Re: [RFC PATCH v3 2/4] arm64: Check the return PC against unreliable code sections

[Ard Biesheuvel <ardb@kernel.org>]

On Mon, 3 May 2021 at 19:38, <madvenka@linux.microsoft.com> wrote:
>
> From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
>
> Create a sym_code_ranges[] array to cover the following text sections that
> contain functions defined as SYM_CODE_*(). These functions are low-level
> functions (and do not have a proper frame pointer prolog and epilog). So,
> they are inherently unreliable from a stack unwinding perspective.
>
>         .entry.text
>         .idmap.text
>         .hyp.idmap.text
>         .hyp.text
>         .hibernate_exit.text
>         .entry.tramp.text
>
> If a return PC falls in any of these, mark the stack trace unreliable.
>
> The only exception to this is - if the unwinder has reached the last
> frame already, it will not mark the stack trace unreliable since there
> is no more unwinding to do. E.g.,
>
>         - ret_from_fork() occurs at the end of the stack trace of
>           kernel tasks.
>
>         - el0_*() functions occur at the end of EL0 exception stack
>           traces. This covers all user task entries into the kernel.
>
> NOTE:
>         - EL1 exception handlers are in .entry.text. So, stack traces that
>           contain those functions will be marked not reliable. This covers
>           interrupts, exceptions and breakpoints encountered while executing
>           in the kernel.
>
>         - At the end of an interrupt, the kernel can preempt the current
>           task if required. So, the stack traces of all preempted tasks will
>           show the interrupt frame and will be considered unreliable.
>
> Signed-off-by: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
> ---
>  arch/arm64/kernel/stacktrace.c | 54 ++++++++++++++++++++++++++++++++++
>  1 file changed, 54 insertions(+)
>
> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
> index c21a1bca28f3..1ff14615a55a 100644
> --- a/arch/arm64/kernel/stacktrace.c
> +++ b/arch/arm64/kernel/stacktrace.c
> @@ -15,9 +15,48 @@
>
>  #include <asm/irq.h>
>  #include <asm/pointer_auth.h>
> +#include <asm/sections.h>
>  #include <asm/stack_pointer.h>
>  #include <asm/stacktrace.h>
>
> +struct code_range {
> +       unsigned long   start;
> +       unsigned long   end;
> +};
> +
> +struct code_range      sym_code_ranges[] =

This should be static and const

> +{
> +       /* non-unwindable ranges */
> +       { (unsigned long)__entry_text_start,
> +         (unsigned long)__entry_text_end },
> +       { (unsigned long)__idmap_text_start,
> +         (unsigned long)__idmap_text_end },
> +       { (unsigned long)__hyp_idmap_text_start,
> +         (unsigned long)__hyp_idmap_text_end },
> +       { (unsigned long)__hyp_text_start,
> +         (unsigned long)__hyp_text_end },
> +#ifdef CONFIG_HIBERNATION
> +       { (unsigned long)__hibernate_exit_text_start,
> +         (unsigned long)__hibernate_exit_text_end },
> +#endif
> +#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
> +       { (unsigned long)__entry_tramp_text_start,
> +         (unsigned long)__entry_tramp_text_end },
> +#endif
> +       { /* sentinel */ }
> +};
> +
> +static struct code_range *lookup_range(unsigned long pc)

const struct code_range *

> +{
> +       struct code_range *range;

const struct code_range *

> +
> +       for (range = sym_code_ranges; range->start; range++) {
> +               if (pc >= range->start && pc < range->end)
> +                       return range;
> +       }
> +       return range;
> +}
> +
>  /*
>   * AArch64 PCS assigns the frame pointer to x29.
>   *
> @@ -43,6 +82,7 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
>  {
>         unsigned long fp = frame->fp;
>         struct stack_info info;
> +       struct code_range *range;

const struct code_range *

>
>         frame->reliable = true;
>
> @@ -103,6 +143,8 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
>                 return 0;
>         }
>
> +       range = lookup_range(frame->pc);
> +
>  #ifdef CONFIG_FUNCTION_GRAPH_TRACER
>         if (tsk->ret_stack &&
>                 frame->pc == (unsigned long)return_to_handler) {
> @@ -118,9 +160,21 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
>                         return -EINVAL;
>                 frame->pc = ret_stack->ret;
>                 frame->pc = ptrauth_strip_insn_pac(frame->pc);
> +               return 0;
>         }
>  #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
>
> +       if (!range->start)
> +               return 0;
> +
> +       /*
> +        * The return PC falls in an unreliable function. If the final frame
> +        * has been reached, no more unwinding is needed. Otherwise, mark the
> +        * stack trace not reliable.
> +        */
> +       if (frame->fp)
> +               frame->reliable = false;
> +
>         return 0;
>  }
>  NOKPROBE_SYMBOL(unwind_frame);
> --
> 2.25.1
>
>
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