Hi Amir, On Sun, 2021-04-04 at 13:27 +0300, Amir Goldstein wrote: > [forking question about security modules] > > > > > Nice thing about vfs_{set,remove}xattr() is that they already have > > several levels of __vfs_ helpers and nfsd already calls those, so > > we can hoist fsnotify_xattr() hooks hooks up from the __vfs_xxx > > helpers to the common vfs_xxx helpers and add fsnotify hooks to > > the very few callers of __vfs_ helpers. > > > > nfsd is consistently calling __vfs_{set,remove}xattr_locked() which > > do generate events, but ecryptfs mixes __vfs_setxattr_locked() with > > __vfs_removexattr(), which does not generate event and does not > > check permissions - it looks like an oversight. > > > > The thing is, right now __vfs_setxattr_noperm() generates events, > > but looking at all the security/* callers, it feels to me like those are > > very internal operations and that "noperm" should also imply "nonotify". > > > > To prove my point, all those callers call __vfs_removexattr() which > > does NOT generate an event. > > > > Also, I *think* the EVM setxattr is something that usually follows > > another file data/metadata change, so some event would have been > > generated by the original change anyway. > > > > Mimi, > > > > Do you have an opinion on that? Right, EVM is re-calculating the EVM HMAC, which is based on other LSM xattrs and includes some misc file metadata (e.g. ino, generation, uid, gid, mode). > > > > The question is if you think it is important for an inotify/fanotify watcher > > that subscribed to IN_ATTRIB/FAN_ATTRIB events on a file to get an > > event when the IMA security blob changes. Probably not. Programs could open files R/W, but never modify the file. Perhaps to detect mutable file changes, but I'm not aware of anyone doing so. > > Guys, > > I was doing some re-factoring of the __vfs_setxattr helpers > and noticed some strange things. > > The wider context is fsnotify_xattr() hooks inside internal > setxattr,removexattr calls. I would like to move those hooks > to the common vfs_{set,remove}xattr() helpers. > > SMACK & SELINUX: > For the callers of __vfs_setxattr_noperm(), > smack_inode_setsecctx() and selinux_inode_setsecctx() > It seems that the only user is nfsd4_set_nfs4_label(), so it > makes sense for me to add the fsnotify_xattr() in nfsd context, > same as I did with other fsnotify_ hooks. > > Are there any other expected callers of security_inode_setsecctx() > except nfsd in the future? If so they would need to also add the > fsnotify_xattr() hook, if at all the user visible FS_ATTRIB event is > considered desirable. > > SMACK: > Just to be sure, is the call to __vfs_setxattr() from smack_d_instantiate() > guaranteed to be called for an inode whose S_NOSEC flag is already > cleared? Because the flag is being cleared by __vfs_setxattr_noperm(). > > EVM: > I couldn't find what's stopping this recursion: > evm_update_evmxattr() => __vfs_setxattr_noperm() => > security_inode_post_setxattr() => evm_inode_post_removexattr() => > evm_update_evmxattr() EVM is triggered when file metadata changes, causing the EVM HMAC to be re-calculated. Before updating security.evm, EVM first verifies, on the evm_inode_setattr/setxattr/removexattr() hooks, that the existing security.evm value is correct. On the _post hooks, security.evm is updated or removed, if no LSM xattr exists. > It looks like the S_NOSEC should already be clear when > evm_update_evmxattr() is called(?), so it seems more logical to me to > call __vfs_setxattr() as there is no ->inode_setsecurity() hook for EVM. > Am I missing something? EVM is triggered when an LSM updates/removes its xattr. The LSM is responsible for taking the inode lock. Thus it is calling __vfs_setxattr_noperm. > > It seems to me that updating the EVM hmac should not generate > a visible FS_ATTRIB event to listeners, because it is an internal > implementation detail and because update EVM hmac happens > following another change to the inode which anyway reports a > visible event to listeners. Ok > Also, please note that evm_update_evmxattr() may also call > __vfs_removexattr() which does not call the fsnotify_xattr() hook. > > IMA: > Similarly, ima_fix_xattr() should be called on an inode without > S_NOSEC flag and no other LSM should be interested in the > IMA hash update, right? So wouldn't it be better to use > __vfs_setxattr() in this case as well? > > ima_fix_xattr() can be called after file data update, which again > will have other visible events, but it can also be called in "fix mode" > I suppose also when reading files? Still, it seems to me like an > internal implementation detail that should not generate a user > visible event. Originally, IMA took the inode lock really early, way before calling setxattr. Taking the inode lock can probably be deferred to setxattr. I have a vague recollection that SELinux also prevented IMA from writing its own xattr label. I don't know if that is still true. thanks, Mimi > > If you agree with my proposed changes, please ACK the > respective bits of your subsystem from the attached patch. > Note that my patch does not contain the proposed change to > use __vfs_setxattr() in IMA/EVM. > > Thanks, > Amir.