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@xxxxxxxxx>
>
> 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
