Messages in this thread | | | Date | Fri, 11 Mar 2016 11:24:26 +0100 | From | Daniel Borkmann <> | Subject | Re: [PATCH net-next] bpf: avoid copying junk bytes in bpf_get_current_comm() |
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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@waldekranz.com> > Signed-off-by: Alexei Starovoitov <ast@kernel.org> [...] > 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.
Thanks, Daniel
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