Messages in this thread Patch in this message | | | Date | Thu, 14 Feb 2019 11:14:29 +0100 | From | Peter Zijlstra <> | Subject | [PATCH] sched/x86: Save [ER]FLAGS on context switch |
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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;
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