From 555724a831b4a146e7bdf16ecc989cda032b076d Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Mon, 12 May 2014 13:56:27 -0400 Subject: kernfs, sysfs, cgroup: restrict extra perm check on open to sysfs The kernfs open method - kernfs_fop_open() - inherited extra permission checks from sysfs. While the vfs layer allows ignoring the read/write permissions checks if the issuer has CAP_DAC_OVERRIDE, sysfs explicitly denied open regardless of the cap if the file doesn't have any of the UGO perms of the requested access or doesn't implement the requested operation. It can be debated whether this was a good idea or not but the behavior is too subtle and dangerous to change at this point. After cgroup got converted to kernfs, this extra perm check also got applied to cgroup breaking libcgroup which opens write-only files with O_RDWR as root. This patch gates the extra open permission check with a new flag KERNFS_ROOT_EXTRA_OPEN_PERM_CHECK and enables it for sysfs. For sysfs, nothing changes. For cgroup, root now can perform any operation regardless of the permissions as it was before kernfs conversion. Note that kernfs still fails unimplemented operations with -EINVAL. While at it, add comments explaining KERNFS_ROOT flags. Signed-off-by: Tejun Heo Reported-by: Andrey Wagin Tested-by: Andrey Wagin Cc: Li Zefan References: http://lkml.kernel.org/g/CANaxB-xUm3rJ-Cbp72q-rQJO5mZe1qK6qXsQM=vh0U8upJ44+A@mail.gmail.com Fixes: 2bd59d48ebfb ("cgroup: convert to kernfs") Signed-off-by: Greg Kroah-Hartman --- fs/kernfs/file.c | 17 ++++++++++------- 1 file changed, 10 insertions(+), 7 deletions(-) (limited to 'fs/kernfs') diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c index e01ea4a14a01..5e9a80cfc3d8 100644 --- a/fs/kernfs/file.c +++ b/fs/kernfs/file.c @@ -610,6 +610,7 @@ static void kernfs_put_open_node(struct kernfs_node *kn, static int kernfs_fop_open(struct inode *inode, struct file *file) { struct kernfs_node *kn = file->f_path.dentry->d_fsdata; + struct kernfs_root *root = kernfs_root(kn); const struct kernfs_ops *ops; struct kernfs_open_file *of; bool has_read, has_write, has_mmap; @@ -624,14 +625,16 @@ static int kernfs_fop_open(struct inode *inode, struct file *file) has_write = ops->write || ops->mmap; has_mmap = ops->mmap; - /* check perms and supported operations */ - if ((file->f_mode & FMODE_WRITE) && - (!(inode->i_mode & S_IWUGO) || !has_write)) - goto err_out; + /* see the flag definition for details */ + if (root->flags & KERNFS_ROOT_EXTRA_OPEN_PERM_CHECK) { + if ((file->f_mode & FMODE_WRITE) && + (!(inode->i_mode & S_IWUGO) || !has_write)) + goto err_out; - if ((file->f_mode & FMODE_READ) && - (!(inode->i_mode & S_IRUGO) || !has_read)) - goto err_out; + if ((file->f_mode & FMODE_READ) && + (!(inode->i_mode & S_IRUGO) || !has_read)) + goto err_out; + } /* allocate a kernfs_open_file for the file */ error = -ENOMEM; -- cgit v1.2.3