CVE-2016-7097 (published 2016-10-16T21:59Z)

The filesystem implementation in the Linux kernel through 4.8.2 preserves the setgid bit during a setxattr call, which allows local users to gain group privileges by leveraging the existence of a setgid program with restrictions on execute permissions.

Severity: LOW
CVSS v2 base score: 3.6 (AV:L/AC:L/Au:N/C:P/I:P/A:N)
CVSS v3 base score: 4.4, MEDIUM (CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N)

Weakness information:
cwe_id: CWE-285
cwe_name: Improper Authorization
description: The product does not perform or incorrectly performs an authorization check when an actor attempts to access a resource or perform an action.
extended_description: 
            Assuming a user with a given identity, authorization is the process of determining whether that user can access a given resource, based on the user's privileges and any permissions or other access-control specifications that apply to the resource.
            When access control checks are not applied consistently - or not at all - users are able to access data or perform actions that they should not be allowed to perform. This can lead to a wide range of problems, including information exposures, denial of service, and arbitrary code execution.
         
         

Fix information:
repo_url: https://github.com/torvalds/linux
commit_details: repo_url: https://github.com/torvalds/linux
author: Jan Kara
author_date: 2016-09-19 17:39:09+02:00
msg: posix_acl: Clear SGID bit when setting file permissions

When file permissions are modified via chmod(2) and the user is not in
the owning group or capable of CAP_FSETID, the setgid bit is cleared in
inode_change_ok().  Setting a POSIX ACL via setxattr(2) sets the file
permissions as well as the new ACL, but doesn't clear the setgid bit in
a similar way; this allows to bypass the check in chmod(2).  Fix that.

References: CVE-2016-7097
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andreas Gruenbacher <agruenba@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: acl.c
change_type: ModificationType.MODIFY
diff: @@ -276,32 +276,26 @@ static int v9fs_xattr_set_acl(const struct xattr_handler *handler,
 	switch (handler->flags) {
 	case ACL_TYPE_ACCESS:
 		if (acl) {
-			umode_t mode = inode->i_mode;
-			retval = posix_acl_equiv_mode(acl, &mode);
-			if (retval < 0)
+			struct iattr iattr;
+
+			retval = posix_acl_update_mode(inode, &iattr.ia_mode, &acl);
+			if (retval)
 				goto err_out;
-			else {
-				struct iattr iattr;
-				if (retval == 0) {
-					/*
-					 * ACL can be represented
-					 * by the mode bits. So don't
-					 * update ACL.
-					 */
-					acl = NULL;
-					value = NULL;
-					size = 0;
-				}
-				/* Updte the mode bits */
-				iattr.ia_mode = ((mode & S_IALLUGO) |
-						 (inode->i_mode & ~S_IALLUGO));
-				iattr.ia_valid = ATTR_MODE;
-				/* FIXME should we update ctime ?
-				 * What is the following setxattr update the
-				 * mode ?
+			if (!acl) {
+				/*
+				 * ACL can be represented
+				 * by the mode bits. So don't
+				 * update ACL.
 				 */
-				v9fs_vfs_setattr_dotl(dentry, &iattr);
+				value = NULL;
+				size = 0;
 			}
+			iattr.ia_valid = ATTR_MODE;
+			/* FIXME should we update ctime ?
+			 * What is the following setxattr update the
+			 * mode ?
+			 */
+			v9fs_vfs_setattr_dotl(dentry, &iattr);
 		}
 		break;
 	case ACL_TYPE_DEFAULT:

programming_language: C
filename: acl.c
change_type: ModificationType.MODIFY
diff: @@ -79,11 +79,9 @@ static int __btrfs_set_acl(struct btrfs_trans_handle *trans,
 	case ACL_TYPE_ACCESS:
 		name = XATTR_NAME_POSIX_ACL_ACCESS;
 		if (acl) {
-			ret = posix_acl_equiv_mode(acl, &inode->i_mode);
-			if (ret < 0)
+			ret = posix_acl_update_mode(inode, &inode->i_mode, &acl);
+			if (ret)
 				return ret;
-			if (ret == 0)
-				acl = NULL;
 		}
 		ret = 0;
 		break;

