Re: [PATCH] sched/x86: Save [ER]FLAGS on context switch

hpa
2019-02-16T04:07:42Z

On February 14, 2019 2:14:29 AM PST, Peter Zijlstra <peterz@infradead.org> wrote:
>On Wed, Feb 13, 2019 at 02:49:47PM -0800, Andy Lutomirski wrote:
>
>> Do we need to backport this thing?
>
>Possibly, just to be safe.
>
>> The problem can’t be too widespread or we would have heard of it
>before.
>
>Yes, so far we've been lucky.
>
>---
>Subject: sched/x86: Save [ER]FLAGS on context switch
>From: Peter Zijlstra <peterz@infradead.org>
>Date: Thu Feb 14 10:30:52 CET 2019
>
>Effectively revert commit:
>
> 2c7577a75837 ("sched/x86_64: Don't save flags on context switch")
>
>Specifically because SMAP uses FLAGS.AC which invalidates the claim
>that the kernel has clean flags.
>
>In particular; while preemption from interrupt return is fine (the
>IRET frame on the exception stack contains FLAGS) it breaks any code
>that does synchonous scheduling, including preempt_enable().
>
>This has become a significant issue ever since commit:
>
>5b24a7a2aa20 ("Add 'unsafe' user access functions for batched
>accesses")
>
>provided for means of having 'normal' C code between STAC / CLAC,
>exposing the FLAGS.AC state. So far this hasn't led to trouble,
>however fix it before it comes apart.
>
>Fixes: 5b24a7a2aa20 ("Add 'unsafe' user access functions for batched
>accesses")
>Acked-by: Andy Lutomirski <luto@amacapital.net>
>Reported-by: Julien Thierry <julien.thierry@arm.com>
>Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>---
> arch/x86/entry/entry_32.S | 2 ++
> arch/x86/entry/entry_64.S | 2 ++
> arch/x86/include/asm/switch_to.h | 1 +
> 3 files changed, 5 insertions(+)
>
>--- 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
>--- a/arch/x86/entry/entry_64.S
>+++ b/arch/x86/entry/entry_64.S
>@@ -291,6 +291,7 @@ ENTRY(__switch_to_asm)
> pushq %r13
> pushq %r14
> pushq %r15
>+ pushfq
>
> /* switch stack */
> movq %rsp, TASK_threadsp(%rdi)
>@@ -313,6 +314,7 @@ ENTRY(__switch_to_asm)
> #endif
>
> /* restore callee-saved registers */
>+ popfq
> popq %r15
> popq %r14
> popq %r13
>--- a/arch/x86/include/asm/switch_to.h
>+++ b/arch/x86/include/asm/switch_to.h
>@@ -40,6 +40,7 @@ asmlinkage void ret_from_fork(void);
> * order of the fields must match the code in __switch_to_asm().
> */
> struct inactive_task_frame {
>+ unsigned long flags;
> #ifdef CONFIG_X86_64
> unsigned long r15;
> unsigned long r14;

This implies we invoke schedule -- a restricted operation (consider may_sleep) during execution of STAC-enabled code, but *not* as an exception or interrupt, since those preserve the flags.

I have serious concerns about this. This is more or less saying that we have left an unlimited gap and have had AC escape.

Does *anyone* see a need to allow this? I got a question at LPC from someone about this, and what they were trying to do once all the layers had been unwound was so far down the wrong track for a root problem that actually has a very simple solution.

--
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