linux-kernel - [PATCH] x86/stackframe/32: repair 32-bit Xen PV

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Date:   Mon, 7 Oct 2019 12:41:41 +0200
From:   Jan Beulich <jbeulich@...e.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     lkml <linux-kernel@...r.kernel.org>
Subject: [PATCH] x86/stackframe/32: repair 32-bit Xen PV

Once again RPL checks have been introduced which don't account for a
32-bit kernel living in ring 1 when running in a PV Xen domain. The
case in FIXUP_FRAME has been preventing boot; adjust BUG_IF_WRONG_CR3
as well just in case.

Fixes: 3c88c692c287 ("x86/stackframe/32: Provide consistent pt_regs")
Signed-off-by: Jan Beulich <jbeulich@...e.com>

--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -48,6 +48,17 @@
 
 #include "calling.h"
 
+#ifndef CONFIG_XEN_PV
+# define USER_SEGMENT_RPL_MASK SEGMENT_RPL_MASK
+#else
+/*
+ * When running paravirtualized on Xen the kernel runs in ring 1, and hence
+ * simple mask based tests (i.e. ones not comparing against USER_RPL) have to
+ * ignore bit 0. See also the C-level get_kernel_rpl().
+ */
+# define USER_SEGMENT_RPL_MASK (SEGMENT_RPL_MASK & ~1)
+#endif
+
 	.section .entry.text, "ax"
 
 /*
@@ -172,7 +183,7 @@
 	ALTERNATIVE "jmp .Lend_\@", "", X86_FEATURE_PTI
 	.if \no_user_check == 0
 	/* coming from usermode? */
-	testl	$SEGMENT_RPL_MASK, PT_CS(%esp)
+	testl	$USER_SEGMENT_RPL_MASK, PT_CS(%esp)
 	jz	.Lend_\@
 	.endif
 	/* On user-cr3? */
@@ -217,7 +228,7 @@
 	testl	$X86_EFLAGS_VM, 4*4(%esp)
 	jnz	.Lfrom_usermode_no_fixup_\@
 #endif
-	testl	$SEGMENT_RPL_MASK, 3*4(%esp)
+	testl	$USER_SEGMENT_RPL_MASK, 3*4(%esp)
 	jnz	.Lfrom_usermode_no_fixup_\@
 
 	orl	$CS_FROM_KERNEL, 3*4(%esp)