Re: [PATCH] clone3: validate stack arguments

[Szabolcs Nagy]

On 31/10/2019 11:36, Christian Brauner wrote:
> diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
> index 99335e1f4a27..25b4fa00bad1 100644
> --- a/include/uapi/linux/sched.h
> +++ b/include/uapi/linux/sched.h
> @@ -51,6 +51,10 @@
>   *               sent when the child exits.
>   * @stack:       Specify the location of the stack for the
>   *               child process.
> + *               Note, @stack is expected to point to the
> + *               lowest address. The stack direction will be
> + *               determined by the kernel and set up
> + *               appropriately based on @stack_size.
>   * @stack_size:  The size of the stack for the child process.
>   * @tls:         If CLONE_SETTLS is set, the tls descriptor
>   *               is set to tls.
> diff --git a/kernel/fork.c b/kernel/fork.c
> index bcdf53125210..55af6931c6ec 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2561,7 +2561,35 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
>  	return 0;
>  }
>  
> -static bool clone3_args_valid(const struct kernel_clone_args *kargs)
> +/**
> + * clone3_stack_valid - check and prepare stack
> + * @kargs: kernel clone args
> + *
> + * Verify that the stack arguments userspace gave us are sane.
> + * In addition, set the stack direction for userspace since it's easy for us to
> + * determine.
> + */
> +static inline bool clone3_stack_valid(struct kernel_clone_args *kargs)
> +{
> +	if (kargs->stack == 0) {
> +		if (kargs->stack_size > 0)
> +			return false;
> +	} else {
> +		if (kargs->stack_size == 0)
> +			return false;
> +
> +		if (!access_ok((void __user *)kargs->stack, kargs->stack_size))
> +			return false;
> +
> +#if !defined(CONFIG_STACK_GROWSUP) && !defined(CONFIG_IA64)
> +		kargs->stack += kargs->stack_size;
> +#endif
> +	}

from the description it is not clear whose
responsibility it is to guarantee the alignment
of sp on entry.

i think 0 stack size may work if signals are
blocked and then prohibiting it might not be
the right thing.

it's not clear how libc should deal with v5.3
kernels which don't have the stack+=stack_size
logic.