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.
