On Tue, 2023-04-11 at 10:38 +0200, Christian Brauner wrote: > On Sun, Apr 09, 2023 at 06:12:09PM -0400, Jeff Layton wrote: > > On Sun, 2023-04-09 at 17:22 +0200, Christian Brauner wrote: > > > On Fri, Apr 07, 2023 at 09:29:29AM -0400, Jeff Layton wrote: > > > > > > > > > > > > > > > > I would ditch the original proposal in favor of this 2-line patch shown here: > > > > > > > > > > > > > > > > https://lore.kernel.org/linux-integrity/a95f62ed-8b8a-38e5-e468-ecbde3b221af@xxxxxxxxxxxxx/T/#m3bd047c6e5c8200df1d273c0ad551c645dd43232 > > > > > > > > > > We should cool it with the quick hacks to fix things. :) > > > > > > > > > > > > > Yeah. It might fix this specific testcase, but I think the way it uses > > > > the i_version is "gameable" in other situations. Then again, I don't > > > > know a lot about IMA in this regard. > > > > > > > > When is it expected to remeasure? If it's only expected to remeasure on > > > > a close(), then that's one thing. That would be a weird design though. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Ok, I think I get it. IMA is trying to use the i_version from the > > > > > > > overlayfs inode. > > > > > > > > > > > > > > I suspect that the real problem here is that IMA is just doing a bare > > > > > > > inode_query_iversion. Really, we ought to make IMA call > > > > > > > vfs_getattr_nosec (or something like it) to query the getattr routine in > > > > > > > the upper layer. Then overlayfs could just propagate the results from > > > > > > > the upper layer in its response. > > > > > > > > > > > > > > That sort of design may also eventually help IMA work properly with more > > > > > > > exotic filesystems, like NFS or Ceph. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Maybe something like this? It builds for me but I haven't tested it. It > > > > > > looks like overlayfs already should report the upper layer's i_version > > > > > > in getattr, though I haven't tested that either: > > > > > > > > > > > > -----------------------8<--------------------------- > > > > > > > > > > > > [PATCH] IMA: use vfs_getattr_nosec to get the i_version > > > > > > > > > > > > IMA currently accesses the i_version out of the inode directly when it > > > > > > does a measurement. This is fine for most simple filesystems, but can be > > > > > > problematic with more complex setups (e.g. overlayfs). > > > > > > > > > > > > Make IMA instead call vfs_getattr_nosec to get this info. This allows > > > > > > the filesystem to determine whether and how to report the i_version, and > > > > > > should allow IMA to work properly with a broader class of filesystems in > > > > > > the future. > > > > > > > > > > > > Reported-by: Stefan Berger <stefanb@xxxxxxxxxxxxx> > > > > > > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx> > > > > > > --- > > > > > > > > > > So, I think we want both; we want the ovl_copyattr() and the > > > > > vfs_getattr_nosec() change: > > > > > > > > > > (1) overlayfs should copy up the inode version in ovl_copyattr(). That > > > > > is in line what we do with all other inode attributes. IOW, the > > > > > overlayfs inode's i_version counter should aim to mirror the > > > > > relevant layer's i_version counter. I wouldn't know why that > > > > > shouldn't be the case. Asking the other way around there doesn't > > > > > seem to be any use for overlayfs inodes to have an i_version that > > > > > isn't just mirroring the relevant layer's i_version. > > > > > > > > It's less than ideal to do this IMO, particularly with an IS_I_VERSION > > > > inode. > > > > > > > > You can't just copy up the value from the upper. You'll need to call > > > > inode_query_iversion(upper_inode), which will flag the upper inode for a > > > > logged i_version update on the next write. IOW, this could create some > > > > (probably minor) metadata write amplification in the upper layer inode > > > > with IS_I_VERSION inodes. > > > > > > I'm likely just missing context and am curious about this so bear with me. Why > > > do we need to flag the upper inode for a logged i_version update? Any required > > > i_version interactions should've already happened when overlayfs called into > > > the upper layer. So all that's left to do is for overlayfs' to mirror the > > > i_version value after the upper operation has returned. > > > > > ovl_copyattr() - which copies the inode attributes - is always called after the > > > operation on the upper inode has finished. So the additional query seems odd at > > > first glance. But there might well be a good reason for it. In my naive > > > approach I would've thought that sm along the lines of: > > > > > > diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c > > > index 923d66d131c1..8b089035b9b3 100644 > > > --- a/fs/overlayfs/util.c > > > +++ b/fs/overlayfs/util.c > > > @@ -1119,4 +1119,5 @@ void ovl_copyattr(struct inode *inode) > > > inode->i_mtime = realinode->i_mtime; > > > inode->i_ctime = realinode->i_ctime; > > > i_size_write(inode, i_size_read(realinode)); > > > + inode_set_iversion_raw(inode, inode_peek_iversion_raw(realinode)); > > > } > > > > > > would've been sufficient. > > > > > > > Nope, because then you wouldn't get any updates to i_version after that > > point. > > > > Note that with an IS_I_VERSION inode we only update the i_version when > > there has been a query since the last update. What you're doing above is > > circumventing that mechanism. You'll get the i_version at the time of of > > the ovl_copyattr, but there won't be any updates of it after that point > > because the QUERIED bit won't end up being set on realinode. > > I get all that. > But my understanding had been that the i_version value at the time of > ovl_copyattr() would be correct. Because when ovl_copyattr() is called > the expected i_version change will have been done in the relevant layer > includig raising the QUERIED bit. Since the layers are not allowed to be > changed outside of the overlayfs mount any change to them can only > originate from overlayfs which would necessarily call ovl_copyattr() > again. IOW, overlayfs would by virtue of its implementation keep the > i_version value in sync. > > Overlayfs wouldn't even raise SB_I_VERSION. It would indeed just be a > cache of i_version of the relevant layer. > > > > > > > > Since overlayfs' does explicitly disallow changes to the upper and lower trees > > > while overlayfs is mounted it seems intuitive that it should just mirror the > > > relevant layer's i_version. > > > > > > > > > If we don't do this, then we should probably document that i_version doesn't > > > have a meaning yet for the inodes of stacking filesystems. > > > > > > > Trying to cache the i_version is counterproductive, IMO, at least with > > an IS_I_VERSION inode. > > > > The problem is that a query against the i_version has a side-effect. It > > has to (atomically) mark the inode for an update on the next change. > > > > If you try to cache that value, you'll likely end up doing more queries > > than you really need to (because you'll need to keep the cache up to > > date) and you'll have an i_version that will necessarily lag the one in > > the upper layer inode. > > > > The whole point of the change attribute is to get the value as it is at > > this very moment so we can check whether there have been changes. A > > laggy value is not terribly useful. > > > > Overlayfs should just always call the upper layer's ->getattr to get the > > version. I wouldn't even bother copying it up in the first place. Doing > > so is just encouraging someone to try use the value in the overlayfs > > inode, when they really need to go through ->getattr and get the one > > from the upper layer. > > That seems reasonable to me. I read this as an agreeing with my earlier > suggestion to document that i_version doesn't have a meaning for the > inodes of stacking filesystems and that we should spell out that > vfs_getattr()/->getattr() needs to be used to interact with i_version. > It really has no meaning in the stacked filesystem's _inode_. The only i_version that has any meaning in a (simple) stacking setup is the upper layer inode. > We need to explain to subsystems such as IMA somwhere what the correct > way to query i_version agnostically is; independent of filesystem > implementation details. > > Looking at IMA, it queries the i_version directly without checking > whether it's an IS_I_VERSION() inode first. This might make a > difference. > IMA should just use getattr. That allows the filesystem to present the i_version to the caller as it sees fit. Fetching i_version directly without testing for IS_I_VERSION is wrong, because you don't know what that field contains, or whether the fs supports it at all. > Afaict, filesystems that persist i_version to disk automatically raise > SB_I_VERSION. I would guess that it be considered a bug if a filesystem > would persist i_version to disk and not raise SB_I_VERSION. If so IMA > should probably be made to check for IS_I_VERSION() and it will probably > get that by switching to vfs_getattr_nosec(). Not quite. SB_I_VERSION tells the vfs that the filesystem wants the kernel to manage the increment of the i_version for it. The filesystem is still responsible for persisting that value to disk (if appropriate). Switching to vfs_getattr_nosec should make it so IMA doesn't need to worry about the gory details of all of this. If STATX_CHANGE_COOKIE is set in the response, then it can trust that value. Otherwise, it's no good. -- Jeff Layton <jlayton@xxxxxxxxxx>
