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 <[email protected]> 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 <[email protected]> Reported-by: Julien Thierry <[email protected]> Signed-off-by: Peter Zijlstra (Intel) <[email protected]> --- 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;
