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

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Message-ID: <7bf271b1-4909-d12d-bc65-d18790f7683e@fb.com>
Date:   Fri, 20 Mar 2020 08:31:03 -0700
From:   Yonghong Song <yhs@...com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alexei Starovoitov <ast@...nel.org>
CC:     Martin KaFai Lau <kafai@...com>, Song Liu <songliubraving@...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 8:24 AM, Daniel Borkmann wrote:
> 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://urldefense.proofpoint.com/v2/url?u=https-3A__android-2Dreview.googlesource.com_c_kernel_common_-2B_1235490&d=DwIDaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=DA8e1B5r073vIqRrFz7MRA&m=Fz_Xc6psG64uMowK2qpH0gTLj0NQE7k1CTWb5fODVeg&s=WKW0vq8WBALfwsSq5xGGWwxuLWKfI0DNN9XVMc1DkcE&e= 
>> 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()

Not sure whether in these places existing approach will cause
kernel failure or not. They did not call CHECK_ATTR, e.g.,
for bpf_prog_info structure.

> 
> Please add these as well to your fix.
> 
>>       if (copy_from_user(&attr, uattr, size) != 0)
>>           return -EFAULT;
>>
>> base-commit: 6c90b86a745a446717fdf408c4a8a4631a5e8ee3
>>
>