Re: [PATCH net-next] bpf: avoid copying junk bytes in bpf_get_current_comm()

[Daniel Borkmann]

On 03/11/2016 06:20 PM, Alexei Starovoitov wrote:
> On 3/11/16 2:24 AM, Daniel Borkmann wrote:
> > On 03/10/2016 05:02 AM, Alexei Starovoitov wrote:
> > > Lots of places in the kernel use memcpy(buf, comm, TASK_COMM_LEN); but
> > > the result is typically passed to print("%s", buf) and extra bytes
> > > after zero don't cause any harm.
> > > In bpf the result of bpf_get_current_comm() is used as the part of
> > > map key and was causing spurious hash map mismatches.
> > > Use strlcpy() to guarantee zero-terminated string.
> > > bpf verifier checks that output buffer is zero-initialized,
> >
> > Sorry for late reply, more below:
> >
> > > so even for short task names the output buffer don't have junk bytes.
> > > Note it's not a security concern, since kprobe+bpf is root only.
> > >
> > > Fixes: ffeedafbf023 ("bpf: introduce current->pid, tgid, uid, gid,
> > > comm accessors")
> > > Reported-by: Tobias Waldekranz <tobias@xxxxxxxxxxxxxx>
> > > Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
> > [...]
> > > diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> > > index 4504ca66118d..50da680c479f 100644
> > > --- a/kernel/bpf/helpers.c
> > > +++ b/kernel/bpf/helpers.c
> > > @@ -166,7 +166,7 @@ static u64 bpf_get_current_comm(u64 r1, u64 size,
> > > u64 r3, u64 r4, u64 r5)
> > >       if (!task)
> > >           return -EINVAL;
> > >
> > > -    memcpy(buf, task->comm, min_t(size_t, size, sizeof(task->comm)));
> > > +    strlcpy(buf, task->comm, min_t(size_t, size, sizeof(task->comm)));
> >
> > If I see this correctly, __set_task_comm() makes sure comm is always zero
> > terminated, so that seems good, but isn't it already sufficient when
> > switching
> > to strlcpy() to simply use:
> >
> >      strlcpy(buf, task->comm, size);
> >
> > The min_t() seems unnecessary work to me, why do we still need it? size
> > is guaranteed to be > 0 through the eBPF verifier, so strlcpy() should take
> > care of the rest.
>
> that's one clever optimization. yep. we can drop min_t.
> btw I wanted to add memset to __set_task_comm, keep memcpy in
> bpf_get_current_comm and optimize perf_event_comm_event
> (which doing: memset+strlcpy and can be replaced with memcpy),
> but figured that such 'fix' is not suitable for stable.
> I guess we can do in the next cycle? strlen is not cheap.
> Especially since it turned out that bpf_get_current_comm() is
> used very often in the hot path in bcc/tools.

Would strscpy() help in this case (see 30035e45753b ("string: provide
strscpy()"))?

> Also for the next cycle I'm planning to extend verifier to
> allow uninitialized stack to be passed to functions like
> bpf_get_current_comm() and they would have to zero it in
> error cases. Then we can save few more cycles from the programs.

That would be useful also for other helpers indeed.