CVE-2016-1237 (published 2016-06-29T14:10Z)

nfsd in the Linux kernel through 4.6.3 allows local users to bypass intended file-permission restrictions by setting a POSIX ACL, related to nfs2acl.c, nfs3acl.c, and nfs4acl.c.

Severity: MEDIUM
CVSS v2 base score: 4.9 (AV:L/AC:L/Au:N/C:C/I:N/A:N)
CVSS v3 base score: 5.5, MEDIUM (CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N)

Weakness information:
cwe_id: CWE-284
cwe_name: Improper Access Control
description: The product does not restrict or incorrectly restricts access to a resource from an unauthorized actor.
extended_description: 
            Access control involves the use of several protection mechanisms such as:
            
               Authentication (proving the identity of an actor)
               Authorization (ensuring that a given actor can access a resource), and
               Accountability (tracking of activities that were performed)
            
            When any mechanism is not applied or otherwise fails, attackers can compromise the security of the product by gaining privileges, reading sensitive information, executing commands, evading detection, etc.
            There are two distinct behaviors that can introduce access control weaknesses:
               
                  Specification: incorrect privileges, permissions, ownership, etc. are explicitly specified for either the user or the resource (for example, setting a password file to be world-writable, or giving administrator capabilities to a guest user). This action could be performed by the program or the administrator.
                  Enforcement: the mechanism contains errors that prevent it from properly enforcing the specified access control requirements (e.g., allowing the user to specify their own privileges, or allowing a syntactically-incorrect ACL to produce insecure settings). This problem occurs within the program itself, in that it does not actually enforce the intended security policy that the administrator specifies.
               
         
		 

Fix information:
repo_url: https://github.com/torvalds/linux
commit_details: repo_url: https://github.com/torvalds/linux
author: Ben Hutchings
author_date: 2016-06-22 19:43:35+01:00
msg: nfsd: check permissions when setting ACLs

Use set_posix_acl, which includes proper permission checks, instead of
calling ->set_acl directly.  Without this anyone may be able to grant
themselves permissions to a file by setting the ACL.

Lock the inode to make the new checks atomic with respect to set_acl.
(Also, nfsd was the only caller of set_acl not locking the inode, so I
suspect this may fix other races.)

This also simplifies the code, and ensures our ACLs are checked by
posix_acl_valid.

The permission checks and the inode locking were lost with commit
4ac7249e, which changed nfsd to use the set_acl inode operation directly
instead of going through xattr handlers.

Reported-by: David Sinquin <david@sinquin.eu>
[agreunba@redhat.com: use set_posix_acl]
Fixes: 4ac7249e
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: stable@vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
repository: repo_url: https://github.com/torvalds/linux
repo_name: torvalds/linux
description: Linux kernel source tree
repo_language: C
owner: torvalds
file_changes: filename: nfs2acl.c
change_type: ModificationType.MODIFY
diff: @@ -104,22 +104,21 @@ static __be32 nfsacld_proc_setacl(struct svc_rqst * rqstp,
 		goto out;
 
 	inode = d_inode(fh->fh_dentry);
-	if (!IS_POSIXACL(inode) || !inode->i_op->set_acl) {
-		error = -EOPNOTSUPP;
-		goto out_errno;
-	}
 
 	error = fh_want_write(fh);
 	if (error)
 		goto out_errno;
 
-	error = inode->i_op->set_acl(inode, argp->acl_access, ACL_TYPE_ACCESS);
+	fh_lock(fh);
+
+	error = set_posix_acl(inode, ACL_TYPE_ACCESS, argp->acl_access);
 	if (error)
-		goto out_drop_write;
-	error = inode->i_op->set_acl(inode, argp->acl_default,
-				     ACL_TYPE_DEFAULT);
+		goto out_drop_lock;
+	error = set_posix_acl(inode, ACL_TYPE_DEFAULT, argp->acl_default);
 	if (error)
-		goto out_drop_write;
+		goto out_drop_lock;
+
+	fh_unlock(fh);
 
 	fh_drop_write(fh);
 
@@ -131,7 +130,8 @@ static __be32 nfsacld_proc_setacl(struct svc_rqst * rqstp,
 	posix_acl_release(argp->acl_access);
 	posix_acl_release(argp->acl_default);
 	return nfserr;
-out_drop_write:
+out_drop_lock:
+	fh_unlock(fh);
 	fh_drop_write(fh);
 out_errno:
 	nfserr = nfserrno(error);

programming_language: C
filename: nfs3acl.c
change_type: ModificationType.MODIFY
diff: @@ -95,22 +95,20 @@ static __be32 nfsd3_proc_setacl(struct svc_rqst * rqstp,
 		goto out;
 
 	inode = d_inode(fh->fh_dentry);
-	if (!IS_POSIXACL(inode) || !inode->i_op->set_acl) {
-		error = -EOPNOTSUPP;
-		goto out_errno;
-	}
 
 	error = fh_want_write(fh);
 	if (error)
 		goto out_errno;
 
-	error = inode->i_op->set_acl(inode, argp->acl_access, ACL_TYPE_ACCESS);
+	fh_lock(fh);
+
+	error = set_posix_acl(inode, ACL_TYPE_ACCESS, argp->acl_access);
 	if (error)
-		goto out_drop_write;
-	error = inode->i_op->set_acl(inode, argp->acl_default,
-				     ACL_TYPE_DEFAULT);
+		goto out_drop_lock;
+	error = set_posix_acl(inode, ACL_TYPE_DEFAULT, argp->acl_default);
 
-out_drop_write:
+out_drop_lock:
+	fh_unlock(fh);
 	fh_drop_write(fh);
 out_errno:
 	nfserr = nfserrno(error);

programming_language: C
filename: nfs4acl.c
change_type: ModificationType.MODIFY
diff: @@ -770,9 +770,6 @@ nfsd4_set_nfs4_acl(struct svc_rqst *rqstp, struct svc_fh *fhp,
 	dentry = fhp->fh_dentry;
 	inode = d_inode(dentry);
 
-	if (!inode->i_op->set_acl || !IS_POSIXACL(inode))
-		return nfserr_attrnotsupp;
-
 	if (S_ISDIR(inode->i_mode))
 		flags = NFS4_ACL_DIR;
 
@@ -782,16 +779,19 @@ nfsd4_set_nfs4_acl(struct svc_rqst *rqstp, struct svc_fh *fhp,
 	if (host_error < 0)
 		goto out_nfserr;
 
-	host_error = inode->i_op->set_acl(inode, pacl, ACL_TYPE_ACCESS);
+	fh_lock(fhp);
+
+	host_error = set_posix_acl(inode, ACL_TYPE_ACCESS, pacl);
 	if (host_error < 0)
-		goto out_release;
+		goto out_drop_lock;
 
 	if (S_ISDIR(inode->i_mode)) {
-		host_error = inode->i_op->set_acl(inode, dpacl,
-						  ACL_TYPE_DEFAULT);
+		host_error = set_posix_acl(inode, ACL_TYPE_DEFAULT, dpacl);
 	}
 
-out_release:
+out_drop_lock:
+	fh_unlock(fhp);
+
 	posix_acl_release(pacl);
 	posix_acl_release(dpacl);
 out_nfserr:

programming_language: C