bpf: avoid copying junk bytes in bpf_get_current_comm()

[ Upstream commit cdc4e47da8f4c32eeb6b2061a8a834f4362a12b7 ]

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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 4504ca66118da0c0735bdb56d9204ba6cb79994b..50da680c479f030588314ed664c8d57200c28990 100644 (file)
--- 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)));
        return 0;
 }