[PATCH 04/10] x86/entry/32: Restore FLAGS on SYSEXIT
From: Andy Lutomirski
Date: Mon Feb 29 2016 - 00:32:29 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
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 263ebde6333f..ed171f938960 100644
@@ -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)
* Return back to the vDSO, which will pop ecx and edx.
* Don't bother with DS and ES (they already contain __USER_DS).