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Message-ID: <CA+CK2bBgeK=rW3=RJSx8zxST8xnJ-VXSSYmKPVK9gHX3pxEWuQ@mail.gmail.com>
Date: Wed, 13 Mar 2024 09:43:50 -0400
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Thomas Gleixner <tglx@...utronix.de>
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Subject: Re: [RFC 11/14] x86: add support for Dynamic Kernel Stacks
On Wed, Mar 13, 2024 at 6:23 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Mon, Mar 11 2024 at 16:46, Pasha Tatashin wrote:
> > @@ -413,6 +413,9 @@ DEFINE_IDTENTRY_DF(exc_double_fault)
> > }
> > #endif
> >
> > + if (dynamic_stack_fault(current, address))
> > + return;
> > +
> > irqentry_nmi_enter(regs);
> > instrumentation_begin();
> > notify_die(DIE_TRAP, str, regs, error_code, X86_TRAP_DF, SIGSEGV);
> > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> > index d6375b3c633b..651c558b10eb 100644
> > --- a/arch/x86/mm/fault.c
> > +++ b/arch/x86/mm/fault.c
> > @@ -1198,6 +1198,9 @@ do_kern_addr_fault(struct pt_regs *regs, unsigned long hw_error_code,
> > if (is_f00f_bug(regs, hw_error_code, address))
> > return;
> >
> > + if (dynamic_stack_fault(current, address))
> > + return;
>
> T1 schedules out with stack used close to the fault boundary.
>
> switch_to(T2)
>
> Now T1 schedules back in
>
> switch_to(T1)
> __switch_to_asm()
> ...
> switch_stacks() <- SP on T1 stack
> ! ...
> ! jmp __switch_to()
> ! __switch_to()
> ! ...
> ! raw_cpu_write(pcpu_hot.current_task, next_p);
>
> After switching SP to T1's stack and up to the point where
> pcpu_hot.current_task (aka current) is updated to T1 a stack fault will
> invoke dynamic_stack_fault(T2, address) which will return false here:
>
> /* check if address is inside the kernel stack area */
> stack = (unsigned long)tsk->stack;
> if (address < stack || address >= stack + THREAD_SIZE)
> return false;
>
> because T2's stack does obviously not cover the faulting address on T1's
> stack. As a consequence double fault will panic the machine.
Hi Thomas,
Thank you, you are absolutely right, we can't trust "current" in the
fault handler.
We can change dynamic_stack_fault() to only accept fault_address as an
argument, and let it determine the right task_struct pointer
internally.
Let's modify dynamic_stack_fault() to accept only the fault_address.
It can then determine the correct task_struct pointer internally.
Here's a potential solution that is fast, avoids locking, and ensures atomicity:
1. Kernel Stack VA Space
Dedicate a virtual address range ([KSTACK_START_VA - KSTACK_END_VA])
exclusively for kernel stacks. This simplifies validation of faulting
addresses to be part of a stack.
2. Finding the faulty task
- Use ALIGN(fault_address, THREAD_SIZE) to calculate the end of the
topmost stack page (since stack addresses are aligned to THREAD_SIZE).
- Store the task_struct pointer as the last word on this topmost page,
that is always present as it is a pre-allcated stack page.
3. Stack Padding
Increase padding to 8 bytes on x86_64 (TOP_OF_KERNEL_STACK_PADDING 8)
to accommodate the task_struct pointer.
Another issue that this race brings is that 3-pages per-cpu might not
be enough, we might need up-to 6 pages: 3 to cover going-away task,
and 3 to cover the new task.
Pasha
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