Xen Security Advisory 174 (CVE-2016-3961) - hugetlbfs use may crash PV Linux guests

Xen Security Advisory 174 (CVE-2016-3961) - hugetlbfs use may crash PV Linux guests

[Xen.org security team]

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            Xen Security Advisory CVE-2016-3961 / XSA-174
                              version 3

                hugetlbfs use may crash PV Linux guests

UPDATES IN VERSION 3
====================

Public release.

ISSUE DESCRIPTION
=================

Huge (2Mb) pages are generally unavailable to PV guests.  Since x86
Linux pvops-based kernels are generally multi purpose, they would
normally be built with hugetlbfs support enabled.  Use of that
functionality by an application in a PV guest would cause an
infinite page fault loop, and an OOPS to occur upon an attempt to
terminate the hung application.

IMPACT
======

Depending on the guest kernel configuration, the OOPS could result
in a kernel crash (guest DoS).

VULNERABLE SYSTEMS
==================

All upstream x86 Linux versions operating as PV Xen guests are
vulnerable.

ARM systems are not vulnerable.  x86 HVM guests are not vulnerable.

x86 Linux versions derived from linux-2.6.18-xen.hg (XenoLinux) are not
vulnerable.

Oracle Unbreakable Enterprise Kernels are not vulnerable.

We believe that non-Linux guests are not vulnerable, as we are not
aware of any with an analogous bug.

MITIGATION
==========

Running only HVM guests will avoid this issue.

Not enabling hugetlbfs use, by not altering the boot time default value
of zero in /proc/sys/vm/nr_hugepages (which can only be written by the
root user) will avoid this issue.

It is possible that disabling (or not enabling) the "panic on OOPS"
behavior (via use of the "oops=panic" command line option or the
"panic_on_oops" sysctl) will also avoid this issue, by limiting the
effect to an application crash.  We are not currently sure whether
this is an effective mitigation, as we are not sure whether any locks
or mutexes are held at the point of the crash.

CREDITS
=======

This issue was discovered by Vitaly Kuznetsov from Red Hat.

RESOLUTION
==========

Applying the attached patch resolves this issue.

xsa174.patch           Linux 4.5.x ... 3.10.x

$ sha256sum xsa174*
cbec70e183f76b4081ebba05c0a8105bd4952d164a2e5c40528c05bf8861ddef  xsa174.patch
$

DEPLOYMENT DURING EMBARGO
=========================

Deployment of patches or mitigations is NOT permitted (except where
all the affected systems and VMs are administered and used only by
organisations which are members of the Xen Project Security Issues
Predisclosure List).  Specifically, deployment on public cloud systems
is NOT permitted.

This is because such host configuration changes would be user mode
visible, which could lead to the rediscovery of the vulnerability.

But: Distribution of updated software is prohibited (except to other
members of the predisclosure list).

Predisclosure list members who wish to deploy significantly different
patches and/or mitigations, please contact the Xen Project Security
Team.


(Note: this during-embargo deployment notice is retained in
post-embargo publicly released Xen Project advisories, even though it
is then no longer applicable.  This is to enable the community to have
oversight of the Xen Project Security Team's decisionmaking.)

For more information about permissible uses of embargoed information,
consult the Xen Project community's agreed Security Policy:
  http://www.xenproject.org/security-policy.html
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[-- Attachment #2: xsa174.patch --]
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x86/xen: suppress hugetlbfs in PV guests

Huge pages are not normally available to PV guests. Not suppressing
hugetlbfs use results in an endless loop of page faults when user mode
code tries to access a hugetlbfs mapped area (since the hypervisor
denies such PTEs to be created, but error indications can't be
propagated out of xen_set_pte_at(), just like for various of its
siblings), and - once killed in an oops like this:

kernel BUG at .../fs/hugetlbfs/inode.c:428!
invalid opcode: 0000 [#1] SMP 
Modules linked in: ...
Supported: Yes
CPU: 2 PID: 6088 Comm: hugetlbfs Tainted: G        W         4.4.0-2016-01-20-pv #2
Hardware name: ...
task: ffff8808059205c0 ti: ffff880803c84000 task.ti: ffff880803c84000
RIP: e030:[<ffffffff811c333b>]  [<ffffffff811c333b>] remove_inode_hugepages+0x25b/0x320
RSP: e02b:ffff880803c879a8  EFLAGS: 00010202
RAX: 000000000077a4db RBX: ffffea001acff000 RCX: 0000000078417d38
RDX: 0000000000000000 RSI: 000000007e154fa7 RDI: ffff880805d70960
RBP: 0000000000000960 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000001 R12: 0000000000000000
R13: ffff880807486018 R14: 0000000000000000 R15: ffff880803c87af0
FS:  00007f85fa8b8700(0000) GS:ffff88080b640000(0000) knlGS:0000000000000000
CS:  e033 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 00007f85fa000000 CR3: 0000000001a0a000 CR4: 0000000000040660
Stack:
 ffff880000000fb0 ffff880803c87a18 ffff880803c87ae8 ffff8808059205c0
 ffff880803c87af0 ffff880803c87ae8 ffff880807486018 0000000000000000
 ffffffff81bf6e60 ffff880807486168 000003ffffffffff 0000000003c87758
Call Trace:
 [<ffffffff811c3415>] hugetlbfs_evict_inode+0x15/0x40
 [<ffffffff81167b3d>] evict+0xbd/0x1b0
 [<ffffffff8116514a>] __dentry_kill+0x19a/0x1f0
 [<ffffffff81165b0e>] dput+0x1fe/0x220
 [<ffffffff81150535>] __fput+0x155/0x200
 [<ffffffff81079fc0>] task_work_run+0x60/0xa0
 [<ffffffff81063510>] do_exit+0x160/0x400
 [<ffffffff810637eb>] do_group_exit+0x3b/0xa0
 [<ffffffff8106e8bd>] get_signal+0x1ed/0x470
 [<ffffffff8100f854>] do_signal+0x14/0x110
 [<ffffffff810030e9>] prepare_exit_to_usermode+0xe9/0xf0
 [<ffffffff814178a5>] retint_user+0x8/0x13

This is XSA-174.

Reported-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Cc: stable@vger.kernel.org
---
v2: Make Xen-inspecific, by using cpu_has_pse.

--- a/arch/x86/include/asm/hugetlb.h
+++ b/arch/x86/include/asm/hugetlb.h
@@ -4,6 +4,7 @@
 #include <asm/page.h>
 #include <asm-generic/hugetlb.h>
 
+#define hugepages_supported() cpu_has_pse
 
 static inline int is_hugepage_only_range(struct mm_struct *mm,
 					 unsigned long addr,

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