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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Mar 20, 2020 at 04:24:32PM +0100, 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@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://android-review.googlesource.com/c/kernel/common/+/1235490
> > Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.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()

Unless I am mistaken, struct bpf_prog_info is packed fully, with no
holes, so this shouldn't be an issue there.

> - bpf_map_get_info_by_fd()

No padding in struct bpf_map_info that I can see, so I doubt this is
needed there.

> - btf_get_info_by_fd()

There is no padding in struct bpf_btf_info, so that's not needed there,
but I can add it if you really want.

I can change these, but I don't think that there currently is a bug in
those functions, unlike with "union bpf_attr" which, as Yonghong points
out, is tripping on the CHECK_ATTR() test later on.

thanks,

greg k-h