[PATCH] xen: Fix possible user space selector corruption

[Frediano Ziglio]

Due to the way kernel is initialized under Xen is possible that the ring1
selector used by the kernel for the boot cpu end up to be copied to
userspace leading to segmentation fault in the userspace.


Xen code in the kernel initialize no-boot cpus with correct selectors (ds
and es set to __USER_DS) but the boot one keep the ring1 (passed by Xen).
On task context switch (switch_to) we assume that ds, es and cs already
point to __USER_DS and __KERNEL_CSso these selector are not changed.

If processor is an Intel that support sysenter instruction sysenter/sysexit
is used so ds and es are not restored switching back from kernel to
userspace. In the case the selectors point to a ring1 instead of __USER_DS
the userspace code will crash on first memory access attempt (to be
precise Xen on the emulated iret used to do sysexit will detect and set ds
and es to zero which lead to GPF anyway).

Now if an userspace process call kernel using sysenter and get rescheduled
(for me it happen on a specific init calling wait4) could happen that the
ring1 selector is set to ds and es.

This is quite hard to detect cause after a while these selectors are fixed
(__USER_DS seems sticky).

Bisecting the code commit 7076aada1040de4ed79a5977dbabdb5e5ea5e249 appears
to be the first one that have this issue.

Signed-off-by: Frediano Ziglio <frediano.ziglio@xxxxxxxxxx>
---
 arch/x86/xen/smp.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/arch/x86/xen/smp.c b/arch/x86/xen/smp.c
index d1e4777..2a47241 100644
--- a/arch/x86/xen/smp.c
+++ b/arch/x86/xen/smp.c
@@ -278,6 +278,18 @@ static void __init xen_smp_prepare_boot_cpu(void)
 		   old memory can be recycled */
 		make_lowmem_page_readwrite(xen_initial_gdt);
 
+#ifdef CONFIG_X86_32
+		/*
+		 * Assure we use segments with user level access.
+		 * During switching of task these segments got not reloaded
+		 * so it could happen that userspace tasks get Xen ring1
+		 * selector causing exit with sysenter failures on next
+		 * userspace memory operation.
+		 */
+		loadsegment(ds, __USER_DS);
+		loadsegment(es, __USER_DS);
+#endif
+
 		xen_filter_cpu_maps();
 		xen_setup_vcpu_info_placement();
 	}
-- 
1.7.10.4


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