[PATCH] vfs: allow unprivileged whiteout creation

From: Miklos Szeredi <miklos@szeredi.hu>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Subject: [PATCH] vfs: allow unprivileged whiteout creation
Date: Thu, 9 Apr 2020 23:28:59 +0200	[thread overview]
Message-ID: <20200409212859.GH28467@miu.piliscsaba.redhat.com> (raw)

From: Miklos Szeredi <mszeredi@redhat.com>

Whiteouts, unlike real device node should not require privileges to create.

The general concern with device nodes is that opening them can have side
effects.  The kernel already avoids zero major (see
Documentation/admin-guide/devices.txt).  To be on the safe side the patch
explicitly forbids registering a char device with 0/0 number (see
cdev_add()).

This guarantees that a non-O_PATH open on a whiteout will fail with ENODEV;
i.e. it won't have any side effect.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
---
 fs/char_dev.c                 |    3 +++
 fs/namei.c                    |   17 ++++-------------
 include/linux/device_cgroup.h |    3 +++
 3 files changed, 10 insertions(+), 13 deletions(-)

--- a/fs/char_dev.c
+++ b/fs/char_dev.c
@@ -483,6 +483,9 @@ int cdev_add(struct cdev *p, dev_t dev,
 	p->dev = dev;
 	p->count = count;
 
+	if (WARN_ON(dev == WHITEOUT_DEV))
+		return -EBUSY;
+
 	error = kobj_map(cdev_map, dev, count, NULL,
 			 exact_match, exact_lock, p);
 	if (error)
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3505,12 +3505,14 @@ EXPORT_SYMBOL(user_path_create);
 
 int vfs_mknod(struct inode *dir, struct dentry *dentry, umode_t mode, dev_t dev)
 {
+	bool is_whiteout = S_ISCHR(mode) && dev == WHITEOUT_DEV;
 	int error = may_create(dir, dentry);
 
 	if (error)
 		return error;
 
-	if ((S_ISCHR(mode) || S_ISBLK(mode)) && !capable(CAP_MKNOD))
+	if ((S_ISCHR(mode) || S_ISBLK(mode)) && !capable(CAP_MKNOD) &&
+	    !is_whiteout)
 		return -EPERM;
 
 	if (!dir->i_op->mknod)
@@ -4345,9 +4347,6 @@ static int do_renameat2(int olddfd, cons
 	    (flags & RENAME_EXCHANGE))
 		return -EINVAL;
 
-	if ((flags & RENAME_WHITEOUT) && !capable(CAP_MKNOD))
-		return -EPERM;
-
 	if (flags & RENAME_EXCHANGE)
 		target_flags = 0;
 
@@ -4485,15 +4484,7 @@ SYSCALL_DEFINE2(rename, const char __use
 
 int vfs_whiteout(struct inode *dir, struct dentry *dentry)
 {
-	int error = may_create(dir, dentry);
-	if (error)
-		return error;
-
-	if (!dir->i_op->mknod)
-		return -EPERM;
-
-	return dir->i_op->mknod(dir, dentry,
-				S_IFCHR | WHITEOUT_MODE, WHITEOUT_DEV);
+	return vfs_mknod(dir, dentry, S_IFCHR | WHITEOUT_MODE, WHITEOUT_DEV);
 }
 EXPORT_SYMBOL(vfs_whiteout);
 
--- a/include/linux/device_cgroup.h
+++ b/include/linux/device_cgroup.h
@@ -51,6 +51,9 @@ static inline int devcgroup_inode_mknod(
 	if (!S_ISBLK(mode) && !S_ISCHR(mode))
 		return 0;
 
+	if (S_ISCHR(mode) && dev == WHITEOUT_DEV)
+		return 0;
+
 	if (S_ISBLK(mode))
 		type = DEVCG_DEV_BLOCK;
 	else
