On Mon, May 12, 2014 at 01:56:27PM -0400, Tejun Heo wrote: > 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. > It works for me. Thank you for the quick response. Tested-by: Andrey Wagin <avagin@xxxxxxxxx> > Signed-off-by: Tejun Heo <tj@xxxxxxxxxx> > Reported-by: Andrey Wagin <avagin@xxxxxxxxx> > Cc: Li Zefan <lizefan@xxxxxxxxxx> > References: http://lkml.kernel.org/g/CANaxB-xUm3rJ-Cbp72q-rQJO5mZe1qK6qXsQM=vh0U8upJ44+A@xxxxxxxxxxxxxx > Fixes: 2bd59d48ebfb ("cgroup: convert to kernfs") > --- > fs/kernfs/file.c | 19 +++++++++++-------- > fs/sysfs/mount.c | 3 ++- > include/linux/kernfs.h | 19 ++++++++++++++++++- > 3 files changed, 31 insertions(+), 10 deletions(-) > > --- a/fs/kernfs/file.c > +++ b/fs/kernfs/file.c > @@ -610,6 +610,7 @@ static void kernfs_put_open_node(struct > 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 > 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; > - > - if ((file->f_mode & FMODE_READ) && > - (!(inode->i_mode & S_IRUGO) || !has_read)) > - 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; > + } > > /* allocate a kernfs_open_file for the file */ > error = -ENOMEM; > --- a/fs/sysfs/mount.c > +++ b/fs/sysfs/mount.c > @@ -63,7 +63,8 @@ int __init sysfs_init(void) > { > int err; > > - sysfs_root = kernfs_create_root(NULL, 0, NULL); > + sysfs_root = kernfs_create_root(NULL, KERNFS_ROOT_EXTRA_OPEN_PERM_CHECK, > + NULL); > if (IS_ERR(sysfs_root)) > return PTR_ERR(sysfs_root); > > --- a/include/linux/kernfs.h > +++ b/include/linux/kernfs.h > @@ -50,7 +50,24 @@ enum kernfs_node_flag { > > /* @flags for kernfs_create_root() */ > enum kernfs_root_flag { > - KERNFS_ROOT_CREATE_DEACTIVATED = 0x0001, > + /* > + * kernfs_nodes are created in the deactivated state and invisible. > + * They require explicit kernfs_activate() to become visible. This > + * can be used to make related nodes become visible atomically > + * after all nodes are created successfully. > + */ > + KERNFS_ROOT_CREATE_DEACTIVATED = 0x0001, > + > + /* > + * For regular flies, if the opener has CAP_DAC_OVERRIDE, open(2) > + * succeeds regardless of the RW permissions. sysfs had an extra > + * layer of enforcement where open(2) fails with -EACCES regardless > + * of CAP_DAC_OVERRIDE if the permission doesn't have the > + * respective read or write access at all (none of S_IRUGO or > + * S_IWUGO) or the respective operation isn't implemented. The > + * following flag enables that behavior. > + */ > + KERNFS_ROOT_EXTRA_OPEN_PERM_CHECK = 0x0002, > }; > > /* type-specific structures for kernfs_node union members */ _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/containers
