linux-kernel - [095/151] xen: use iret for return from 64b kernel to 32b usermode

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Message-Id: <20091217035655.322127547@mini.kroah.org>
Date:	Wed, 16 Dec 2009 19:56:32 -0800
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Jan Beulich <jbeulich@...ell.com>
Subject: [095/151] xen: use iret for return from 64b kernel to 32b usermode

2.6.32-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>

commit 6aaf5d633bb6cead81b396d861d7bae4b9a0ba7e upstream.

If Xen wants to return to a 32b usermode with sysret it must use the
right form.  When using VCGF_in_syscall to trigger this, it looks at
the code segment and does a 32b sysret if it is FLAT_USER_CS32.
However, this is different from __USER32_CS, so it fails to return
properly if we use the normal Linux segment.

So avoid the whole mess by dropping VCGF_in_syscall and simply use
plain iret to return to usermode.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Acked-by: Jan Beulich <jbeulich@...ell.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

---
 arch/x86/xen/xen-asm_64.S |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/arch/x86/xen/xen-asm_64.S
+++ b/arch/x86/xen/xen-asm_64.S
@@ -96,7 +96,7 @@ ENTRY(xen_sysret32)
 	pushq $__USER32_CS
 	pushq %rcx
 
-	pushq $VGCF_in_syscall
+	pushq $0
 1:	jmp hypercall_iret
 ENDPATCH(xen_sysret32)
 RELOC(xen_sysret32, 1b+1)
@@ -151,7 +151,7 @@ ENTRY(xen_syscall32_target)
 ENTRY(xen_sysenter_target)
 	lea 16(%rsp), %rsp	/* strip %rcx, %r11 */
 	mov $-ENOSYS, %rax
-	pushq $VGCF_in_syscall
+	pushq $0
 	jmp hypercall_iret
 ENDPROC(xen_syscall32_target)
 ENDPROC(xen_sysenter_target)


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