[RFC][PATCH] bpf: Check xattr name/value pair from bpf_lsm_inode_init_security()

[Alexei Starovoitov]

On Fri, Oct 21, 2022 at 9:57 AM Roberto Sassu
<roberto.sassu at huaweicloud.com> wrote:
>
> From: Roberto Sassu <roberto.sassu at huawei.com>
>
> BPF LSM allows security modules to directly attach to the security hooks,
> with the potential of not meeting the kernel expectation.
>
> This is the case for the inode_init_security hook, for which the kernel
> expects that name and value are set if the hook implementation returns
> zero.
>
> Consequently, not meeting the kernel expectation can cause the kernel to
> crash. One example is evm_protected_xattr_common() which expects the
> req_xattr_name parameter to be always not NULL.

Sounds like a bug in evm_protected_xattr_common.

> Introduce a level of indirection in BPF LSM, for the inode_init_security
> hook, to check the validity of the name and value set by security modules.

Doesn't make sense.
You probably meant security_old_inode_init_security,
because the hook without _old_ doesn't have such args:
int security_inode_init_security(struct inode *inode, struct inode *dir,
                                 const struct qstr *qstr,
                                 initxattrs initxattrs, void *fs_data);

> Encapsulate bpf_lsm_inode_init_security(), the existing attachment point,
> with bpf_inode_init_security(), the new function. After the attachment
> point is called, return -EOPNOTSUPP if the xattr name is not set, -ENOMEM
> if the xattr value is not set.
>
> As the name still cannot be set, rely on future patches to the eBPF
> verifier or introducing new kfuncs/helpers to ensure its correctness.
>
> Finally, as proposed by Nicolas, update the LSM hook documentation for the
> inode_init_security hook, to reflect the current behavior (only the xattr
> value is allocated).
>
> Cc: stable at vger.kernel.org
> Fixes: 520b7aa00d8cd ("bpf: lsm: Initialize the BPF LSM hooks")
> Reported-by: Nicolas Bouchinet <nicolas.bouchinet at clip-os.org>
> Signed-off-by: Roberto Sassu <roberto.sassu at huawei.com>
> ---
>  include/linux/lsm_hooks.h |  4 ++--
>  security/bpf/hooks.c      | 25 +++++++++++++++++++++++++
>  2 files changed, 27 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> index 4ec80b96c22e..f44d45f4737f 100644
> --- a/include/linux/lsm_hooks.h
> +++ b/include/linux/lsm_hooks.h
> @@ -229,8 +229,8 @@
>   *     This hook is called by the fs code as part of the inode creation
>   *     transaction and provides for atomic labeling of the inode, unlike
>   *     the post_create/mkdir/... hooks called by the VFS.  The hook function
> - *     is expected to allocate the name and value via kmalloc, with the caller
> - *     being responsible for calling kfree after using them.
> + *     is expected to allocate the value via kmalloc, with the caller
> + *     being responsible for calling kfree after using it.

must be an obsolete comment.

>   *     If the security module does not use security attributes or does
>   *     not wish to put a security attribute on this particular inode,
>   *     then it should return -EOPNOTSUPP to skip this processing.
> diff --git a/security/bpf/hooks.c b/security/bpf/hooks.c
> index e5971fa74fd7..492c07ba6722 100644
> --- a/security/bpf/hooks.c
> +++ b/security/bpf/hooks.c
> @@ -6,11 +6,36 @@
>  #include <linux/lsm_hooks.h>
>  #include <linux/bpf_lsm.h>
>
> +static int bpf_inode_init_security(struct inode *inode, struct inode *dir,
> +                                  const struct qstr *qstr, const char **name,
> +                                  void **value, size_t *len)
> +{
> +       int ret;
> +
> +       ret = bpf_lsm_inode_init_security(inode, dir, qstr, name, value, len);
> +       if (ret)
> +               return ret;
> +
> +       /*
> +        * As the name cannot be set by the eBPF programs directly, eBPF will
> +        * be responsible for its correctness through the verifier or
> +        * appropriate kfuncs/helpers.
> +        */
> +       if (name && !*name)
> +               return -EOPNOTSUPP;

bpf cannot write into such pointers.
It won't be able to use kfuncs to kmalloc and write into them either.
None of it makes sense to me.

> +
> +       if (value && !*value)
> +               return -ENOMEM;
> +
> +       return 0;
> +}
> +
>  static struct security_hook_list bpf_lsm_hooks[] __lsm_ro_after_init = {
>         #define LSM_HOOK(RET, DEFAULT, NAME, ...) \
>         LSM_HOOK_INIT(NAME, bpf_lsm_##NAME),
>         #include <linux/lsm_hook_defs.h>
>         #undef LSM_HOOK
> +       LSM_HOOK_INIT(inode_init_security, bpf_inode_init_security),
>         LSM_HOOK_INIT(inode_free_security, bpf_inode_storage_free),
>         LSM_HOOK_INIT(task_free, bpf_task_storage_free),
>  };
> --
> 2.25.1
>