Re: [tip:x86/asm] x86/asm/entry/64: Remove unused thread_struct::usersp
From: Denys Vlasenko
Date: Tue Mar 17 2015 - 08:51:36 EST
On 03/17/2015 01:22 PM, Denys Vlasenko wrote:
> On 03/17/2015 08:39 AM, Borislav Petkov wrote:
>> On Tue, Mar 17, 2015 at 08:21:18AM +0100, Ingo Molnar wrote:
>>> Assuming this does not fix the regression, could you apply the minimal
>>> patch below - which reverts the old_rsp handling change.
>>>
>>> (The rest of the commit are in a third patch, but those are only
>>> comment changes.)
>>>
>>> So my theory is that this change is what will revert the regression.
>>
>> Yep, it does. Below is the diff that works (it is the rough revert
>> without the comments :-)):
>>
> ...
>> @@ -395,6 +398,8 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
>> /*
>> * Switch the PDA and FPU contexts.
>> */
>> + prev->usersp = this_cpu_read(old_rsp);
>> + this_cpu_write(old_rsp, next->usersp);
>
> I have a theory. There is a time window when user's sp
> is in PER_CPU_VAR(old_rsp) but not yet in pt_regs->sp,
> and *interrupts are enabled*:
>
> ENTRY(system_call)
> SWAPGS_UNSAFE_STACK
> movq %rsp,PER_CPU_VAR(old_rsp)
> movq PER_CPU_VAR(kernel_stack),%rsp
> ENABLE_INTERRUPTS(CLBR_NONE)
> ALLOC_PT_GPREGS_ON_STACK 8 /* +8: space for orig_ax */
> movq %rcx,RIP(%rsp)
> movq PER_CPU_VAR(old_rsp),%rcx
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> movq %r11,EFLAGS(%rsp)
> movq %rcx,RSP(%rsp)
>
> Before indicated insn, interrupts are already enabled.
> If preempt would hit now, next task can clobber PER_CPU_VAR(old_rsp).
> Then, when we return to this task, a bogus user's sp will be stored
> in pt_regs, restores on exit to userspace, and next attempt
> to, say, execute RETQ will try to pop a bogus, likely noncanonical
> address into RIP -> #GP -> SEGV!
>
> The theory can be tested by just moving interrupt enable a bit down:
>
> ENTRY(system_call)
> SWAPGS_UNSAFE_STACK
> movq %rsp,PER_CPU_VAR(old_rsp)
> movq PER_CPU_VAR(kernel_stack),%rsp
> - ENABLE_INTERRUPTS(CLBR_NONE)
> ALLOC_PT_GPREGS_ON_STACK 8 /* +8: space for orig_ax */
> movq %rcx,RIP(%rsp)
> movq PER_CPU_VAR(old_rsp),%rcx
> movq %r11,EFLAGS(%rsp)
> movq %rcx,RSP(%rsp)
> + ENABLE_INTERRUPTS(CLBR_NONE)
>
> If I'm right, segfaults should be gone.
> Borislav, can you try this?
I managed to reproduce the segfault, and the fix shown above works.
I see that Ingo removed the failing commit from his tree.
I'll send two patches: one which moves ENABLE_INTERRUPTS(CLBR_NONE) down,
and another which tries to remove thread_struct::usersp again.
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