x86/entry/64/compat: Fix Xen PV SYSENTER frame setup

The SYSENTER frame setup was nonsense.  It worked by accident because the
normal code into which the Xen asm jumped (entry_SYSENTER_32/compat) threw
away SP without touching the stack.  entry_SYSENTER_compat was recently
modified such that it relied on having a valid stack pointer, so now the
Xen asm needs to invoke it with a valid stack.

Fix it up like SYSCALL: use the Xen-provided frame and skip the bare
metal prologue.

Fixes: 1c3e5d3f60 ("x86/entry: Make entry_64_compat.S objtool clean")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lkml.kernel.org/r/947880c41ade688ff4836f665d0c9fcaa9bd1201.1593191971.git.luto@kernel.org

arch/x86/xen/xen-asm_64.S

@@ -161,10 +161,22 @@ SYM_FUNC_END(xen_syscall32_target)
 
 /* 32-bit compat sysenter target */
 SYM_FUNC_START(xen_sysenter_target)
-	mov 0*8(%rsp), %rcx
-	mov 1*8(%rsp), %r11
-	mov 5*8(%rsp), %rsp
-	jmp entry_SYSENTER_compat
+	/*
+	 * NB: Xen is polite and clears TF from EFLAGS for us.  This means
+	 * that we don't need to guard against single step exceptions here.
+	 */
+	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_SYSENTER_compat_after_hwframe
 SYM_FUNC_END(xen_sysenter_target)
 
 #else /* !CONFIG_IA32_EMULATION */