On Mon, Mar 18, 2019 at 09:58:20AM -0700, Linus Torvalds wrote:
> On Mon, Mar 18, 2019 at 8:54 AM Peter Zijlstra <pet...@infradead.org> wrote:
> >
> > We rely on objtool to verify AC=1 doesn't escape. However there is no
> > objtool support for x86_32, and thus we cannot guarantee the
> > correctness of the 32bit code.
>
> Absolutely not.
>
> This is just crazy. We had working SMAP long before objtool, and we
> will have it regardless of objtool.
Well, 'working', because as shown with this work, there's been a number
of bugs around this.
Anyway, since you mentioned it, I checked, about 16 months: Broadwell
was Oct'14, objtool was Feb'16.
> This is like saying "ok, I don't have an oxygen sensor, so I don't
> know that the air I'm breathing is sufficient to maintain life, so
> I'll just stop breathing".
With PTI, booting a 32bit kernel on a Broadwell or later will already
complain about it being sub-optimal for missing out on PCID. But sure;
if you want to enable this..
> So no way in hell do we make SMAP go away on 32-bit for no sane reason
> what-so-ever.
then I'd sleep much better if we make 32bit context switch EFLAGS.
Because, yes, x86_64 objtool validates a lot of the x86_32 code too, but
unless we have 100% coverage, there's always the chance an AC=1 escapes.
---
arch/x86/entry/entry_32.S | 2 ++
arch/x86/include/asm/switch_to.h | 1 +
arch/x86/kernel/process_32.c | 5 +++++
3 files changed, 8 insertions(+)
diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index d309f30cf7af..5fc76b755510 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -650,6 +650,7 @@ ENTRY(__switch_to_asm)
pushl %ebx
pushl %edi
pushl %esi
+ pushfl
/* switch stack */
movl %esp, TASK_threadsp(%eax)
@@ -672,6 +673,7 @@ ENTRY(__switch_to_asm)
#endif
/* restore callee-saved registers */
+ popfl
popl %esi
popl %edi
popl %ebx
diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
index 7cf1a270d891..18a4b6890fa8 100644
--- a/arch/x86/include/asm/switch_to.h
+++ b/arch/x86/include/asm/switch_to.h
@@ -46,6 +46,7 @@ struct inactive_task_frame {
unsigned long r13;
unsigned long r12;
#else
+ unsigned long flags;
unsigned long si;
unsigned long di;
#endif
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index e471d8e6f0b2..d28e9a74b736 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -127,6 +127,11 @@ int copy_thread_tls(unsigned long clone_flags, unsigned
long sp,
struct task_struct *tsk;
int err;
+ /*
+ * We start a new task with the RESET value of EFLAGS, in particular we
+ * context switch with IRQs disabled.
+ */
+ frame->flags = X86_EFLAGS_FIXED;
frame->bp = 0;
frame->ret_addr = (unsigned long) ret_from_fork;
p->thread.sp = (unsigned long) fork_frame;
