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

From: Peter Zijlstra
Date: Thu Feb 14 2019 - 05:14:45 EST

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 canât 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@xxxxxxxxxxxxx>
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@xxxxxxxxxxxxxx>
Reported-by: Julien Thierry <julien.thierry@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
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)

/* 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)

/* 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;