[PATCH] x86/xen/64: Fix the reported SS and CS in SYSCALL

[Andy Lutomirski <luto@kernel.org>]

When I cleaned up the Xen SYSCALL entries, I inadvertently changed
the reported segment registers.  Before my patch, regs->ss was
__USER(32)_DS and regs->cs was __USER(32)_CS.  After the patch, they
are FLAT_USER_CS/DS(32).

This had a couple unfortunate effects.  It confused the
opportunistic fast return logic.  It also significantly increased
the risk of triggering a nasty glibc bug:

https://sourceware.org/bugzilla/show_bug.cgi?id=21269

Update the Xen entry code to change it back.

Reported-by: Brian Gerst <brgerst@gmail.com>
Fixes: 8a9949bc71a7 ("x86/xen/64: Rearrange the SYSCALL entries")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/xen/xen-asm_64.S | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/arch/x86/xen/xen-asm_64.S b/arch/x86/xen/xen-asm_64.S
index a8a4f4c460a6..c5fee2680abc 100644
--- a/arch/x86/xen/xen-asm_64.S
+++ b/arch/x86/xen/xen-asm_64.S
@@ -88,6 +88,15 @@ RELOC(xen_sysret64, 1b+1)
 ENTRY(xen_syscall_target)
 	popq %rcx
 	popq %r11
+
+	/*
+	 * Neither Xen nor the kernel really knows what the old SS and
+	 * CS were.  The kernel expects __USER_DS and __USER_CS, so
+	 * report those values even though Xen will guess its own values.
+	 */
+	movq $__USER_DS, 4*8(%rsp)
+	movq $__USER_CS, 1*8(%rsp)
+
 	jmp entry_SYSCALL_64_after_hwframe
 ENDPROC(xen_syscall_target)
 
@@ -97,6 +106,15 @@ ENDPROC(xen_syscall_target)
 ENTRY(xen_syscall32_target)
 	popq %rcx
 	popq %r11
+
+	/*
+	 * Neither Xen nor the kernel really knows what the old SS and
+	 * CS were.  The kernel expects __USER32_DS and __USER32_CS, so
+	 * report those values even though Xen will guess its own values.
+	 */
+	movq $__USER32_DS, 4*8(%rsp)
+	movq $__USER32_CS, 1*8(%rsp)
+
 	jmp entry_SYSCALL_compat_after_hwframe
 ENDPROC(xen_syscall32_target)
 
-- 
2.13.3