Re: [PATCH 1/3] bpf: Add BPF_F_VERIFY_ELEM to require signature verification on map values

[KP Singh]

On Wed, May 25, 2022 at 3:21 PM Roberto Sassu <roberto.sassu@xxxxxxxxxx> wrote:
>
> In some cases, it is desirable to ensure that a map contains data from
> authenticated sources, for example if map data are used for making security
> decisions.

I am guessing this comes from the discussion we had about digilim.
I remember we discussed a BPF helper that could verify signatures.
Why would that approach not work?

>
>
> Such restriction is achieved by verifying the signature of map values, at
> the time those values are added to the map with the bpf() system call (more
> specifically, when the commands passed to bpf() are BPF_MAP_UPDATE_ELEM or
> BPF_MAP_UPDATE_BATCH). Mmappable maps are not allowed in this case.
>
> Signature verification is initially done with keys in the primary and
> secondary kernel keyrings, similarly to kernel modules. This allows system
> owners to enforce a system-wide policy based on the keys they trust.
> Support for additional keyrings could be added later, based on use case
> needs.
>
> Signature verification is done only for those maps for which the new map
> flag BPF_F_VERIFY_ELEM is set. When the flag is set, the kernel expects map
> values to be in the following format:
>
> +-------------------------------+---------------+-----+-----------------+
> | verified data+sig size (be32) | verified data | sig | unverified data |
> +-------------------------------+---------------+-----+-----------------+
>
> where sig is a module-style appended signature as generated by the
> sign-file tool. The verified data+sig size (in big endian) must be
> explicitly provided (it is not generated by sign-file), as it cannot be
> determined in other ways (currently, the map value size is fixed). It can
> be obtained from the size of the file created by sign-file.
>
> Introduce the new map flag BPF_F_VERIFY_ELEM, and additionally call the new
> function bpf_map_verify_value_sig() from bpf_map_update_value() if the flag
> is set. bpf_map_verify_value_sig(), declared as global for a new helper, is
> basically equivalent to mod_verify_sig(). It additionally does the marker
> check, that for kernel modules is done in module_sig_check(), and the
> parsing of the verified data+sig size.
>
> Currently, enable the usage of the flag only for the array map. Support for
> more map types can be added later.
>
> Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> ---
>  include/linux/bpf.h            |  7 ++++
>  include/uapi/linux/bpf.h       |  3 ++
>  kernel/bpf/arraymap.c          |  2 +-
>  kernel/bpf/syscall.c           | 70 ++++++++++++++++++++++++++++++++++
>  tools/include/uapi/linux/bpf.h |  3 ++
>  5 files changed, 84 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index a7080c86fa76..8f5c042e70a7 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1825,6 +1825,8 @@ static inline bool unprivileged_ebpf_enabled(void)
>         return !sysctl_unprivileged_bpf_disabled;
>  }
>
> +int bpf_map_verify_value_sig(const void *mod, size_t modlen, bool verify);
> +
>  #else /* !CONFIG_BPF_SYSCALL */
>  static inline struct bpf_prog *bpf_prog_get(u32 ufd)
>  {
> @@ -2034,6 +2036,11 @@ static inline bool unprivileged_ebpf_enabled(void)
>         return false;
>  }
>
> +static inline int bpf_map_verify_value_sig(const void *mod, size_t modlen,
> +                                          bool verify)
> +{
> +       return -EOPNOTSUPP;
> +}
>  #endif /* CONFIG_BPF_SYSCALL */
>
>  void __bpf_free_used_btfs(struct bpf_prog_aux *aux,
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index f4009dbdf62d..a8e7803d2593 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -1226,6 +1226,9 @@ enum {
>
>  /* Create a map that is suitable to be an inner map with dynamic max entries */
>         BPF_F_INNER_MAP         = (1U << 12),
> +
> +/* Verify map value (fmt: ver data+sig size(be32), ver data, sig, unver data) */
> +       BPF_F_VERIFY_ELEM       = (1U << 13)
>  };
>
>  /* Flags for BPF_PROG_QUERY. */
> diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
> index fe40d3b9458f..b430fdd0e892 100644
> --- a/kernel/bpf/arraymap.c
> +++ b/kernel/bpf/arraymap.c
> @@ -17,7 +17,7 @@
>
