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

From: Daniel Borkmann
Date: Fri Mar 11 2016 - 05:24:44 EST

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.