[PATCH] r/o bind mounts: elevate write count for xattr_permission() callers

This basically audits the callers of xattr_permission(), which calls
permission() and can perform writes to the filesystem.

[AV: add missing parts - removexattr() and nfsd posix acls, plug for a leak
spotted by Miklos]
Acked-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/nfsd/nfs4proc.c

@@ -658,14 +658,19 @@ nfsd4_setattr(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 			return status;
 		}
 	}
+	status = mnt_want_write(cstate->current_fh.fh_export->ex_path.mnt);
+	if (status)
+		return status;
 	status = nfs_ok;
 	if (setattr->sa_acl != NULL)
 		status = nfsd4_set_nfs4_acl(rqstp, &cstate->current_fh,
 					    setattr->sa_acl);
 	if (status)
-		return status;
+		goto out;
 	status = nfsd_setattr(rqstp, &cstate->current_fh, &setattr->sa_iattr,
 				0, (time_t)0);
+out:
+	mnt_drop_write(cstate->current_fh.fh_export->ex_path.mnt);
 	return status;
 }
 

fs/nfsd/vfs.c

@@ -2086,6 +2086,9 @@ nfsd_set_posix_acl(struct svc_fh *fhp, int type, struct posix_acl *acl)
 	} else
 		size = 0;
 
+	error = mnt_want_write(fhp->fh_export->ex_path.mnt);
+	if (error)
+		goto getout;
 	if (size)
 		error = vfs_setxattr(fhp->fh_dentry, name, value, size, 0);
 	else {
@@ -2097,6 +2100,7 @@ nfsd_set_posix_acl(struct svc_fh *fhp, int type, struct posix_acl *acl)
 				error = 0;
 		}
 	}
+	mnt_drop_write(fhp->fh_export->ex_path.mnt);
 
 getout:
 	kfree(value);

fs/xattr.c

@@ -11,6 +11,7 @@
 #include <linux/slab.h>
 #include <linux/file.h>
 #include <linux/xattr.h>
+#include <linux/mount.h>
 #include <linux/namei.h>
 #include <linux/security.h>
 #include <linux/syscalls.h>
@@ -32,8 +33,6 @@ xattr_permission(struct inode *inode, const char *name, int mask)
 	 * filesystem  or on an immutable / append-only inode.
 	 */
 	if (mask & MAY_WRITE) {
-		if (IS_RDONLY(inode))
-			return -EROFS;
 		if (IS_IMMUTABLE(inode) || IS_APPEND(inode))
 			return -EPERM;
 	}
@@ -262,7 +261,11 @@ sys_setxattr(char __user *path, char __user *name, void __user *value,
 	error = user_path_walk(path, &nd);
 	if (error)
 		return error;
+	error = mnt_want_write(nd.path.mnt);
+	if (!error) {
 		error = setxattr(nd.path.dentry, name, value, size, flags);
+		mnt_drop_write(nd.path.mnt);
+	}
 	path_put(&nd.path);
 	return error;
 }
@@ -277,7 +280,11 @@ sys_lsetxattr(char __user *path, char __user *name, void __user *value,
 	error = user_path_walk_link(path, &nd);
 	if (error)
 		return error;
+	error = mnt_want_write(nd.path.mnt);
+	if (!error) {
 		error = setxattr(nd.path.dentry, name, value, size, flags);
+		mnt_drop_write(nd.path.mnt);
+	}
 	path_put(&nd.path);
 	return error;
 }
@@ -295,7 +302,12 @@ sys_fsetxattr(int fd, char __user *name, void __user *value,
 		return error;
 	dentry = f->f_path.dentry;
 	audit_inode(NULL, dentry);
+	error = mnt_want_write(f->f_path.mnt);
+	if (!error) {
 		error = setxattr(dentry, name, value, size, flags);
+		mnt_drop_write(f->f_path.mnt);
+	}
+out_fput:
 	fput(f);
 	return error;
 }
@@ -482,7 +494,11 @@ sys_removexattr(char __user *path, char __user *name)
 	error = user_path_walk(path, &nd);
 	if (error)
 		return error;
+	error = mnt_want_write(nd.path.mnt);
+	if (!error) {
 		error = removexattr(nd.path.dentry, name);
+		mnt_drop_write(nd.path.mnt);
+	}
 	path_put(&nd.path);
 	return error;
 }
@@ -496,7 +512,11 @@ sys_lremovexattr(char __user *path, char __user *name)
 	error = user_path_walk_link(path, &nd);
 	if (error)
 		return error;
+	error = mnt_want_write(nd.path.mnt);
+	if (!error) {
 		error = removexattr(nd.path.dentry, name);
+		mnt_drop_write(nd.path.mnt);
+	}
 	path_put(&nd.path);
 	return error;
 }
@@ -513,7 +533,11 @@ sys_fremovexattr(int fd, char __user *name)
 		return error;
 	dentry = f->f_path.dentry;
 	audit_inode(NULL, dentry);
+	error = mnt_want_write(f->f_path.mnt);
+	if (!error) {
 		error = removexattr(dentry, name);
+		mnt_drop_write(f->f_path.mnt);
+	}
 	fput(f);
 	return error;
 }