On Thu, Apr 08, 2021 at 06:34:01PM +0300, Amir Goldstein wrote: > On Thu, Apr 8, 2021 at 3:55 PM Christian Brauner > <christian.brauner@xxxxxxxxxx> wrote: > > > > On Thu, Apr 08, 2021 at 02:44:47PM +0300, Amir Goldstein wrote: > > > > One thing your patch > > > > > > > > commit ea31e84fda83c17b88851de399f76f5d9fc1abf4 > > > > Author: Amir Goldstein <amir73il@xxxxxxxxx> > > > > Date: Sat Mar 20 12:58:12 2021 +0200 > > > > > > > > fs: allow open by file handle inside userns > > > > > > > > open_by_handle_at(2) requires CAP_DAC_READ_SEARCH in init userns, > > > > where most filesystems are mounted. > > > > > > > > Relax the requirement to allow a user with CAP_DAC_READ_SEARCH > > > > inside userns to open by file handle in filesystems that were > > > > mounted inside that userns. > > > > > > > > In addition, also allow open by handle in an idmapped mount, which is > > > > mapped to the userns while verifying that the returned open file path > > > > is under the root of the idmapped mount. > > > > > > > > This is going to be needed for setting an fanotify mark on a filesystem > > > > and watching events inside userns. > > > > > > > > Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx> > > > > > > > > Requires fs/exportfs/expfs.c to be made idmapped mounts aware. > > > > open_by_handle_at() uses exportfs_decode_fh() which e.g. has the > > > > following and other callchains: > > > > > > > > exportfs_decode_fh() > > > > -> exportfs_decode_fh_raw() > > > > -> lookup_one_len() > > > > -> inode_permission(mnt_userns, ...) > > > > > > > > That's not a huge problem though I did all these changes for the > > > > overlayfs support for idmapped mounts I have in a branch from an earlier > > > > version of the idmapped mounts patchset. Basically lookup_one_len(), > > > > lookup_one_len_unlocked(), and lookup_positive_unlocked() need to take > > > > the mnt_userns into account. I can rebase my change and send it for > > > > consideration next cycle. If you can live without the > > > > open_by_handle_at() support for now in this patchset (Which I think you > > > > said you could.) then it's not a blocker either. Sorry for the > > > > inconvenience. > > > > > > > > > > Christian, > > > > > > I think making exportfs_decode_fh() idmapped mount aware is not > > > enough, because when a dentry alias is found in dcache, none of > > > those lookup functions are called. > > > > > > I think we will also need something like this: > > > https://github.com/amir73il/linux/commits/fhandle_userns > > > > > > I factored-out a helper from nfsd_apcceptable() which implements > > > the "subtree_check" nfsd logic and uses it for open_by_handle_at(). > > > > > > I've also added a small patch to name_to_handle_at() with a UAPI > > > change that could make these changes usable by userspace nfs > > > server inside userns, but I have no demo nor tests for that and frankly, > > > I have little incentive to try and promote this UAPI change without > > > anybody asking for it... > > > > Ah, at first I was confused about why this would matter but it matters > > because nfsd already implements a check of that sort directly in nfsd > > independent of idmapped mounts: > > https://github.com/amir73il/linux/commit/4bef9ff1718935b7b42afbae71cfaab7770e8436 > > > > The check is needed for slightly different reasons. > nfsd "subtree_check" feature explicitly meant to forbid access in case > file was moved "out of reach", for example, out of the export path. > Note the nfsd "subtree_check" affects both file handle encoding > (i.e. "connectable") and file handle decoding (i.e. nfsd_acceptable()). > > open_by_handle_at() in idmapped mount needs to verify that the ancestry > inode owners can be mapped to the userns, because we already checked > that user has CAP_DAC_READ_SEARCH in userns, but it's nicer to do > the full inode_permission() check IMO. > > > Afaict, an nfs server can't be mounted inside of userns right now. That > > is something that folks from Netflix and from Kinvolk have been > > interested in enabling. They also want the ability to use idmapped > > mounts + nfs. Understandable that you don't want to drive this of > > course. I'll sync with them about this. > > > > Independent of that, I thought our last understanding was that you > > wouldn't need to handle open_by_handle_at() for now. > > > > I don't need it. But I realized that the fanotify_userns demo branch > I provided you is buggy in terms of security, so I wanted to give you > (or whoever wants to pursue this) a better reference. > It was one of those things that are easier to code than to explain ;-) And I highly appreciate it! :) Please keep those branches for reference somewhere. I've noted them down to make sure to look at them before I do anything! Christian
