Messages in this thread | | | Date | Sun, 16 Apr 2023 21:57:10 -0400 | Subject | Re: [PATCH] overlayfs: Trigger file re-evaluation by IMA / EVM after writes | From | Stefan Berger <> |
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On 4/7/23 09:29, 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@linux.ibm.com/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.
IMA should remeasure the file when it has visibly changed for another thread or process.
>>> -----------------------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@linux.ibm.com> >>> Signed-off-by: Jeff Layton <jlayton@kernel.org> >>> --- >> >> 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. > > >> (2) Jeff's changes for ima to make it rely on vfs_getattr_nosec(). >> Currently, ima assumes that it will get the correct i_version from >> an inode but that just doesn't hold for stacking filesystem. >> >> While (1) would likely just fix the immediate bug (2) is correct and >> _robust_. If we change how attributes are handled vfs_*() helpers will >> get updated and ima with it. Poking at raw inodes without using >> appropriate helpers is much more likely to get ima into trouble. > > This will fix it the right way, I think (assuming it actually works), > and should open the door for IMA to work properly with networked > filesystems that support i_version as well. > > Note that there Stephen is correct that calling getattr is probably > going to be less efficient here since we're going to end up calling > generic_fillattr unnecessarily, but I still think it's the right thing > to do.
I was wondering whether to use the existing inode_eq_iversion() for all other filesystems than overlayfs, nfs, and possibly other ones (which ones?) where we would use the vfs_getattr_nosec() via a case on inode->i_sb->s_magic? If so, would this function be generic enough to be a public function for libfs.c?
I'll hopefully be able to test the proposed patch tomorrow.
> > If it turns out to cause measurable performance regressions though, > maybe we can look at adding a something that still calls ->getattr if it > exists but only returns the change_cookie value.
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