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

From: Daniel Borkmann
Date: Fri Mar 11 2016 - 13:02:51 EST

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
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

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.