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[Xen-devel] [PATCH] 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>] 
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
 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 CVE-2016-3961 / XSA-174.

Reported-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx 
 arch/x86/include/asm/hugetlb.h |    1 +
 1 file changed, 1 insertion(+)

--- 4.6-rc4/arch/x86/include/asm/hugetlb.h
+++ 4.6-rc4-xsa174/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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