programming_language: C
filename: acl.c
change_type: ModificationType.MODIFY
diff: @@ -95,11 +95,9 @@ int ceph_set_acl(struct inode *inode, struct posix_acl *acl, int type)
 	case ACL_TYPE_ACCESS:
 		name = XATTR_NAME_POSIX_ACL_ACCESS;
 		if (acl) {
-			ret = posix_acl_equiv_mode(acl, &new_mode);
-			if (ret < 0)
+			ret = posix_acl_update_mode(inode, &new_mode, &acl);
+			if (ret)
 				goto out;
-			if (ret == 0)
-				acl = NULL;
 		}
 		break;
 	case ACL_TYPE_DEFAULT:

programming_language: C
filename: acl.c
change_type: ModificationType.MODIFY
diff: @@ -190,15 +190,11 @@ ext2_set_acl(struct inode *inode, struct posix_acl *acl, int type)
 		case ACL_TYPE_ACCESS:
 			name_index = EXT2_XATTR_INDEX_POSIX_ACL_ACCESS;
 			if (acl) {
-				error = posix_acl_equiv_mode(acl, &inode->i_mode);
-				if (error < 0)
+				error = posix_acl_update_mode(inode, &inode->i_mode, &acl);
+				if (error)
 					return error;
-				else {
-					inode->i_ctime = CURRENT_TIME_SEC;
-					mark_inode_dirty(inode);
-					if (error == 0)
-						acl = NULL;
-				}
+				inode->i_ctime = CURRENT_TIME_SEC;
+				mark_inode_dirty(inode);
 			}
 			break;
 

programming_language: C
filename: acl.c
change_type: ModificationType.MODIFY
diff: @@ -193,15 +193,11 @@ __ext4_set_acl(handle_t *handle, struct inode *inode, int type,
 	case ACL_TYPE_ACCESS:
 		name_index = EXT4_XATTR_INDEX_POSIX_ACL_ACCESS;
 		if (acl) {
-			error = posix_acl_equiv_mode(acl, &inode->i_mode);
-			if (error < 0)
+			error = posix_acl_update_mode(inode, &inode->i_mode, &acl);
+			if (error)
 				return error;
-			else {
-				inode->i_ctime = ext4_current_time(inode);
-				ext4_mark_inode_dirty(handle, inode);
-				if (error == 0)
-					acl = NULL;
-			}
+			inode->i_ctime = ext4_current_time(inode);
+			ext4_mark_inode_dirty(handle, inode);
 		}
 		break;
 

programming_language: C
filename: acl.c
change_type: ModificationType.MODIFY
diff: @@ -210,12 +210,10 @@ static int __f2fs_set_acl(struct inode *inode, int type,
 	case ACL_TYPE_ACCESS:
 		name_index = F2FS_XATTR_INDEX_POSIX_ACL_ACCESS;
 		if (acl) {
-			error = posix_acl_equiv_mode(acl, &inode->i_mode);
-			if (error < 0)
+			error = posix_acl_update_mode(inode, &inode->i_mode, &acl);
+			if (error)
 				return error;
 			set_acl_inode(inode, inode->i_mode);
-			if (error == 0)
-				acl = NULL;
 		}
 		break;
 

programming_language: C
filename: acl.c
change_type: ModificationType.MODIFY
diff: @@ -92,17 +92,11 @@ int __gfs2_set_acl(struct inode *inode, struct posix_acl *acl, int type)
 	if (type == ACL_TYPE_ACCESS) {
 		umode_t mode = inode->i_mode;
 
