Re: [PATCH v2] fork: check exit_signal passed in clone3() call

[Eric W. Biederman]

Eugene Syromiatnikov <esyr@xxxxxxxxxx> writes:

> Previously, higher 32 bits of exit_signal fields were lost when
> copied to the kernel args structure (that uses int as a type for the
> respective field).  Moreover, as Oleg has noted[1], exit_signal is used
> unchecked, so it has to be checked for sanity before use; for the legacy
> syscalls, applying CSIGNAL mask guarantees that it is at least non-negative;
> however, there's no such thing is done in clone3() code path, and that can
> break at least thread_group_leader.
>
> Checking user-passed exit_signal against ~CSIGNAL mask solves both
> of these problems.
>
> [1] https://lkml.org/lkml/2019/9/10/467
>
> * kernel/fork.c (copy_clone_args_from_user): Fail with -EINVAL if
> args.exit_signal has bits set outside CSIGNAL mask.
> (_do_fork): Note that exit_signal is expected to be checked for the
> sanity by the caller.
>
> Fixes: 7f192e3cd316 ("fork: add clone3")
> Reported-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> Signed-off-by: Eugene Syromiatnikov <esyr@xxxxxxxxxx>

Acked-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>

> ---
>  kernel/fork.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 2852d0e..9dee2ab 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2338,6 +2338,8 @@ struct mm_struct *copy_init_mm(void)
>   *
>   * It copies the process, and if successful kick-starts
>   * it and waits for it to finish using the VM if required.
> + *
> + * args->exit_signal is expected to be checked for sanity by the caller.
>   */
>  long _do_fork(struct kernel_clone_args *args)
>  {
> @@ -2562,6 +2564,16 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
>  	if (copy_from_user(&args, uargs, size))
>  		return -EFAULT;
>  
> +	/*
> +	 * exit_signal is confined to CSIGNAL mask in legacy syscalls,
> +	 * so it is used unchecked deeper in syscall handling routines;
> +	 * moreover, copying to struct kernel_clone_args.exit_signals
> +	 * trims higher 32 bits, so it is has to be checked that they
> +	 * are zero.
> +	 */
> +	if (unlikely(args.exit_signal & ~((u64)CSIGNAL)))
> +		return -EINVAL;
> +
>  	*kargs = (struct kernel_clone_args){
>  		.flags		= args.flags,
>  		.pidfd		= u64_to_user_ptr(args.pidfd),