linux-kernel - [PATCH v2 04/10] x86/entry/32: Restore FLAGS on SYSEXIT

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Date:	Sat,  5 Mar 2016 21:52:17 -0800
From:	Andy Lutomirski <luto@...nel.org>
To:	x86@...nel.org
Cc:	linux-kernel@...r.kernel.org, Borislav Petkov <bp@...en8.de>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Brian Gerst <brgerst@...il.com>,
	Andy Lutomirski <luto@...nel.org>
Subject: [PATCH v2 04/10] x86/entry/32: Restore FLAGS on SYSEXIT

We weren't restoring FLAGS at all on SYSEXIT.  Apparently no one cared.

With this patch applied, native kernels should always honor
task_pt_regs()->flags, which opens the door for some sys_iopl
cleanups.  I'll do those as a separate series, though, since getting
it right will involve tweaking some paravirt ops.

(The short version is that, before this patch, sys_iopl, invoked via
 SYSENTER, wasn't guaranteed to ever transfer the updated
 regs->flags, so sys_iopl had to change the hardware flags register
 as well.)

Reported-by: Brian Gerst <brgerst@...il.com>
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/entry/entry_32.S | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 289a17bf0c71..a8c3424c3392 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -343,6 +343,15 @@ sysenter_past_esp:
 	popl	%eax			/* pt_regs->ax */
 
 	/*
+	 * Restore all flags except IF (we restore IF separately because
+	 * STI gives a one-instruction window in which we won't be interrupted,
+	 * whereas POPF does not.
+	 */
+	addl	$PT_EFLAGS-PT_DS, %esp	/* point esp at pt_regs->flags */
+	btr	$X86_EFLAGS_IF_BIT, (%esp)
+	popfl
+
+	/*
 	 * Return back to the vDSO, which will pop ecx and edx.
 	 * Don't bother with DS and ES (they already contain __USER_DS).
 	 */
-- 
2.5.0