linux-kernel - Re: [PATCH] bpf: explicitly memset the bpf

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Date:   Fri, 20 Mar 2020 16:24:32 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alexei Starovoitov <ast@...nel.org>
Cc:     Martin KaFai Lau <kafai@...com>, Song Liu <songliubraving@...com>,
        Yonghong Song <yhs@...com>, Andrii Nakryiko <andriin@...com>,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Maciej Żenczykowski <maze@...gle.com>,
        John Stultz <john.stultz@...aro.org>,
        Alexander Potapenko <glider@...gle.com>,
        Alistair Delva <adelva@...gle.com>
Subject: Re: [PATCH] bpf: explicitly memset the bpf_attr structure

On 3/20/20 10:48 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.
> 
> Fix this by explicitly memsetting the structure to 0 before using it.
> 
> Reported-by: Maciej Żenczykowski <maze@...gle.com>
> Reported-by: John Stultz <john.stultz@...aro.org>
> Reported-by: Alexander Potapenko <glider@...gle.com>
> Reported-by: Alistair Delva <adelva@...gle.com>
> Cc: stable <stable@...r.kernel.org>
> Link: https://android-review.googlesource.com/c/kernel/common/+/1235490
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> ---
>   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));

Thanks for the fix, there are a few more of these places. We would also need
to cover:

- bpf_prog_get_info_by_fd()
- bpf_map_get_info_by_fd()
- btf_get_info_by_fd()

Please add these as well to your fix.

>   	if (copy_from_user(&attr, uattr, size) != 0)
>   		return -EFAULT;
>   
> 
> base-commit: 6c90b86a745a446717fdf408c4a8a4631a5e8ee3
>