-		error = posix_acl_equiv_mode(acl, &mode);
-		if (error < 0)
+		error = posix_acl_update_mode(inode, &inode->i_mode, &acl);
+		if (error)
 			return error;
-
-		if (error == 0)
-			acl = NULL;
-
-		if (mode != inode->i_mode) {
-			inode->i_mode = mode;
+		if (mode != inode->i_mode)
 			mark_inode_dirty(inode);
-		}
 	}
 
 	if (acl) {

programming_language: C
filename: posix_acl.c
change_type: ModificationType.MODIFY
diff: @@ -65,8 +65,8 @@ int hfsplus_set_posix_acl(struct inode *inode, struct posix_acl *acl,
 	case ACL_TYPE_ACCESS:
 		xattr_name = XATTR_NAME_POSIX_ACL_ACCESS;
 		if (acl) {
-			err = posix_acl_equiv_mode(acl, &inode->i_mode);
-			if (err < 0)
+			err = posix_acl_update_mode(inode, &inode->i_mode, &acl);
+			if (err)
 				return err;
 		}
 		err = 0;

programming_language: C
filename: acl.c
change_type: ModificationType.MODIFY
diff: @@ -233,9 +233,10 @@ int jffs2_set_acl(struct inode *inode, struct posix_acl *acl, int type)
 	case ACL_TYPE_ACCESS:
 		xprefix = JFFS2_XPREFIX_ACL_ACCESS;
 		if (acl) {
-			umode_t mode = inode->i_mode;
-			rc = posix_acl_equiv_mode(acl, &mode);
-			if (rc < 0)
+			umode_t mode;
+
+			rc = posix_acl_update_mode(inode, &mode, &acl);
+			if (rc)
 				return rc;
 			if (inode->i_mode != mode) {
 				struct iattr attr;
@@ -247,8 +248,6 @@ int jffs2_set_acl(struct inode *inode, struct posix_acl *acl, int type)
 				if (rc < 0)
 					return rc;
 			}
-			if (rc == 0)
-				acl = NULL;
 		}
 		break;
 	case ACL_TYPE_DEFAULT:

programming_language: C
filename: acl.c
change_type: ModificationType.MODIFY
diff: @@ -78,13 +78,11 @@ static int __jfs_set_acl(tid_t tid, struct inode *inode, int type,
 	case ACL_TYPE_ACCESS:
 		ea_name = XATTR_NAME_POSIX_ACL_ACCESS;
 		if (acl) {
-			rc = posix_acl_equiv_mode(acl, &inode->i_mode);
-			if (rc < 0)
+			rc = posix_acl_update_mode(inode, &inode->i_mode, &acl);
+			if (rc)
 				return rc;
 			inode->i_ctime = CURRENT_TIME;
 			mark_inode_dirty(inode);
-			if (rc == 0)
-				acl = NULL;
 		}
 		break;
 	case ACL_TYPE_DEFAULT:

programming_language: C
filename: acl.c
change_type: ModificationType.MODIFY
diff: @@ -241,13 +241,11 @@ int ocfs2_set_acl(handle_t *handle,
 	case ACL_TYPE_ACCESS:
 		name_index = OCFS2_XATTR_INDEX_POSIX_ACL_ACCESS;
 		if (acl) {
-			umode_t mode = inode->i_mode;
-			ret = posix_acl_equiv_mode(acl, &mode);
-			if (ret < 0)
-				return ret;
+			umode_t mode;
 
-			if (ret == 0)
-				acl = NULL;
+			ret = posix_acl_update_mode(inode, &mode, &acl);
+			if (ret)
+				return ret;
 
 			ret = ocfs2_acl_set_mode(inode, di_bh,
 						 handle, mode);

programming_language: C
filename: acl.c
change_type: ModificationType.MODIFY
diff: @@ -73,14 +73,11 @@ int orangefs_set_acl(struct inode *inode, struct posix_acl *acl, int type)
 	case ACL_TYPE_ACCESS:
 		name = XATTR_NAME_POSIX_ACL_ACCESS;
 		if (acl) {
-			umode_t mode = inode->i_mode;
-			/*
-			 * can we represent this with the traditional file
-			 * mode permission bits?
-			 */
-			error = posix_acl_equiv_mode(acl, &mode);
-			if (error < 0) {
-				gossip_err("%s: posix_acl_equiv_mode err: %d\n",
+			umode_t mode;
+
+			error = posix_acl_update_mode(inode, &mode, &acl);
+			if (error) {
+				gossip_err("%s: posix_acl_update_mode err: %d\n",
 					   __func__,
 					   error);
 				return error;
@@ -90,8 +87,6 @@ int orangefs_set_acl(struct inode *inode, struct posix_acl *acl, int type)
 				SetModeFlag(orangefs_inode);
 			inode->i_mode = mode;
 			mark_inode_dirty_sync(inode);
-			if (error == 0)
-				acl = NULL;
 		}
 		break;
 	case ACL_TYPE_DEFAULT:

