netdev - Re: [PATCH net-next] bpf: avoid copying junk bytes in bpf_get_current

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Date:	Fri, 11 Mar 2016 11:24:26 +0100
From:	Daniel Borkmann <daniel@...earbox.net>
To:	Alexei Starovoitov <ast@...com>,
	"David S . Miller" <davem@...emloft.net>
CC:	Tobias Waldekranz <tobias@...dekranz.com>,
	Brendan Gregg <brendan.d.gregg@...il.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	kernel-team@...com
Subject: Re: [PATCH net-next] bpf: avoid copying junk bytes in bpf_get_current_comm()

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@...dekranz.com>
> Signed-off-by: Alexei Starovoitov <ast@...nel.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.

Thanks,
Daniel