[PATCH v13 bpf-next 1/6] bpf: Add kfunc bpf_get_file_xattr

[Song Liu]

On Thu, Nov 23, 2023 at 3:40 PM Song Liu <song at kernel.org> 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.
>
> Signed-off-by: Song Liu <song at kernel.org>
> ---
>  kernel/trace/bpf_trace.c | 63 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 63 insertions(+)
>
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index f0b8b7c29126..55758a6fbe90 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -24,6 +24,7 @@
>  #include <linux/key.h>
>  #include <linux/verification.h>
>  #include <linux/namei.h>
> +#include <linux/fileattr.h>
>
>  #include <net/bpf_sk_storage.h>
>
> @@ -1431,6 +1432,68 @@ static int __init bpf_key_sig_kfuncs_init(void)
>  late_initcall(bpf_key_sig_kfuncs_init);
>  #endif /* CONFIG_KEYS */
>
> +/* filesystem kfuncs */
> +__bpf_kfunc_start_defs();
> +
> +/**
> + * bpf_get_file_xattr - get xattr of a file
> + * @file: file to get xattr from
> + * @name__str: name of the xattr
> + * @value_ptr: output buffer of the xattr value
> + *
> + * Get xattr *name__str* of *file* and store the output in *value_ptr*.
> + *
> + * 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);
> +}
> +
> +__bpf_kfunc_end_defs();
> +
> +BTF_SET8_START(fs_kfunc_set_ids)
> +BTF_ID_FLAGS(func, bpf_get_file_xattr, KF_SLEEPABLE | KF_TRUSTED_ARGS)
> +BTF_SET8_END(fs_kfunc_set_ids)
> +
> +static int bpf_get_file_xattr_filter(const struct bpf_prog *prog, u32 kfunc_id)
> +{
> +       if (!btf_id_set8_contains(&fs_kfunc_set_ids, kfunc_id))
> +               return 0;
> +
> +       /* Only allow to attach from LSM hooks, to avoid recursion */
> +       return prog->type != BPF_PROG_TYPE_LSM ? -EACCES : 0;
> +}
> +
> +const struct btf_kfunc_id_set bpf_fs_kfunc_set = {

Missed static here. If there will be v14, I will fix it here.

Thanks,
Song

> +       .owner = THIS_MODULE,
> +       .set = &fs_kfunc_set_ids,
> +       .filter = bpf_get_file_xattr_filter,
> +};
> +
> +static int __init bpf_fs_kfuncs_init(void)
> +{
> +       return register_btf_kfunc_id_set(BPF_PROG_TYPE_LSM, &bpf_fs_kfunc_set);
> +}
> +
> +late_initcall(bpf_fs_kfuncs_init);
> +
>  static const struct bpf_func_proto *
>  bpf_tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>  {
> --
> 2.34.1
>