Messages in this thread | | | Date | Fri, 11 Mar 2016 19:02:38 +0100 | From | Daniel Borkmann <> | Subject | Re: [PATCH net-next] bpf: avoid copying junk bytes in bpf_get_current_comm() |
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On 03/11/2016 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@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. > > 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.
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