Re: [PATCH] bpf: btf: Fix a missing-check bug

[Wenwen Wang]

On Wed, Oct 24, 2018 at 4:39 AM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
>
> Hi Wenwen,
>
> On 10/22/2018 05:57 PM, Y Song wrote:
> > On Fri, Oct 19, 2018 at 3:30 PM Wenwen Wang <wang6495@xxxxxxx> wrote:
> >>
> >> In btf_parse(), the header of the user-space btf data 'btf_data' is firstly
> >> parsed and verified through btf_parse_hdr(). In btf_parse_hdr(), the header
> >> is copied from user-space 'btf_data' to kernel-space 'btf->hdr' and then
> >> verified. If no error happens during the verification process, the whole
> >> data of 'btf_data', including the header, is then copied to 'data' in
> >> btf_parse(). It is obvious that the header is copied twice here. More
> >> importantly, no check is enforced after the second copy to make sure the
> >> headers obtained in these two copies are same. Given that 'btf_data'
> >> resides in the user space, a malicious user can race to modify the header
> >> between these two copies. By doing so, the user can inject inconsistent
> >> data, which can cause undefined behavior of the kernel and introduce
> >> potential security risk.
> >>
> >> To avoid the above issue, this patch rewrites the header after the second
> >> copy, using 'btf->hdr', which is obtained in the first copy.
> >>
> >> Signed-off-by: Wenwen Wang <wang6495@xxxxxxx>
> >> ---
> >>  kernel/bpf/btf.c | 3 +++
> >>  1 file changed, 3 insertions(+)
> >>
> >> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> >> index 138f030..2a85f91 100644
> >> --- a/kernel/bpf/btf.c
> >> +++ b/kernel/bpf/btf.c
> >> @@ -2202,6 +2202,9 @@ static struct btf *btf_parse(void __user *btf_data, u32 btf_data_size,
> >>                 goto errout;
> >>         }
> >>
> >> +       memcpy(data, &btf->hdr,
> >> +               min_t(u32, btf->hdr.hdr_len, sizeof(btf->hdr)));
> >
> > Could you restructure the code to memcpy the header followed by copying
> > the rest of btf_data with copy_from_user? This way, each byte is only
> > copied once.
> > Could you add some comments right before memcpy so later people will know
> > why we implement this way?
>
> Thanks for the fix! Agree with Yonghong that we should rework this a bit, so
> please respin a v2 with the feedback addressed, thanks.

Hi Yonghong and Daniel,

Thanks for your suggestions! No problem, I will work on the v2 and
resubmit the patch.

Wenwen