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@gmail.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/3f98b207472dc9784838eb5ca2b89dcc845ce269.1457578375.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 8daa812..7610906 100644 (file)
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -342,6 +342,15 @@ sysenter_past_esp:
        popl    %ebp                    /* pt_regs->bp */
        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).