Re: [PATCH] bpf: explicitly memset the bpf_attr structure

[Yonghong Song <yhs@fb.com>]

On 3/20/20 2:48 AM, Greg Kroah-Hartman wrote:
> For the bpf syscall, we are relying on the compiler to properly zero out
> the bpf_attr union that we copy userspace data into.  Unfortunately that
> doesn't always work properly, padding and other oddities might not be
> correctly zeroed, and in some tests odd things have been found when the
> stack is pre-initialized to other values.

Maybe add more contexts about the failure itself so it could be clear
why we need this patch.

As far as I know from the link below, the failure happens in
CHECK_ATTR() which checks any unused *area* for a particular subcommand
must be 0, and this patch tries to provide this guarantee beyond
area beyond min(uattr_size, sizeof(attr)).

> 
> Fix this by explicitly memsetting the structure to 0 before using it.
> 
> Reported-by: Maciej Żenczykowski <maze@google.com>
> Reported-by: John Stultz <john.stultz@linaro.org>
> Reported-by: Alexander Potapenko <glider@google.com>
> Reported-by: Alistair Delva <adelva@google.com>
> Cc: stable <stable@vger.kernel.org>
> Link: https://urldefense.proofpoint.com/v2/url?u=https-3A__android-2Dreview.googlesource.com_c_kernel_common_-2B_1235490&d=DwIDaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=DA8e1B5r073vIqRrFz7MRA&m=LW31qE_U9SQxf_FnwMUaEXeM9h54NJ1fOf44hk_QDWk&s=HJ-aQi8Ho6V6ZegmWlPYJqnY7e3KRKfFjFj6C2yEN04&e=
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Acked-by: Yonghong Song <yhs@fb.com>

> ---
>   kernel/bpf/syscall.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index a91ad518c050..a4b1de8ea409 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -3354,7 +3354,7 @@ static int bpf_map_do_batch(const union bpf_attr *attr,
>   
>   SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, uattr, unsigned int, size)
>   {
> -	union bpf_attr attr = {};
> +	union bpf_attr attr;
>   	int err;
>   
>   	if (sysctl_unprivileged_bpf_disabled && !capable(CAP_SYS_ADMIN))
> @@ -3366,6 +3366,7 @@ SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, uattr, unsigned int, siz
>   	size = min_t(u32, size, sizeof(attr));
>   
>   	/* copy attributes from user space, may be less than sizeof(bpf_attr) */
> +	memset(&attr, 0, sizeof(attr));
>   	if (copy_from_user(&attr, uattr, size) != 0)
>   		return -EFAULT;
>   
> 
> base-commit: 6c90b86a745a446717fdf408c4a8a4631a5e8ee3
>