On Thu, Apr 8, 2021 at 5:15 PM J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote: > > On Thu, Apr 08, 2021 at 02:55:30PM +0200, Christian Brauner 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 > > Only in the NFSEXP_NOSUBTREECHECK case. Taking a quick look, I think > Amir's not proposing a check like that by default, so, fine. (I assume > problems with e.g. subtreechecking and cross-directory renames are > understood....) > They are understood to me :) but I didn't want to get into it, because it is complicated to explain and I wasn't sure if anyone cared... I started working on open_by_handle_at() in userns for fanotify and fanotify mostly reports directory fhandle, so no issues with cross-directory renames. In any case, fanotify never reports "connectable" non-dir file handles. Because my proposed change ALSO makes it possible to start talking about userspace nfs server inside userns (in case anyone cares), I wanted to lay out the path towards a userspace "subtree_check" like solution. Another thing I am contemplating is, if and when idmapped mount support is added to overlayfs, we can store an additional "connectable" file handle in the overlayfs index (whose key is the non-connectable fhandle) and fix ovl_acceptable() similar to nfsd_acceptable() and then we will be able to mount an overlayfs inside userns with nfs_export support. I've included a two liner patch on the fhandle_userns branch to allow overlayfs inside userns with nfs_export support in the case that underlying filesystem was mounted inside userns, but that is not such an interesting use case IMO. Thanks, Amir.
