Re: [PATCH bpf-next 01/11] bpf: make bpf_d_path() helper use probe-read semantics

[Christian Brauner <brauner@kernel.org>]

On Tue, Feb 20, 2024 at 01:22:14PM +0000, Matt Bobrowski wrote:
> On Tue, Feb 20, 2024 at 10:48:10AM +0100, Christian Brauner wrote:
> > On Tue, Feb 20, 2024 at 09:27:23AM +0000, Matt Bobrowski wrote:
> > > There has now been several reported instances [0, 1, 2] where the
> > > usage of the BPF helper bpf_d_path() has led to some form of memory
> > > corruption issue.
> > > 
> > > The fundamental reason behind why we repeatedly see bpf_d_path() being
> > > susceptible to such memory corruption issues is because it only
> > > enforces ARG_PTR_TO_BTF_ID constraints onto it's struct path
> > > argument. This essentially means that it only requires an in-kernel
> > > pointer of type struct path to be provided to it. Depending on the
> > > underlying context and where the supplied struct path was obtained
> > > from and when, depends on whether the struct path is fully intact or
> > > not when calling bpf_d_path(). It's certainly possible to call
> > > bpf_d_path() and subsequently d_path() from contexts where the
> > > supplied struct path to bpf_d_path() has already started being torn
> > > down by __fput() and such. An example of this is perfectly illustrated
> > > in [0].
> > > 
> > > Moving forward, we simply cannot enforce KF_TRUSTED_ARGS semantics
> > > onto struct path of bpf_d_path(), as this approach would presumably
> > > lead to some pretty wide scale and highly undesirable BPF program
> > > breakage. To avoid breaking any pre-existing BPF program that is
> > > dependent on bpf_d_path(), I propose that we take a different path and
> > > re-implement an incredibly minimalistic and bare bone version of
> > > d_path() which is entirely backed by kernel probe-read semantics. IOW,
> > > a version of d_path() that is backed by
> > > copy_from_kernel_nofault(). This ensures that any reads performed
> > > against the supplied struct path to bpf_d_path() which may end up
> > > faulting for whatever reason end up being gracefully handled and fixed
> > > up.
> > > 
> > > The caveats with such an approach is that we can't fully uphold all of
> > > d_path()'s path resolution capabilities. Resolving a path which is
> > > comprised of a dentry that make use of dynamic names via isn't
> > > possible as we can't enforce probe-read semantics onto indirect
> > > function calls performed via d_op as they're implementation
> > > dependent. For such cases, we just return -EOPNOTSUPP. This might be a
> > > little surprising to some users, especially those that are interested
> > > in resolving paths that involve a dentry that resides on some
> > > non-mountable pseudo-filesystem, being pipefs/sockfs/nsfs, but it's
> > > arguably better than enforcing KF_TRUSTED_ARGS onto bpf_d_path() and
> > > causing an unnecessary shemozzle for users. Additionally, we don't
> > 
> > NAK. We're not going to add a semi-functional reimplementation of
> > d_path() for bpf. This relied on VFS internals and guarantees that were
> > never given. Restrict it to KF_TRUSTED_ARGS as it was suggested when
> > this originally came up or fix it another way. But we're not adding a
> > bunch of kfuncs to even more sensitive VFS machinery and then build a
> > d_path() clone just so we can retroactively justify broken behavior.
> 
> OK, I agree, having a semi-functional re-implementation of d_path() is
> indeed suboptimal. However, also understand that slapping the

The ugliness of the duplicated code made me start my mail with NAK. It
would've been enough to just say no.

> KF_TRUSTED_ARGS constraint onto the pre-existing BPF helper
> bpf_d_path() would outright break a lot of BPF programs out there, so
> I can't see how taht would be an acceptable approach moving forward
> here either.
> 
> Let's say that we decided to leave the pre-existing bpf_d_path()
> implementation as is, accepting that it is fundamentally succeptible
> to memory corruption issues, are you saying that you're also not for
> adding the KF_TRUSTED_ARGS d_path() variant as I've done so here

No, that's fine and was the initial proposal anyway. You're already
using the existing d_path() anway in that bpf_d_path() thing. So
exposing another variant with KF_TRUSTED_ARGS restriction is fine. But
not hacking up a custom d_path() variant.

> [0]. Or, is it the other supporting reference counting based BPF
> kfuncs [1, 2] that have irked you and aren't supportive of either?

Yes, because you're exposing fs_root, fs_pwd, path_put() and fdput(),
get_task_exe_file(), get_mm_exe_file(). None of that I see being turned
into kfuncs.