Re: [PATCH v2] x86: ptrace.h: Make regs_get_kernel_stack_nth() not fault on bad stack

[Joel Fernandes <joel@joelfernandes.org>]

On Wed, Oct 17, 2018 at 04:59:51PM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> 
> Andy had some concerns about using regs_get_kernel_stack_nth() in a new
> function regs_get_kernel_argument() as if there's any error in the stack
> code, it could cause a bad memory access. To be on the safe side, call
> probe_kernel_read() on the stack address to be extra careful in accessing
> the memory. A helper function, regs_get_kernel_stack_nth_addr(), was added
> to just return the stack address (or NULL if not on the stack), that will be
> used to find the address (and could be used by other functions) and read the
> address with kernel_probe_read().
> 
> Link: http://lkml.kernel.org/r/CALCETrXn9zKTb9i1LP3qoFcpqZHF34BdkuZ5D3N0uCmRr+VnbA@mail.gmail.com
> Requested-by: Andy Lutomirski <luto@amacapital.net>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> ---

Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

thanks,

 - Joel


> Changes since v1:
> 
>    - Make regs_get_kernel_stack_nth() not fault, and not have a
>      separate function. Only tracing uses it anyway.
> 
>  arch/x86/include/asm/ptrace.h | 43 ++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 36 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
> index c2304b25e2fd..055f632ce711 100644
> --- a/arch/x86/include/asm/ptrace.h
> +++ b/arch/x86/include/asm/ptrace.h
> @@ -237,23 +237,52 @@ static inline int regs_within_kernel_stack(struct pt_regs *regs,
>  }
>  
>  /**
> + * regs_get_kernel_stack_nth_addr() - get the address of the Nth entry on stack
> + * @regs:	pt_regs which contains kernel stack pointer.
> + * @n:		stack entry number.
> + *
> + * regs_get_kernel_stack_nth() returns the address of the @n th entry of the
> + * kernel stack which is specified by @regs. If the @n th entry is NOT in
> + * the kernel stack, this returns NULL.
> + */
> +static inline unsigned long *regs_get_kernel_stack_nth_addr(struct pt_regs *regs,
> +							    unsigned int n)
> +{
> +	unsigned long *addr = (unsigned long *)kernel_stack_pointer(regs);
> +
> +	addr += n;
> +	if (regs_within_kernel_stack(regs, (unsigned long)addr))
> +		return addr;
> +	else
> +		return NULL;
> +}
> +
> +/* To avoid include hell, we can't include uaccess.h */
> +extern long probe_kernel_read(void *dst, const void *src, size_t size);
> +
> +/**
>   * regs_get_kernel_stack_nth() - get Nth entry of the stack
>   * @regs:	pt_regs which contains kernel stack pointer.
>   * @n:		stack entry number.
>   *
>   * regs_get_kernel_stack_nth() returns @n th entry of the kernel stack which
> - * is specified by @regs. If the @n th entry is NOT in the kernel stack,
> + * is specified by @regs. If the @n th entry is NOT in the kernel stack
>   * this returns 0.
>   */
>  static inline unsigned long regs_get_kernel_stack_nth(struct pt_regs *regs,
>  						      unsigned int n)
>  {
> -	unsigned long *addr = (unsigned long *)kernel_stack_pointer(regs);
> -	addr += n;
> -	if (regs_within_kernel_stack(regs, (unsigned long)addr))
> -		return *addr;
> -	else
> -		return 0;
> +	unsigned long *addr;
> +	unsigned long val;
> +	long ret;
> +
> +	addr = regs_get_kernel_stack_nth_addr(regs, n);
> +	if (addr) {
> +		ret = probe_kernel_read(&val, addr, sizeof(val));
> +		if (!ret)
> +			return val;
> +	}
> +	return 0;
>  }
>  
>  /**
> -- 
> 2.13.6
>