Subject: Re: [PATCH net-next v1 2/2] bpf: Extend check_uarg_tail_zero() checks

From: Daniel Borkmann <[email-masked]>
Date: Mon, 07 Aug 2017 20:34:22 +0200

On 08/07/2017 06:36 PM, Mickaël Salaün wrote:
> The function check_uarg_tail_zero() was created from bpf(2) for
> BPF_OBJ_GET_INFO_BY_FD without taking the access_ok() nor the PAGE_SIZE
> checks. Make this checks more generally available while unlikely to be
> triggered, extend the memory range check and add an explanation
> including why the ToCToU should not be a security concern.
>
> Signed-off-by: Mickaël Salaün <[email-masked]>
> Cc: Alexei Starovoitov <[email-masked]>
> Cc: Daniel Borkmann <[email-masked]>
> Cc: David S. Miller <[email-masked]>
> Cc: Kees Cook <[email-masked]>
> Cc: Martin KaFai Lau <[email-masked]>
> Link: https://lkml.kernel.org/r/CAGXu5j+vRGFvJZmjtAcT8Hi8B+Wz0e1b6VKYZHfQP_=[email-masked]
> ---
>   kernel/bpf/syscall.c | 26 +++++++++++++++-----------
>   1 file changed, 15 insertions(+), 11 deletions(-)
>
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index c653ee0bd162..b884fdc371e0 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -48,6 +48,15 @@ static const struct bpf_map_ops * const bpf_map_types[] = {
>   #undef BPF_MAP_TYPE
>   };
>
> +/*
> + * If we're handed a bigger struct than we know of, ensure all the unknown bits
> + * are 0 - i.e. new user-space does not rely on any kernel feature extensions
> + * we dont know about yet.

Nit: don't

> + *
> + * There is a ToCToU between this function call and the following
> + * copy_from_user() call. However, this should not be a concern since this

Lets make it a bit more clear to the reader: s/should not/is not/

> + * function is meant to be a future-proofing of bits.
> + */
>   static int check_uarg_tail_zero(void __user *uaddr,
>   				size_t expected_size,
>   				size_t actual_size)
> @@ -57,6 +66,12 @@ static int check_uarg_tail_zero(void __user *uaddr,
>   	unsigned char val;
>   	int err;
>
> +	if (unlikely(!access_ok(VERIFY_READ, uaddr, actual_size)))
> +		return -EFAULT;
> +
> +	if (unlikely(actual_size > PAGE_SIZE))	/* silly large */
> +		return -E2BIG;
> +

Yeah, moving the checks into check_uarg_tail_zero() is
fine by me. Can we make the 'silly large' test first, so
we don't generate unnecessary work if we bail out later
anyway?

Other than that:

Acked-by: Daniel Borkmann <[email-masked]>

Thanks,
Daniel

>   	if (actual_size <= expected_size)
>   		return 0;
>
> @@ -1393,17 +1408,6 @@ SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, uattr, unsigned int, siz
>   	if (!capable(CAP_SYS_ADMIN) && sysctl_unprivileged_bpf_disabled)
>   		return -EPERM;
>
> -	if (!access_ok(VERIFY_READ, uattr, 1))
> -		return -EFAULT;
> -
> -	if (size > PAGE_SIZE)	/* silly large */
> -		return -E2BIG;
> -
> -	/* If we're handed a bigger struct than we know of,
> -	 * ensure all the unknown bits are 0 - i.e. new
> -	 * user-space does not rely on any kernel feature
> -	 * extensions we dont know about yet.
> -	 */
>   	err = check_uarg_tail_zero(uattr, sizeof(attr), size);
>   	if (err)
>   		return err;
>