Re: [PATCH] x86/dumpstack/64: Don't evaluate exception stacks before setup

[Josh Poimboeuf]

On Wed, Oct 23, 2019 at 08:05:49PM +0200, Thomas Gleixner wrote:
> Cyrill reported the following crash:
> 
>   BUG: unable to handle page fault for address: 0000000000001ff0
>   #PF: supervisor read access in kernel mode
>   RIP: 0010:get_stack_info+0xb3/0x148
> 
> It turns out that if the stack tracer is invoked before the exception stack
> mappings are initialized in_exception_stack() can erroneously classify an
> invalid address as an address inside of an exception stack:
> 
>     begin = this_cpu_read(cea_exception_stacks);  <- 0
>     end = begin + sizeof(exception stacks);
> 
> i.e. any address between 0 and end will be considered as exception stack
> address and the subsequent code will then try to derefence the resulting
> stack frame at a non mapped address.
> 
>  end = begin + (unsigned long)ep->size;
>      ==> end = 0x2000
> 
>  regs = (struct pt_regs *)end - 1;
>      ==> regs = 0x2000 - sizeof(struct pt_regs *) = 0x1ff0
> 
>  info->next_sp   = (unsigned long *)regs->sp;
>      ==> Crashes due to accessing 0x1ff0
> 
> Prevent this by checking the validity of the cea_exception_stack base
> address and bailing out if it is zero.
> 
> Fixes: afcd21dad88b ("x86/dumpstack/64: Use cpu_entry_area instead of orig_ist")
> Reported-by: Cyrill Gorcunov <gorcunov@xxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Tested-by: Cyrill Gorcunov <gorcunov@xxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx

Acked-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>

-- 
Josh