Re: [PATCH v3 48/51] x86/unwind: warn if stack grows up

[Andy Lutomirski <luto@amacapital.net>]

On Fri, Aug 12, 2016 at 7:29 AM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> Add a sanity check to ensure the stack only grows down, and print a
> warning if the check fails.
>
> Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
> ---
>  arch/x86/kernel/unwind_frame.c | 26 ++++++++++++++++++++++++--
>  1 file changed, 24 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/unwind_frame.c b/arch/x86/kernel/unwind_frame.c
> index 5496462..f21b7ef 100644
> --- a/arch/x86/kernel/unwind_frame.c
> +++ b/arch/x86/kernel/unwind_frame.c
> @@ -32,6 +32,15 @@ unsigned long unwind_get_return_address(struct unwind_state *state)
>  }
>  EXPORT_SYMBOL_GPL(unwind_get_return_address);
>
> +static size_t regs_size(struct pt_regs *regs)
> +{
> +       /* x86_32 regs from kernel mode are two words shorter */
> +       if (IS_ENABLED(CONFIG_X86_32) && !user_mode(regs))
> +               return sizeof(*regs) - (2*sizeof(long));
> +
> +       return sizeof(*regs);
> +}
> +
>  static bool is_last_task_frame(struct unwind_state *state)
>  {
>         unsigned long bp = (unsigned long)state->bp;
> @@ -85,6 +94,7 @@ bool unwind_next_frame(struct unwind_state *state)
>         struct pt_regs *regs;
>         unsigned long *next_bp, *next_sp;
>         size_t next_len;
> +       enum stack_type prev_type = state->stack_info.type;
>
>         if (unwind_done(state))
>                 return false;
> @@ -140,6 +150,18 @@ bool unwind_next_frame(struct unwind_state *state)
>         if (!update_stack_state(state, next_sp, next_len))
>                 goto bad_address;
>
> +       /* make sure it only unwinds up and doesn't overlap the last frame */
> +       if (state->stack_info.type == prev_type) {
> +               if (state->regs &&
> +                   (void *)next_sp < (void *)state->regs +
> +                                     regs_size(state->regs))
> +                       goto bad_address;
> +
> +               if (state->bp &&
> +                   (void *)next_sp < (void *)state->bp + FRAME_HEADER_SIZE)
> +                       goto bad_address;
> +       }
> +

Maybe this is obvious in context, but does something prevent this
error from firing if the stack switched?  That is:

pushq $rbp
movq $rsp, $rbp
...
movq [irq stack], $rsp
<- rsp and rbp have no particular relationship right now.