posix_acl: Clear SGID bit when setting file permissions

commit 07393101 upstream.

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>
[bwh: Backported to 3.2:
 - Drop changes to ceph, f2fs, hfsplus, orangefs
 - Use capable() instead of capable_wrt_inode_uidgid()
 - Update ext3 and generic_acl.c as well
 - In gfs2, jfs, and xfs, take care to avoid leaking the allocated ACL if
   posix_acl_update_mode() determines it's not needed
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/9p/acl.c

@@ -319,32 +319,26 @@ static int v9fs_xattr_set_acl(struct dentry *dentry, const char *name,
 	case ACL_TYPE_ACCESS:
 		name = POSIX_ACL_XATTR_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:

fs/btrfs/acl.c

@@ -118,11 +118,9 @@ static int btrfs_set_acl(struct btrfs_trans_handle *trans,
 	case ACL_TYPE_ACCESS:
 		name = POSIX_ACL_XATTR_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;

fs/ext2/acl.c

@@ -194,15 +194,11 @@ ext2_set_acl(struct inode *inode, int type, struct posix_acl *acl)
 		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;
 

fs/ext3/acl.c

@@ -199,15 +199,11 @@ ext3_set_acl(handle_t *handle, struct inode *inode, int type,
 		case ACL_TYPE_ACCESS:
 			name_index = EXT3_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;
-					ext3_mark_inode_dirty(handle, inode);
-					if (error == 0)
-						acl = NULL;
-				}
+				inode->i_ctime = CURRENT_TIME_SEC;
+				ext3_mark_inode_dirty(handle, inode);
 			}
 			break;
 

fs/ext4/acl.c

@@ -198,15 +198,11 @@ 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;
 

fs/generic_acl.c

@@ -86,16 +86,17 @@ generic_acl_set(struct dentry *dentry, const char *name, const void *value,
 		if (error)
 			goto failed;
 		switch (type) {
-		case ACL_TYPE_ACCESS:
-			error = posix_acl_equiv_mode(acl, &inode->i_mode);
-			if (error < 0)
+		case ACL_TYPE_ACCESS: {
+			struct posix_acl *saved_acl = acl;
+
+			error = posix_acl_update_mode(inode, &inode->i_mode, &acl);
+			if (acl == NULL)
+				posix_acl_release(saved_acl);
+			if (error)
 				goto failed;
 			inode->i_ctime = CURRENT_TIME;
-			if (error == 0) {
-				posix_acl_release(acl);
-				acl = NULL;
-			}
 			break;
+		}
 		case ACL_TYPE_DEFAULT:
 			if (!S_ISDIR(inode->i_mode)) {
 				error = -EINVAL;

fs/gfs2/acl.c

@@ -277,16 +277,14 @@ static int gfs2_xattr_system_set(struct dentry *dentry, const char *name,
 		goto out_release;
 
 	if (type == ACL_TYPE_ACCESS) {
-		umode_t mode = inode->i_mode;
-		error = posix_acl_equiv_mode(acl, &mode);
+		struct posix_acl *saved_acl = acl;
+		umode_t mode;
 
-		if (error <= 0) {
-			posix_acl_release(acl);
-			acl = NULL;
-
-			if (error < 0)
-				return error;
-		}
+		error = posix_acl_update_mode(inode, &mode, &acl);
+		if (error || acl == NULL)
+			posix_acl_release(saved_acl);
+		if (error)
+			return error;
 
 		error = gfs2_set_mode(inode, mode);
 		if (error)

fs/jffs2/acl.c

@@ -227,9 +227,10 @@ static int jffs2_set_acl(struct inode *inode, int type, struct posix_acl *acl)
 	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;
@@ -241,8 +242,6 @@ static int jffs2_set_acl(struct inode *inode, int type, struct posix_acl *acl)
 				if (rc < 0)
 					return rc;
 			}
-			if (rc == 0)
-				acl = NULL;
 		}
 		break;
 	case ACL_TYPE_DEFAULT:

fs/jfs/xattr.c

@@ -693,9 +693,11 @@ static int can_set_system_xattr(struct inode *inode, const char *name,
 			return rc;
 		}
 		if (acl) {
-			rc = posix_acl_equiv_mode(acl, &inode->i_mode);
+			struct posix_acl *dummy = acl;
+
+			rc = posix_acl_update_mode(inode, &inode->i_mode, &dummy);
 			posix_acl_release(acl);
-			if (rc < 0) {
+			if (rc) {
 				printk(KERN_ERR
 				       "posix_acl_equiv_mode returned %d\n",
 				       rc);

fs/ocfs2/acl.c

@@ -247,14 +247,11 @@ static 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)
+			umode_t mode;
+			ret = posix_acl_update_mode(inode, &mode, &acl);
+			if (ret)
 				return ret;
 			else {
-				if (ret == 0)
-					acl = NULL;
-
 				ret = ocfs2_acl_set_mode(inode, di_bh,
 							 handle, mode);
 				if (ret)

fs/posix_acl.c

@@ -341,6 +341,36 @@ static int posix_acl_create_masq(struct posix_acl *acl, umode_t *mode_p)
         return not_equiv;
 }
 
+/**
+ * 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(CAP_FSETID))
+		mode &= ~S_ISGID;
+	*mode_p = mode;
+	return 0;
+}
+EXPORT_SYMBOL(posix_acl_update_mode);
+
 /*
  * Modify the ACL for the chmod syscall.
  */

fs/reiserfs/xattr_acl.c

@@ -272,13 +272,9 @@ reiserfs_set_acl(struct reiserfs_transaction_handle *th, struct inode *inode,
 	case ACL_TYPE_ACCESS:
 		name = POSIX_ACL_XATTR_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:

fs/xfs/xfs_acl.c

@@ -384,17 +384,14 @@ xfs_xattr_acl_set(struct dentry *dentry, const char *name,
 		goto out_release;
 
 	if (type == ACL_TYPE_ACCESS) {
-		umode_t mode = inode->i_mode;
-		error = posix_acl_equiv_mode(acl, &mode);
-
-		if (error <= 0) {
-			posix_acl_release(acl);
-			acl = NULL;
-
-			if (error < 0)
-				return error;
-		}
+		struct posix_acl *saved_acl = acl;
+		umode_t mode;
 
+		error = posix_acl_update_mode(inode, &mode, &acl);
+		if (error || acl == NULL)
+			posix_acl_release(saved_acl);
+		if (error)
+			return error;
 		error = xfs_set_mode(dentry, inode, mode);
 		if (error)
 			goto out_release;

include/linux/posix_acl.h

@@ -83,6 +83,7 @@ extern struct posix_acl *posix_acl_from_mode(umode_t, gfp_t);
 extern int posix_acl_equiv_mode(const struct posix_acl *, umode_t *);
 extern int posix_acl_create(struct posix_acl **, gfp_t, umode_t *);
 extern int posix_acl_chmod(struct posix_acl **, gfp_t, umode_t);
+extern int posix_acl_update_mode(struct inode *, umode_t *, struct posix_acl **);
 
 extern struct posix_acl *get_posix_acl(struct inode *, int);
 extern int set_posix_acl(struct inode *, int, struct posix_acl *);