Re: [PATCH RFC v12 8/20] ipe: add userspace interface

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On 2/5/2024 3:10 PM, Paul Moore wrote:
On Mon, Feb 5, 2024 at 6:01 PM Fan Wu <wufan@xxxxxxxxxxxxxxxxxxx> wrote:
On 2/3/2024 2:25 PM, Paul Moore wrote:
On Jan 30, 2024 Fan Wu <wufan@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Fan Wu <wufan@xxxxxxxxxxxxxxxxxxx>
---
v2:
    + Split evaluation loop, access control hooks,
      and evaluation loop from policy parser and userspace
      interface to pass mailing list character limit

v3:
    + Move policy load and activation audit event to 03/12
    + Fix a potential panic when a policy failed to load.
    + use pr_warn for a failure to parse instead of an
      audit record
    + Remove comments from headers
    + Add lockdep assertions to ipe_update_active_policy and
      ipe_activate_policy
    + Fix up warnings with checkpatch --strict
    + Use file_ns_capable for CAP_MAC_ADMIN for securityfs
      nodes.
    + Use memdup_user instead of kzalloc+simple_write_to_buffer.
    + Remove strict_parse command line parameter, as it is added
      by the sysctl command line.
    + Prefix extern variables with ipe_

v4:
    + Remove securityfs to reverse-dependency
    + Add SHA1 reverse dependency.
    + Add versioning scheme for IPE properties, and associated
      interface to query the versioning scheme.
    + Cause a parser to always return an error on unknown syntax.
    + Remove strict_parse option
    + Change active_policy interface from sysctl, to securityfs,
      and change scheme.

v5:
    + Cause an error if a default action is not defined for each
      operation.
    + Minor function renames

v6:
    + No changes

v7:
    + Propagating changes to support the new ipe_context structure in the
      evaluation loop.

    + Further split the parser and userspace interface changes into
      separate commits.

    + "raw" was renamed to "pkcs7" and made read only
    + "raw"'s write functionality (update a policy) moved to "update"
    + introduced "version", "policy_name" nodes.
    + "content" renamed to "policy"
    + changes to allow the compiled-in policy to be treated
      identical to deployed-after-the-fact policies.

v8:
    + Prevent securityfs initialization if the LSM is disabled

v9:
    + Switch to securityfs_recursive_remove for policy folder deletion

v10:
    + Simplify and correct concurrency
    + Fix typos

v11:
    + Correct code comments

v12:
    + Correct locking and remove redundant code
---
   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    | 123 ++++++++++
   security/ipe/policy.h    |   9 +
   security/ipe/policy_fs.c | 469 +++++++++++++++++++++++++++++++++++++++
   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 f22a576a6d68..61fea3e38e11 100644
--- a/security/ipe/policy.c
+++ b/security/ipe/policy.c
@@ -43,6 +71,68 @@ 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.
+ * @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.
+ *
+ * Context: Requires root->i_rwsem to be held.
+ * Return:
+ * * !IS_ERR        - The existing policy saved in the inode before update
+ * * -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;
+
+    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;
+    swap(new->policyfs, old->policyfs);

Should the swap() take place with @ipe_policy_lock held?

I think we are safe here because root->i_rwsem is held. Other two
operations set_active and delete are also depending on the inode lock.
+    mutex_lock(&ipe_policy_lock);
+    ap = rcu_dereference_protected(ipe_active_policy,
+                                   lockdep_is_held(&ipe_policy_lock));
+    if (old == ap) {
+            rcu_assign_pointer(ipe_active_policy, new);
+            mutex_unlock(&ipe_policy_lock);
+            synchronize_rcu();

I'm guessing you are forcing a synchronize_rcu() here because you are
free()'ing @old in the caller, yes?  Looking at the code, I only see
one caller, update_policy().  With only one caller, why not free @old
directly in ipe_update_policy()?  Do you see others callers that would
do something different?

The call of synchronize_rcu() is because we are updating the current
active policy so we need to set the new policy as active.

Unless I'm mistaken, a syncronize_rcu() call only ensures that the
current task will see the updated value by waiting until all current
RCU critical sections have finished.  Given the mutex involved here I
don't believe this is necessary, but please correct me if I'm wrong.

Sorry for the confusion. I think your previous comment was right, the call of synchronize_rcu() is to free the old one. And I should put the free of old just after the synchronize_rcu() call.

Thanks,
Fan




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