Re: [PATCH RESEND v3 2/4] clone3: switch to copy_struct_from_user()

Kees Cook
2019-09-30T23:42:42Z

On Tue, Oct 01, 2019 at 05:15:24AM +1000, Aleksa Sarai wrote:
> From: Aleksa Sarai <cyphar@cyphar.com>
>
> The change is very straightforward, and helps unify the syscall
> interface for struct-from-userspace syscalls. Additionally, explicitly
> define CLONE_ARGS_SIZE_VER0 to match the other users of the
> struct-extension pattern.
>
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> ---
> include/uapi/linux/sched.h | 2 ++
> kernel/fork.c | 34 +++++++---------------------------
> 2 files changed, 9 insertions(+), 27 deletions(-)
>
> diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
> index b3105ac1381a..0945805982b4 100644
> --- a/include/uapi/linux/sched.h
> +++ b/include/uapi/linux/sched.h
> @@ -47,6 +47,8 @@ struct clone_args {
> __aligned_u64 tls;
> };
>
> +#define CLONE_ARGS_SIZE_VER0 64 /* sizeof first published struct */
> +
> /*
> * Scheduling policies
> */
> diff --git a/kernel/fork.c b/kernel/fork.c
> index f9572f416126..2ef529869c64 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2525,39 +2525,19 @@ SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp,
> #ifdef __ARCH_WANT_SYS_CLONE3
> noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
> struct clone_args __user *uargs,
> - size_t size)
> + size_t usize)
> {
> + int err;
> struct clone_args args;
>
> - if (unlikely(size > PAGE_SIZE))
> + if (unlikely(usize > PAGE_SIZE))
> return -E2BIG;

I quickly looked through the earlier threads and couldn't find it, but
I have a memory of some discussion about moving this test into the
copy_struct_from_user() function itself? That would seems like a
reasonable idea? ("4k should be enough for any structure!")

Either way:

Reviewed-by: Kees Cook <keescook@chromium.org>

> -
> - if (unlikely(size < sizeof(struct clone_args)))
> + if (unlikely(usize < CLONE_ARGS_SIZE_VER0))
> return -EINVAL;
>
> - if (unlikely(!access_ok(uargs, size)))
> - return -EFAULT;
> -
> - if (size > sizeof(struct clone_args)) {
> - unsigned char __user *addr;
> - unsigned char __user *end;
> - unsigned char val;
> -
> - addr = (void __user *)uargs + sizeof(struct clone_args);
> - end = (void __user *)uargs + size;
> -
> - for (; addr < end; addr++) {
> - if (get_user(val, addr))
> - return -EFAULT;
> - if (val)
> - return -E2BIG;
> - }
> -
> - size = sizeof(struct clone_args);
> - }
> -
> - if (copy_from_user(&args, uargs, size))
> - return -EFAULT;
> + err = copy_struct_from_user(&args, sizeof(args), uargs, usize);
> + if (err)
> + return err;
>
> /*
> * Verify that higher 32bits of exit_signal are unset and that
> --
> 2.23.0
>

--
Kees Cook