FreeBSD.org>
CommitDate: 2021-04-06 18:56:37 +0000
vm_fault: Shoot down multiply mapped COW source page mappings
Reviewed by: kib, rlibby
Discussed with: alc
Approved by: so
Security: CVE-2021-29626
Security: FreeBSD-SA-21:08.vm
---
sys/vm/vm_fault.c | 27 +++++++++++++++++++++++++++
1 file changed, 27 insertions(+)
diff --git a/sys/vm/vm_fault.c b/sys/vm/vm_fault.c
index cf2db256eaa3..7829b3691d83 100644
--- a/sys/vm/vm_fault.c
+++ b/sys/vm/vm_fault.c
@@ -1298,6 +1298,33 @@ readrest:
vm_page_unwire(fs.m, PQ_INACTIVE);
vm_page_unlock(fs.m);
}
+
+ /*
+ * Typically, the shadow object is either
+ * private to this address space
+ * (OBJ_ONEMAPPING) or its pages are read only.
+ * In the highly unusual case where the pages of
+ * a shadow object are read/write shared between
+ * this and other address spaces, we need to
+ * ensure that any pmap-level mappings to the
+ * original, copy-on-write page from the backing
+ * object are removed from those other address
+ * spaces.
+ *
+ * The flag check is racy, but this is
+ * tolerable: if OBJ_ONEMAPPING is cleared after
+ * the check, the busy state ensures that new
+ * mappings of fs.m can't be created.
+ * pmap_enter() will replace an existing mapping
+ * in the current address space. If
+ * OBJ_ONEMAPPING is set after the check,
+ * removing mappings will at worse trigger some
+ * unnecessary page faults.
+ */
+ vm_page_assert_xbusied(fs.m);
+ if ((fs.first_object->flags & OBJ_ONEMAPPING) == 0)
+ pmap_remove_all(fs.m);
+
/*
* We no longer need the old page or object.
*/