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vfs: allow unprivileged whiteout creation

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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>
This commit is contained in:
Miklos Szeredi
2020-05-14 16:44:23 +02:00
parent 0e698dfa28
commit a3c751a50f
4 changed files with 14 additions and 19 deletions
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21
fs/namei.c
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@@ -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)) && !is_whiteout &&
!capable(CAP_MKNOD))
return -EPERM;
if (!dir->i_op->mknod)
@@ -4345,9 +4347,6 @@ static int do_renameat2(int olddfd, const char __user *oldname, int newdfd,
(flags & RENAME_EXCHANGE))
return -EINVAL;
if ((flags & RENAME_WHITEOUT) && !capable(CAP_MKNOD))
return -EPERM;
if (flags & RENAME_EXCHANGE)
target_flags = 0;
@@ -4483,20 +4482,6 @@ SYSCALL_DEFINE2(rename, const char __user *, oldname, const char __user *, newna
return do_renameat2(AT_FDCWD, oldname, AT_FDCWD, newname, 0);
}
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);
}
EXPORT_SYMBOL(vfs_whiteout);
int readlink_copy(char __user *buffer, int buflen, const char *link)
{
int len = PTR_ERR(link);