Re: [Xen-devel] [PATCH 1/2] x86/traps: guard top-of-stack reads

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 31/05/2019 10:17, Jan Beulich wrote:
> Nothing (afaics) guarantees that the original frame's stack pointer
> points at readable memory.

Having hit just the scenario described here, the answer is "nothing".

>  Avoid a (likely nested) crash by attaching
> exception recovery to the read (making it a single read at the same
> time). Don't even invoke _show_trace() in case of a non-readable top
> slot.
>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>
> --- a/xen/arch/x86/traps.c
> +++ b/xen/arch/x86/traps.c
> @@ -484,16 +484,23 @@ static void _show_trace(unsigned long sp
>  
>  static void show_trace(const struct cpu_user_regs *regs)
>  {
> -    unsigned long *sp = ESP_BEFORE_EXCEPTION(regs);
> +    unsigned long *sp = ESP_BEFORE_EXCEPTION(regs), tos = 0;
>  
>      printk("Xen call trace:\n");
>  

/* Probe the stack for readability. */

> +    asm ( "1: mov %2,%0; 2:\n"
> +          ".pushsection .fixup,\"ax\"\n"
> +          "3: xor %k1,%k1; jmp 2b\n"

Can we use some named parameters, so the asm can actually be followed?

Also, you can't do this by zeroing sp, because it aliases with "sp was
at zero and readable".  A better option would be to get an explicit
fault boolean out of the asm.

> +          ".popsection\n"
> +          _ASM_EXTABLE(1b, 3b)
> +          : "+r" (tos), "+r" (sp) : "m" (*sp) );
> +
>      /*
>       * If RIP looks sensible, or the top of the stack doesn't, print RIP at
>       * the top of the stack trace.
>       */
>      if ( is_active_kernel_text(regs->rip) ||
> -         !is_active_kernel_text(*sp) )
> +         !is_active_kernel_text(tos) )
>          printk("   [<%p>] %pS\n", _p(regs->rip), _p(regs->rip));
>      /*
>       * Else RIP looks bad but the top of the stack looks good.  Perhaps we
> @@ -501,12 +508,15 @@ static void show_trace(const struct cpu_
>       * return address; print it and skip past so _show_trace() doesn't print
>       * it again.
>       */
> -    else
> +    else if ( sp )
>      {
> -        printk("   [<%p>] %pS\n", _p(*sp), _p(*sp));
> +        printk("   [<%p>] %pS\n", _p(tos), _p(tos));
>          sp++;
>      }
>  
> +    if ( !sp )
> +        return;

Along with the alias mentioned above, this also has a boundary case when
sp is -8, due to the sp++ above.

It would probably be better to fit an

else if ( fault )
{
    printk("   [Fault on access]\n");
    return;
}

into the middle of the existing if/else.

~Andrew

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