linux-kernel - Re: [PATCH v3 09/51] x86/dumpstack: fix x86_32 kernel_stack

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Date:	Sun, 14 Aug 2016 06:42:03 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Josh Poimboeuf <jpoimboe@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	"H . Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Kees Cook <keescook@...omium.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Byungchul Park <byungchul.park@....com>,
	Nilay Vaish <nilayvaish@...il.com>
Subject: Re: [PATCH v3 09/51] x86/dumpstack: fix x86_32 kernel_stack_pointer()
 previous stack access

On Sun, Aug 14, 2016 at 5:55 AM, Brian Gerst <brgerst@...il.com> wrote:
> On Sun, Aug 14, 2016 at 3:26 AM, Andy Lutomirski <luto@...capital.net> wrote:
>> On Fri, Aug 12, 2016 at 7:28 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>>> On x86_32, when an interrupt happens from kernel space, SS and SP aren't
>>> pushed and the existing stack is used.  So pt_regs is effectively two
>>> words shorter, and the previous stack pointer is normally the memory
>>> after the shortened pt_regs, aka '&regs->sp'.
>>>
>>> But in the rare case where the interrupt hits right after the stack
>>> pointer has been changed to point to an empty stack, like for example
>>> when call_on_stack() is used, the address immediately after the
>>> shortened pt_regs is no longer on the stack.  In that case, instead of
>>> '&regs->sp', the previous stack pointer should be retrieved from the
>>> beginning of the current stack page.
>>>
>>> kernel_stack_pointer() wants to do that, but it forgets to dereference
>>> the pointer.  So instead of returning a pointer to the previous stack,
>>> it returns a pointer to the beginning of the current stack.
>>>
>>> Fixes: 0788aa6a23cb ("x86: Prepare removal of previous_esp from i386 thread_info structure")
>>> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
>>
>> This seems like a valid fix, but I'm not sure I agree with the intent
>> of the code.  &regs->sp really is the previous stack pointer in the
>> sense that the stack pointer was &regs->sp when the entry happened.
>> From an unwinder's perspective, how is:
>>
>> movl [whatever], $esp
>> <-- interrupt
>>
>> any different from:
>>
>> movl [whatever], $esp
>> pushl [something]
>> <-- interrupt
>>
>> Also, does x86_32 do this type of stack switching at all?  AFAICS
>> 32-bit kernels don't use IRQ stacks in the first place.  Do they?  Am
>> I just missing the code that does it?
>
> 32-bit uses a software-based stack switch to run on the IRQ stack.
> See execute_on_irq_stack() in irq_32.c.
>

Indeed, thanks.

I'm still not convinced that kernel_stack_pojnter() needs to handle this.