On Fri, Feb 25, 2022 at 03:43:31PM -0800, Hao Luo wrote:
> This patch allows bpf_syscall prog to perform some basic filesystem
> operations: create, remove directories and unlink files. Three bpf
> helpers are added for this purpose. When combined with the following
> patches that allow pinning and getting bpf objects from bpf prog,
> this feature can be used to create directory hierarchy in bpffs that
> help manage bpf objects purely using bpf progs.
>
> The added helpers subject to the same permission checks as their syscall
> version. For example, one can not write to a read-only file system;
> The identity of the current process is checked to see whether it has
> sufficient permission to perform the operations.
>
> Only directories and files in bpffs can be created or removed by these
> helpers. But it won't be too hard to allow these helpers to operate
> on files in other filesystems, if we want.
In which contexts can those be called?
> +BPF_CALL_2(bpf_rmdir, const char *, pathname, int, pathname_sz)
> +{
> + struct user_namespace *mnt_userns;
> + struct path parent;
> + struct dentry *dentry;
> + int err;
> +
> + if (pathname_sz <= 1 || pathname[pathname_sz - 1])
> + return -EINVAL;
> +
> + err = kern_path(pathname, 0, &parent);
> + if (err)
> + return err;
> +
> + if (!bpf_path_is_bpf_dir(&parent)) {
> + err = -EPERM;
> + goto exit1;
> + }
> +
> + err = mnt_want_write(parent.mnt);
> + if (err)
> + goto exit1;
> +
> + dentry = kern_path_locked(pathname, &parent);
This can't be right. Ever. There is no promise whatsoever
that these two lookups will resolve to the same place.
> +BPF_CALL_2(bpf_unlink, const char *, pathname, int, pathname_sz)
> +{
> + struct user_namespace *mnt_userns;
> + struct path parent;
> + struct dentry *dentry;
> + struct inode *inode = NULL;
> + int err;
> +
> + if (pathname_sz <= 1 || pathname[pathname_sz - 1])
> + return -EINVAL;
> +
> + err = kern_path(pathname, 0, &parent);
> + if (err)
> + return err;
> +
> + err = mnt_want_write(parent.mnt);
> + if (err)
> + goto exit1;
> +
> + dentry = kern_path_locked(pathname, &parent);
> + if (IS_ERR(dentry)) {
> + err = PTR_ERR(dentry);
> + goto exit2;
> + }
Ditto. NAK; if you want to poke into fs/namei.c guts, do it right.
Or at least discuss that on fsdevel. As it is, it's completely broken.
It's racy *and* it blatantly leaks both vfsmount and dentry references.
NAKed-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
