[PATCH v2 18/30] evm: simplify evm_xattr_acl_change()

From: Christian Brauner <brauner@kernel.org>
To: linux-fsdevel@vger.kernel.org
Cc: Christian Brauner <brauner@kernel.org>,
	Seth Forshee <sforshee@kernel.org>,
	Christoph Hellwig <hch@lst.de>, Al Viro <viro@zeniv.linux.org.uk>,
	Mimi Zohar <zohar@linux.ibm.com>,
	linux-integrity@vger.kernel.org,
	linux-security-module@vger.kernel.org
Subject: [PATCH v2 18/30] evm: simplify evm_xattr_acl_change()
Date: Mon, 26 Sep 2022 16:08:15 +0200	[thread overview]
Message-ID: <20220926140827.142806-19-brauner@kernel.org> (raw)
In-Reply-To: <20220926140827.142806-1-brauner@kernel.org>

The posix acl api provides a dedicated security and integrity hook for
setting posix acls. This means that

evm_protect_xattr()
-> evm_xattr_change()
   -> evm_xattr_acl_change()

is now only hit during vfs_remove_acl() at which point we are guaranteed
that xattr_value and xattr_value_len are NULL and 0. In this case evm
always used to return 1. Simplify this function to do just that.

Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
---

Notes:
    /* v2 */
    unchanged

 security/integrity/evm/evm_main.c | 62 +++++++------------------------
 1 file changed, 14 insertions(+), 48 deletions(-)

diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index 15aa5995fff4..1fbe1b8d0364 100644
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -436,62 +436,29 @@ static enum integrity_status evm_verify_current_integrity(struct dentry *dentry)
 
 /*
  * evm_xattr_acl_change - check if passed ACL changes the inode mode
- * @mnt_userns: user namespace of the idmapped mount
- * @dentry: pointer to the affected dentry
  * @xattr_name: requested xattr
  * @xattr_value: requested xattr value
  * @xattr_value_len: requested xattr value length
  *
- * Check if passed ACL changes the inode mode, which is protected by EVM.
+ * This is only hit during xattr removal at which point we always return 1.
+ * Splat a warning in case someone managed to pass data to this function. That
+ * should never happen.
  *
  * Returns 1 if passed ACL causes inode mode change, 0 otherwise.
  */
-static int evm_xattr_acl_change(struct user_namespace *mnt_userns,
-				struct dentry *dentry, const char *xattr_name,
-				const void *xattr_value, size_t xattr_value_len)
+static int evm_xattr_acl_change(const void *xattr_value, size_t xattr_value_len)
 {
-#ifdef CONFIG_FS_POSIX_ACL
-	umode_t mode;
-	struct posix_acl *acl = NULL, *acl_res;
-	struct inode *inode = d_backing_inode(dentry);
-	int rc;
-
-	/*
-	 * An earlier comment here mentioned that the idmappings for
-	 * ACL_{GROUP,USER} don't matter since EVM is only interested in the
-	 * mode stored as part of POSIX ACLs. Nonetheless, if it must translate
-	 * from the uapi POSIX ACL representation to the VFS internal POSIX ACL
-	 * representation it should do so correctly. There's no guarantee that
-	 * we won't change POSIX ACLs in a way that ACL_{GROUP,USER} matters
-	 * for the mode at some point and it's difficult to keep track of all
-	 * the LSM and integrity modules and what they do to POSIX ACLs.
-	 *
-	 * Frankly, EVM shouldn't try to interpret the uapi struct for POSIX
-	 * ACLs it received. It requires knowledge that only the VFS is
-	 * guaranteed to have.
-	 */
-	acl = vfs_set_acl_prepare(mnt_userns, i_user_ns(inode),
-				  xattr_value, xattr_value_len);
-	if (IS_ERR_OR_NULL(acl))
-		return 1;
-
-	acl_res = acl;
-	/*
-	 * Passing mnt_userns is necessary to correctly determine the GID in
-	 * an idmapped mount, as the GID is used to clear the setgid bit in
-	 * the inode mode.
-	 */
-	rc = posix_acl_update_mode(mnt_userns, inode, &mode, &acl_res);
-
-	posix_acl_release(acl);
-
-	if (rc)
-		return 1;
+	int rc = 0;
 
-	if (inode->i_mode != mode)
-		return 1;
+#ifdef CONFIG_FS_POSIX_ACL
+	WARN_ONCE(xattr_value != NULL,
+		  "Passing xattr value for POSIX ACLs not supported\n");
+	WARN_ONCE(xattr_value_len != 0,
+		  "Passing non-zero length for POSIX ACLs not supported\n");
+	rc = 1;
 #endif
-	return 0;
+
+	return rc;
 }
 
 /*
@@ -514,8 +481,7 @@ static int evm_xattr_change(struct user_namespace *mnt_userns,
 	int rc = 0;
 
 	if (posix_xattr_acl(xattr_name))
-		return evm_xattr_acl_change(mnt_userns, dentry, xattr_name,
-					    xattr_value, xattr_value_len);
+		return evm_xattr_acl_change(xattr_value, xattr_value_len);
 
 	rc = vfs_getxattr_alloc(&init_user_ns, dentry, xattr_name, &xattr_data,
 				0, GFP_NOFS);
-- 
2.34.1

