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

[Daniel Xu]

On Wed Nov 4, 2020 at 2:36 PM PST, Daniel Borkmann wrote:
> 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@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").
>
> 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?

I'm a little reluctant to do that b/c it would introduce less efficient,
duplicated code. The word-at-a-time API already has the zero_bytemask()
API so it's clear that it was designed to handle this issue -- we're not
really hacking anything here.

I'll send out a V2 with the selftest shortly. Happy to change things
after that.

Thanks,
Daniel