Re: [PATCH v3 09/51] x86/dumpstack: fix x86_32 kernel_stack_pointer() previous stack access

[Josh Poimboeuf]

On Sun, Aug 14, 2016 at 12:26:29AM -0700, Andy Lutomirski wrote:
> On Fri, Aug 12, 2016 at 7:28 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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@xxxxxxxxxx>
> 
> 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

In the first case, the stack is empty, so reading the value pointed to
by %esp would result in accessing outside the bounds of the stack.

-- 
Josh