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

[Yonghong Song]

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 Åenczykowski <maze@xxxxxxxxxx>
> > Reported-by: John Stultz <john.stultz@xxxxxxxxxx>
> > Reported-by: Alexander Potapenko <glider@xxxxxxxxxx>
> > Reported-by: Alistair Delva <adelva@xxxxxxxxxx>
> > Cc: stable <stable@xxxxxxxxxxxxxxx>
> > 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@xxxxxxxxxxxxxxxxxxx>
> > ---
> > Â kernel/bpf/syscall.c | 3 ++-
> > Â 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,
> > Â SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, uattr, 
> > unsigned int, size)
> > Â {
> > -ÂÂÂ union bpf_attr attr = {};
> > +ÂÂÂ union bpf_attr attr;
> > ÂÂÂÂÂ int err;
> > ÂÂÂÂÂ 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
> > ÂÂÂÂÂ size = min_t(u32, size, sizeof(attr));
> > ÂÂÂÂÂ /* copy attributes from user space, may be less than 
> > sizeof(bpf_attr) */
> > +ÂÂÂ 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.
>
> > ÂÂÂÂÂ if (copy_from_user(&attr, uattr, size) != 0)
> > ÂÂÂÂÂÂÂÂÂ return -EFAULT;
> >
> > base-commit: 6c90b86a745a446717fdf408c4a8a4631a5e8ee3