[PATCH 4.2.y-ckt 199/218] bpf: avoid copying junk bytes in bpf_get_current_comm()

[Kamal Mostafa <kamal@canonical.com>]

4.2.8-ckt7 -stable review patch.  If anyone has any objections, please let me know.

---8<------------------------------------------------------------

From: Alexei Starovoitov <ast@fb.com>

commit cdc4e47da8f4c32eeb6b2061a8a834f4362a12b7 upstream.

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,
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>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
---
 kernel/bpf/helpers.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 1447ec0..c2e2bad 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -171,7 +171,7 @@ 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)));
 	return 0;
 }
 
-- 
2.7.4