[PATCH v2 04/12] x86/entry/32: Restore FLAGS on SYSEXIT

From: Andy Lutomirski
Date: Wed Mar 09 2016 - 22:01:11 EST

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@xxxxxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
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 86665f93ff5d..d1437fdbf597 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).