>  #define ARRAY_CREATE_FLAG_MASK \
>         (BPF_F_NUMA_NODE | BPF_F_MMAPABLE | BPF_F_ACCESS_MASK | \
> -        BPF_F_PRESERVE_ELEMS | BPF_F_INNER_MAP)
> +        BPF_F_PRESERVE_ELEMS | BPF_F_INNER_MAP | BPF_F_VERIFY_ELEM)
>
>  static void bpf_array_free_percpu(struct bpf_array *array)
>  {
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 2b69306d3c6e..ca9e4a284120 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -35,6 +35,8 @@
>  #include <linux/rcupdate_trace.h>
>  #include <linux/memcontrol.h>
>  #include <linux/trace_events.h>
> +#include <linux/verification.h>
> +#include <linux/module_signature.h>
>
>  #define IS_FD_ARRAY(map) ((map)->map_type == BPF_MAP_TYPE_PERF_EVENT_ARRAY || \
>                           (map)->map_type == BPF_MAP_TYPE_CGROUP_ARRAY || \
> @@ -180,6 +182,13 @@ static int bpf_map_update_value(struct bpf_map *map, struct fd f, void *key,
>  {
>         int err;
>
> +       if (map->map_flags & BPF_F_VERIFY_ELEM) {
> +               err = bpf_map_verify_value_sig(value, bpf_map_value_size(map),
> +                                              true);
> +               if (err < 0)
> +                       return err;
> +       }
> +
>         /* Need to create a kthread, thus must support schedule */
>         if (bpf_map_is_dev_bound(map)) {
>                 return bpf_map_offload_update_elem(map, key, value, flags);
> @@ -1057,6 +1066,11 @@ static int map_create(union bpf_attr *attr)
>         if (err)
>                 return -EINVAL;
>
> +       /* Allow signed data to go through update/push methods only. */
> +       if ((attr->map_flags & BPF_F_VERIFY_ELEM) &&
> +           (attr->map_flags & BPF_F_MMAPABLE))
> +               return -EINVAL;
> +
>         if (attr->btf_vmlinux_value_type_id) {
>                 if (attr->map_type != BPF_MAP_TYPE_STRUCT_OPS ||
>                     attr->btf_key_type_id || attr->btf_value_type_id)
> @@ -1353,6 +1367,62 @@ static int map_lookup_elem(union bpf_attr *attr)
>         return err;
>  }
>
> +int bpf_map_verify_value_sig(const void *mod, size_t modlen, bool verify)
> +{
> +       const size_t marker_len = strlen(MODULE_SIG_STRING);
> +       struct module_signature ms;
> +       size_t sig_len;
> +       u32 _modlen;
> +       int ret;
> +
> +       /*
> +        * Format of mod:
> +        *
> +        * verified data+sig size (be32), verified data, sig, unverified data
> +        */
> +       if (modlen <= sizeof(u32))
> +               return -ENOENT;
> +
> +       _modlen = be32_to_cpu(*(u32 *)(mod));
> +
> +       if (_modlen > modlen - sizeof(u32))
> +               return -EINVAL;
> +
> +       modlen = _modlen;
> +       mod += sizeof(u32);
> +
> +       if (modlen <= marker_len)
> +               return -ENOENT;
> +
> +       if (memcmp(mod + modlen - marker_len, MODULE_SIG_STRING, marker_len))
> +               return -ENOENT;
> +
> +       modlen -= marker_len;
> +
> +       if (modlen <= sizeof(ms))
> +               return -EBADMSG;
> +
> +       memcpy(&ms, mod + (modlen - sizeof(ms)), sizeof(ms));
> +
> +       ret = mod_check_sig(&ms, modlen, "bpf_map_value");
> +       if (ret)
> +               return ret;
> +
> +       sig_len = be32_to_cpu(ms.sig_len);
> +       modlen -= sig_len + sizeof(ms);
> +
> +       if (verify) {
> +               ret = verify_pkcs7_signature(mod, modlen, mod + modlen, sig_len,
> +                                            VERIFY_USE_SECONDARY_KEYRING,
> +                                            VERIFYING_UNSPECIFIED_SIGNATURE,
> +                                            NULL, NULL);
> +               if (ret < 0)
> +                       return ret;
> +       }
> +
> +       return modlen;
> +}
> +EXPORT_SYMBOL_GPL(bpf_map_verify_value_sig);
>
>  #define BPF_MAP_UPDATE_ELEM_LAST_FIELD flags
>
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index f4009dbdf62d..a8e7803d2593 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -1226,6 +1226,9 @@ enum {
>
>  /* Create a map that is suitable to be an inner map with dynamic max entries */
>         BPF_F_INNER_MAP         = (1U << 12),
> +
> +/* Verify map value (fmt: ver data+sig size(be32), ver data, sig, unver data) */
> +       BPF_F_VERIFY_ELEM       = (1U << 13)
>  };
>
>  /* Flags for BPF_PROG_QUERY. */
> --
> 2.25.1
>