programming_language: C
filename: posix_acl.c
change_type: ModificationType.MODIFY
diff: @@ -626,6 +626,37 @@ posix_acl_create(struct inode *dir, umode_t *mode,
 }
 EXPORT_SYMBOL_GPL(posix_acl_create);
 
+/**
+ * posix_acl_update_mode  -  update mode in set_acl
+ *
+ * Update the file mode when setting an ACL: compute the new file permission
+ * bits based on the ACL.  In addition, if the ACL is equivalent to the new
+ * file mode, set *acl to NULL to indicate that no ACL should be set.
+ *
+ * As with chmod, clear the setgit bit if the caller is not in the owning group
+ * or capable of CAP_FSETID (see inode_change_ok).
+ *
+ * Called from set_acl inode operations.
+ */
+int posix_acl_update_mode(struct inode *inode, umode_t *mode_p,
+			  struct posix_acl **acl)
+{
+	umode_t mode = inode->i_mode;
+	int error;
+
+	error = posix_acl_equiv_mode(*acl, &mode);
+	if (error < 0)
+		return error;
+	if (error == 0)
+		*acl = NULL;
+	if (!in_group_p(inode->i_gid) &&
+	    !capable_wrt_inode_uidgid(inode, CAP_FSETID))
+		mode &= ~S_ISGID;
+	*mode_p = mode;
+	return 0;
+}
+EXPORT_SYMBOL(posix_acl_update_mode);
+
 /*
  * Fix up the uids and gids in posix acl extended attributes in place.
  */

programming_language: C
filename: xattr_acl.c
change_type: ModificationType.MODIFY
diff: @@ -242,13 +242,9 @@ __reiserfs_set_acl(struct reiserfs_transaction_handle *th, struct inode *inode,
 	case ACL_TYPE_ACCESS:
 		name = XATTR_NAME_POSIX_ACL_ACCESS;
 		if (acl) {
-			error = posix_acl_equiv_mode(acl, &inode->i_mode);
-			if (error < 0)
+			error = posix_acl_update_mode(inode, &inode->i_mode, &acl);
+			if (error)
 				return error;
-			else {
-				if (error == 0)
-					acl = NULL;
-			}
 		}
 		break;
 	case ACL_TYPE_DEFAULT:

programming_language: C
filename: xfs_acl.c
change_type: ModificationType.MODIFY
diff: @@ -257,16 +257,11 @@ xfs_set_acl(struct inode *inode, struct posix_acl *acl, int type)
 		return error;
 
 	if (type == ACL_TYPE_ACCESS) {
-		umode_t mode = inode->i_mode;
-		error = posix_acl_equiv_mode(acl, &mode);
-
-		if (error <= 0) {
-			acl = NULL;
-
-			if (error < 0)
-				return error;
-		}
+		umode_t mode;
 
+		error = posix_acl_update_mode(inode, &mode, &acl);
+		if (error)
+			return error;
 		error = xfs_set_mode(inode, mode);
 		if (error)
 			return error;

programming_language: C
filename: posix_acl.h
change_type: ModificationType.MODIFY
diff: @@ -93,6 +93,7 @@ extern int set_posix_acl(struct inode *, int, struct posix_acl *);
 extern int posix_acl_chmod(struct inode *, umode_t);
 extern int posix_acl_create(struct inode *, umode_t *, struct posix_acl **,
 		struct posix_acl **);
+extern int posix_acl_update_mode(struct inode *, umode_t *, struct posix_acl **);
 
 extern int simple_set_acl(struct inode *, struct posix_acl *, int);
 extern int simple_acl_create(struct inode *, struct inode *);

programming_language: C