Re: [PATCH RFC v10 7/17] ipe: add userspace interface

Paul Moore
2023-07-08T05:38:06Z

On Jun 28, 2023 Fan Wu <wufan@linux.microsoft.com> wrote:
>
> As is typical with LSMs, IPE uses securityfs as its interface with
> userspace. for a complete list of the interfaces and the respective
> inputs/outputs, please see the documentation under
> admin-guide/LSM/ipe.rst
>
> Signed-off-by: Deven Bowers <deven.desai@linux.microsoft.com>
> Signed-off-by: Fan Wu <wufan@linux.microsoft.com>
> ---
> security/ipe/Makefile | 2 +
> security/ipe/fs.c | 101 ++++++++
> security/ipe/fs.h | 16 ++
> security/ipe/ipe.c | 3 +
> security/ipe/ipe.h | 2 +
> security/ipe/policy.c | 111 +++++++++
> security/ipe/policy.h | 9 +
> security/ipe/policy_fs.c | 481 +++++++++++++++++++++++++++++++++++++++
> 8 files changed, 725 insertions(+)
> create mode 100644 security/ipe/fs.c
> create mode 100644 security/ipe/fs.h
> create mode 100644 security/ipe/policy_fs.c

...

> diff --git a/security/ipe/policy.c b/security/ipe/policy.c
> index 4069ff075093..3e8e4a06a044 100644
> --- a/security/ipe/policy.c
> +++ b/security/ipe/policy.c
> @@ -7,9 +7,36 @@
> #include <linux/verification.h>
>
> #include "ipe.h"
> +#include "eval.h"
> +#include "fs.h"
> #include "policy.h"
> #include "policy_parser.h"
>
> +/* lock for synchronizing writers across ipe policy */
> +DEFINE_MUTEX(ipe_policy_lock);
> +
> +/**
> + * ver_to_u64 - Convert an internal ipe_policy_version to a u64.
> + * @p: Policy to extract the version from.
> + *
> + * Bits (LSB is index 0):
> + * [48,32] -> Major
> + * [32,16] -> Minor
> + * [16, 0] -> Revision
> + *
> + * Return: u64 version of the embedded version structure.
> + */
> +static inline u64 ver_to_u64(const struct ipe_policy *const p)
> +{
> + u64 r;
> +
> + r = (((u64)p->parsed->version.major) << 32)
> + | (((u64)p->parsed->version.minor) << 16)
> + | ((u64)(p->parsed->version.rev));
> +
> + return r;
> +}
> +
> /**
> * ipe_free_policy - Deallocate a given IPE policy.
> * @p: Supplies the policy to free.
> @@ -21,6 +48,7 @@ void ipe_free_policy(struct ipe_policy *p)
> if (IS_ERR_OR_NULL(p))
> return;
>
> + ipe_del_policyfs_node(p);
> free_parsed_policy(p->parsed);
> if (!p->pkcs7)
> kfree(p->text);
> @@ -39,6 +67,65 @@ static int set_pkcs7_data(void *ctx, const void *data, size_t len,
> return 0;
> }
>
> +/**
> + * ipe_update_policy - parse a new policy and replace @old with it.

What does "@old" refer to? I'm guessing you want to drop the "@".

> + * @root: Supplies a pointer to the securityfs inode saved the policy.
> + * @text: Supplies a pointer to the plain text policy.
> + * @textlen: Supplies the length of @text.
> + * @pkcs7: Supplies a pointer to a buffer containing a pkcs7 message.
> + * @pkcs7len: Supplies the length of @pkcs7len.
> + *
> + * @text/@textlen is mutually exclusive with @pkcs7/@pkcs7len - see
> + * ipe_new_policy.
> + *
> + * Return:
> + * * !IS_ERR - The old policy

"The old policy" is what?

> + * * -ENOENT - Policy doesn't exist
> + * * -EINVAL - New policy is invalid
> + */
> +struct ipe_policy *ipe_update_policy(struct inode *root,
> + const char *text, size_t textlen,
> + const char *pkcs7, size_t pkcs7len)
> +{
> + int rc = 0;
> + struct ipe_policy *old, *ap, *new = NULL;
> +
> + lockdep_assert_held(&ipe_policy_lock);
> +
> + old = (struct ipe_policy *)root->i_private;
> + if (!old)
> + return ERR_PTR(-ENOENT);
> +
> + new = ipe_new_policy(text, textlen, pkcs7, pkcs7len);
> + if (IS_ERR(new))
> + return new;
> +
> + if (strcmp(new->parsed->name, old->parsed->name)) {
> + rc = -EINVAL;
> + goto err;
> + }
> +
> + if (ver_to_u64(old) > ver_to_u64(new)) {
> + rc = -EINVAL;
> + goto err;
> + }
> +
> + root->i_private = new;
> +
> + ap = rcu_dereference_protected(ipe_active_policy,
> + lockdep_is_held(&ipe_policy_lock));
> + if (old == ap)
> + rcu_assign_pointer(ipe_active_policy, new);
> +
> + swap(new->policyfs, old->policyfs);

We don't have to worry about @new, but is there a guarantee that this
function is the only one attempting to modify @old?

*EDIT*: I found that @root is locked by the caller, that's good. I
would suggest adding this assumption/requirement to the function's
description. In general whenever a function requires something from
a caller it should be documented in the function's description.

> +out:
> + return (rc < 0) ? ERR_PTR(rc) : old;
> +err:
> + ipe_free_policy(new);
> + goto out;
> +}
> +

...

> diff --git a/security/ipe/policy_fs.c b/security/ipe/policy_fs.c
> new file mode 100644
> index 000000000000..52a120118cda
> --- /dev/null
> +++ b/security/ipe/policy_fs.c
> @@ -0,0 +1,481 @@

...

> +/**
> + * getactive - Read handler for "ipe/policies/$name/active".
> + * @f: Supplies a file structure representing the securityfs node.
> + * @data: Suppleis a buffer passed to the write syscall.
> + * @len: Supplies the length of @data.
> + * @offset: unused.
> + *
> + * @data will be populated with the 1 or 0 depending on if the
> + * corresponding policy is active.
> + *
> + * Return:
> + * * >0 - Success, Length of buffer written
> + * * <0 - Error
> + */
> +static ssize_t getactive(struct file *f, char __user *data,
> + size_t len, loff_t *offset)
> +{
> + int rc = 0;
> + const char *str;
> + struct inode *root = NULL;
> + const struct ipe_policy *p = NULL;
> +
> + root = d_inode(f->f_path.dentry->d_parent);
> +
> + inode_lock_shared(root);
> + p = (struct ipe_policy *)root->i_private;
> + if (!p) {
> + inode_unlock_shared(root);
> + return -ENOENT;
> + }
> + inode_unlock_shared(root);
> +
> + str = (p == rcu_access_pointer(ipe_active_policy)) ? "1" : "0";

The line above should be wrapped with a RCU lock.

> + rc = simple_read_from_buffer(data, len, offset, str, 1);
> +
> + return rc;
> +}

--
paul-moore.com