Hi Alexei, On Sat, Nov 4, 2023 at 2:11 AM Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote: > > On Fri, Nov 3, 2023 at 5:13 PM Song Liu <song@xxxxxxxxxx> wrote: > > > > It is common practice for security solutions to store tags/labels in > > xattrs. To implement similar functionalities in BPF LSM, add new kfunc > > bpf_get_file_xattr(). > > > > The first use case of bpf_get_file_xattr() is to implement file > > verifications with asymmetric keys. Specificially, security applications > > could use fsverity for file hashes and use xattr to store file signatures. > > (kfunc for fsverity hash will be added in a separate commit.) > > > > Currently, only xattrs with "user." prefix can be read with kfunc > > bpf_get_file_xattr(). As use cases evolve, we may add a dedicated prefix > > for bpf_get_file_xattr(). > > > > To avoid recursion, bpf_get_file_xattr can be only called from LSM hooks. > > Song, > > have you studied the prior attempt to add this kfunc? > https://lore.kernel.org/bpf/20220628161948.475097-1-kpsingh@xxxxxxxxxx/ I studied this thread, and I think I addressed the concerns. > file instead of (dentry,inode) pair makes sense, > and restricting to "user." prefix makes sense to me as well, > but you need to cc vfs experts in the future. I will cc vfs experts and lists for future versions. > > > + * For security reasons, only *name__str* with prefix "user." is allowed. > > + * > > + * Return: 0 on success, a negative value on error. > > + */ > > +__bpf_kfunc int bpf_get_file_xattr(struct file *file, const char *name__str, > > + struct bpf_dynptr_kern *value_ptr) > > +{ > > + struct dentry *dentry; > > + u32 value_len; > > + void *value; > > + > > + if (strncmp(name__str, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN)) > > + return -EPERM; > > + > > + value_len = __bpf_dynptr_size(value_ptr); > > + value = __bpf_dynptr_data_rw(value_ptr, value_len); > > + if (!value) > > + return -EINVAL; > > + > > + dentry = file_dentry(file); > > + return __vfs_getxattr(dentry, dentry->d_inode, name__str, value, value_len); > > +} > > + > > +__diag_pop(); > > + > > +BTF_SET8_START(fs_kfunc_set_ids) > > +BTF_ID_FLAGS(func, bpf_get_file_xattr, KF_SLEEPABLE | KF_TRUSTED_ARGS) > > See earlier Al's point. Just sleepable is not enough. In bpf_get_file_xattr_filter() we only allow calling bpf_get_file_xattr from LSM hooks. AFAICT, this is enough to avoid deadlock. We still need KF_SLEEPABLE here because xattr_handler->get() may block (lock xattr_sem). Did I miss something? Thanks, Song
