[PATCH] security: fix the logic in security_inode_getsecctx()

[Ondrej Mosnacek <omosnace@redhat.com>]

The inode_getsecctx LSM hook has previously been corrected to have
-EOPNOTSUPP instead of 0 as the default return value to fix BPF LSM
behavior. However, the call_int_hook()-generated loop in
security_inode_getsecctx() was left treating 0 as the neutral value, so
after an LSM returns 0, the loop continues to try other LSMs, and if one
of them returns a non-zero value, the function immediately returns with
said value. So in a situation where SELinux and the BPF LSMs registered
this hook, -EOPNOTSUPP would be incorrectly returned whenever SELinux
returned 0.

Fix this by open-coding the call_int_hook() loop and making it use the
correct LSM_RET_DEFAULT() value as the neutral one, similar to what
other hooks do.

Reported-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Link: https://lore.kernel.org/selinux/CAEjxPJ4ev-pasUwGx48fDhnmjBnq_Wh90jYPwRQRAqXxmOKD4Q@mail.gmail.com/
Fixes: b36995b8609a ("lsm: fix default return value for inode_getsecctx")
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
---

I ran 'tools/nfs.sh' on the patch and even though it fixes the most
serious issue that Stephen reported, some of the tests are still
failing under NFS (but I will presume that these are pre-existing issues
not caused by the patch).

I can also see an opportunity to clean up the hook implementations in
security/security.c - I plan to have a go at it and send it as a
separate patch later.

 security/security.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/security/security.c b/security/security.c
index 0144a98d3712..6196ccaba433 100644
--- a/security/security.c
+++ b/security/security.c
@@ -4255,7 +4255,19 @@ EXPORT_SYMBOL(security_inode_setsecctx);
  */
 int security_inode_getsecctx(struct inode *inode, void **ctx, u32 *ctxlen)
 {
-	return call_int_hook(inode_getsecctx, -EOPNOTSUPP, inode, ctx, ctxlen);
+	struct security_hook_list *hp;
+	int rc;
+
+	/*
+	 * Only one module will provide a security context.
+	 */
+	hlist_for_each_entry(hp, &security_hook_heads.inode_getsecctx, list) {
+		rc = hp->hook.inode_getsecctx(inode, ctx, ctxlen);
+		if (rc != LSM_RET_DEFAULT(inode_getsecctx))
+			return rc;
+	}
+
+	return LSM_RET_DEFAULT(inode_getsecctx);
 }
 EXPORT_SYMBOL(security_inode_getsecctx);
 
-- 
2.43.0