Re: [PATCH bpf-next] lib/strncpy_from_user.c: Don't overcopy bytes after NUL terminator

From: "Daniel Xu" <dxu@dxuuu.xyz>
To: "Daniel Borkmann" <daniel@iogearbox.net>, <bpf@vger.kernel.org>,
	<linux-kernel@vger.kernel.org>, <ast@kernel.org>
Cc: <kernel-team@fb.com>
Subject: Re: [PATCH bpf-next] lib/strncpy_from_user.c: Don't overcopy bytes after NUL terminator
Date: Wed, 04 Nov 2020 12:18:49 -0800	[thread overview]
Message-ID: <C6UR9QUUYXKW.3PHSMQ3EXUYI3@maharaja> (raw)
In-Reply-To: <7831c092-5ab4-033e-8fb3-ad9702332d79@iogearbox.net>

Hi Daniel,

On Wed Nov 4, 2020 at 8:24 AM PST, Daniel Borkmann wrote:
> On 11/4/20 3:29 AM, Daniel Xu wrote:
> > do_strncpy_from_user() may copy some extra bytes after the NUL
> > terminator into the destination buffer. This usually does not matter for
> > normal string operations. However, when BPF programs key BPF maps with
> > strings, this matters a lot.
> > 
> > A BPF program may read strings from user memory by calling the
> > bpf_probe_read_user_str() helper which eventually calls
> > do_strncpy_from_user(). The program can then key a map with the
> > resulting string. BPF map keys are fixed-width and string-agnostic,
> > meaning that map keys are treated as a set of bytes.
> > 
> > The issue is when do_strncpy_from_user() overcopies bytes after the NUL
> > terminator, it can result in seemingly identical strings occupying
> > multiple slots in a BPF map. This behavior is subtle and totally
> > unexpected by the user.
> > 
> > This commit uses the proper word-at-a-time APIs to avoid overcopying.
> > 
> > Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
>
> It looks like this is a regression from the recent refactoring of the
> mem probing
> util functions?

I think it was like this from the beginning, at 6ae08ae3dea2 ("bpf: Add
probe_read_{user, kernel} and probe_read_{user, kernel}_str helpers").
The old bpf_probe_read_str() used the kernel's byte-by-byte copying
routine. bpf_probe_read_user_str() started using strncpy_from_user()
which has been doing the long-sized strides since ~2012 or earlier.

I tried to build and test the kernel at that commit but it seems my
compiler is too new to build that old code. Bunch of build failures.

I assume the refactor you're referring to is 8d92db5c04d1 ("bpf: rework
the compat kernel probe handling").

> Could we add a Fixes tag and then we'd also need to target the fix
> against bpf tree instead of bpf-next, no?

Sure, will do in v2.

>
> Moreover, a BPF kselftest would help to make sure it doesn't regress in
> future again.

Ditto.

[..]

Thanks,
Daniel