This is a note to let you know that I've just added the patch titled
xen: Fix possible user space selector corruption
to the 3.10-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
xen-fix-possible-user-space-selector-corruption.patch
and it can be found in the queue-3.10 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 7cde9b27e7b3a2e09d647bb4f6d94e842698d2d5 Mon Sep 17 00:00:00 2001
From: Frediano Ziglio <frediano.ziglio@xxxxxxxxxx>
Date: Thu, 10 Oct 2013 14:39:37 +0000
Subject: xen: Fix possible user space selector corruption
From: Frediano Ziglio <frediano.ziglio@xxxxxxxxxx>
commit 7cde9b27e7b3a2e09d647bb4f6d94e842698d2d5 upstream.
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>
Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/x86/xen/smp.c | 9 +++++++++
1 file changed, 9 insertions(+)
--- a/arch/x86/xen/smp.c
+++ b/arch/x86/xen/smp.c
@@ -245,6 +245,15 @@ static void __init xen_smp_prepare_boot_
old memory can be recycled */
make_lowmem_page_readwrite(xen_initial_gdt);
+#ifdef CONFIG_X86_32
+ /*
+ * Xen starts us with XEN_FLAT_RING1_DS, but linux code
+ * expects __USER_DS
+ */
+ loadsegment(ds, __USER_DS);
+ loadsegment(es, __USER_DS);
+#endif
+
xen_filter_cpu_maps();
xen_setup_vcpu_info_placement();
}
Patches currently in stable-queue which might be from frediano.ziglio@xxxxxxxxxx are
queue-3.10/xen-fix-possible-user-space-selector-corruption.patch
--
To unsubscribe from this list: send the line "unsubscribe stable" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html