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

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Mon, Aug 15, 2016 at 10:05:58AM -0500, Josh Poimboeuf wrote:
> 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@redhat.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@redhat.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
> 
> 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.

...but maybe your point is that following the previous stack pointer is
outside the scope of kernel_stack_pointer() and should instead be done
by its caller.  Especially considering the fact that the x86_64 version
of this function doesn't have this "feature".  In which case I think I
would agree.

However I think fixing that is outside the scope of this
already-way-too-big patch set.

-- 
Josh