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

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Date:   Wed, 4 Nov 2020 23:36:21 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Daniel Xu <dxu@...uu.xyz>, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, ast@...nel.org
Cc:     kernel-team@...com, mhiramat@...nel.org
Subject: Re: [PATCH bpf-next] lib/strncpy_from_user.c: Don't overcopy bytes
 after NUL terminator

On 11/4/20 9:18 PM, Daniel Xu wrote:
> 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@...uu.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").

Ah I see, it was just reusing 3d7081822f7f ("uaccess: Add non-pagefault user-space
read functions"). Potentially it might be safer choice to just rework the
strncpy_from_user_nofault() to mimic strncpy_from_kernel_nofault() in that
regard?

>> 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
>