On Mon, Jan 11, 2021 at 08:55:43PM +0800, 吴异 wrote: > Yes, the safest way for users is that the first exported file system is on > a different hard disk with "\", and then does not contain subdirectories, > so I seem to have nothing to do. However, I did not find these tips in the > configuration files related and man exports . It is best to recommend the > best configuration method to users if it is updated in the future. I agree that the documentation needs improvement. I also think we should warn by default when a user tries to export something with no_subtree_check that's not at a mountpoint. (Really, "at a mountpoint" should be "at the root of a filesystem", but I don't know how to check for that, at least not from userspace. I'm not sure whether the warning belongs in the kernel or elsewhere.) > Back to the present situation, have you considered removing the following > piece of code in the nfsd_acceptable function： > if (exp->ex_flags & NFSEXP_NOSUBTREECHECK) > return 1; That would turn on subtree_check unconditionally. Currently we default to no_subtree_check, and for good reason: subtree_check causes unexpected behavior, like ESTALE on attempts to access open files after cross-directory rename. > In this way, all problems seem to be solved. Even if an attacker can get or > guess some file handles, these handles will not actually work.Fix > compose_entry_fh and nfsd_lookup_dentry functions is actually not that > urgent. > > One more question, do I need to copy all the previous emails to > linux-nfs@xxxxxxxxxxxxxxx, this is a bit too much, right? I don't think that's necessary. To summarize issue you've raised; if a client exports only a subdirectory of a filesystem: - NFSv2/v3 clients can discover the filehandle of the parent of the export point using readdir_plus. I agree that we should fix this; I'll follow up with a patch. (It's already not too hard to guess filehandles, though.) - Once a single filehandle outside the export directory is found, it's easy to access the rest of the filesystem. This issue has been known for decades. But I don't think it's well-enough understood by users. - If only a subdirectory of a filesystem is exported, an attacker might be able to gain access to *other* filesystems as well, by replacing some component along the exported path by a symlink and waiting for the server to restart. (In the v2/v3 case, export path symlinks are followed on the server side.) The main defense against all of this is to *only* export the root of a filesystem. If you're exporting "/export/home/", then you should create a separate filesystem and mount that at "/export/home/". But this isn't as well understood as it should be, and we should do a better job of preventing users from choosing dangerous configurations. And then mitigate the danger where we can. --b. > > Best regards, > > J. Bruce Fields <bfields@xxxxxxxxxxxx> 于2021年1月11日周一 上午4:28写道： > > > On Sun, Jan 10, 2021 at 03:17:40PM -0500, J. Bruce Fields wrote: > > > On Sat, Jan 09, 2021 at 10:52:33PM +0800, 吴异 wrote: > > > > I do try it,because of the default no_subtreecheck in nfsd, I can > > delete > > > > /nfs as long as I can get the handle of /fs. This takes advantage of > > the > > > > vulnerability I submitted before. I have added my poc and traffic > > > > information in the attachment. > > > > > > > > My server filesystem configuration is as follows： > > > > > > > > /dev/sda3 on / type ext4 (rw,relatime,errors=remount-ro) > > > > > > > > /dev/sda1 on /fs type ext4 (rw,relatime) > > > > > > > > The configuration of /etc/export is > > > > > > > > /fs/nfs *(rw,sync,no_root_squash,insecure) > > > > > > Again, you're exporting a subdirectory, which we discourage. The safe > > > and recommended configuration would be to mount /dev/sda1 on /fs/nfs and > > > export /fs/nfs. > > > > > > > As you can see, the file system where the shared directory is located > > has > > > > been separated from the “\” directory.What I did was to delete the > > mount > > > > point directory > > > > > > It should not be possible to delete /fs/nfs/ while you had something > > > mounted there. > > > > Whoops, I'm sorry, there's only a mountpoint at /fs/, so, yes, you can > > probably delete /fs/nfs and replace it by a symlink, got it. > > > > Sure, that's interesting. But, again, we don't recommend exporting > > filesystem subdirectories. > > > > Good to keep the public nfs list (linux-nfs@xxxxxxxxxxxxxxx) cc'd on > > these things, by the way. > > > > --b. > > > > > > > > --b. > > > > > > > and restart the server-side nfsd service. At this time, the > > > > server-side shared directory became "/", and the configuration > > information > > > > did not change. > > > > > > > > > > > > For the convenience I set no_root_squash on the server, but in fact, as > > > > long as the server gives the user the permission to write in the /fs > > > > directory, the above operation can be completed. > > > > > > > > > > > > Best regards, > > > > > > > > > > > > > > > > J. Bruce Fields <bfields@xxxxxxxxxxxx> 于2021年1月9日周六 上午12:44写道： > > > > > > > > > On Sat, Jan 09, 2021 at 12:30:58AM +0800, 吴异 wrote: > > > > > > when the shared directory /fs/nfs of the server is mounted on the > > device > > > > > > /dev/sda3, and the "/" directory is mounted on the device > > /dev/sda1, then > > > > > > the file system of the shared directory and the "/" file system > > are In > > > > > > isolation, an attacker can obtain the ability of any file in the > > > > > /dev/sda3 > > > > > > file system by using a file handle, but cannot access the "/" file > > system > > > > > > by using an NFS file handle, even if he already knows the "/" > > handle is > > > > > > "0100010000000000", which will return the error code nfserr_stale. > > > > > > > > > > > > In this case, the attacker deletes the mount point directory > > /fs/nfs in > > > > > > the file system under its control., and create a symbolic link > > with the > > > > > > same name pointing to the "/" directory. > > > > > > > > > > Access to the filesystem mounted at /fs/nfs/ should not give you the > > > > > ability to delete the directory /fs/nfs. > > > > > > > > > > Have you actually tried this? > > > > > > > > > > Please explain what access you assume the attacker starts with, what > > > > > they're able to do it, why you think that isn't expected, and please > > > > > test to make sure your idea works. > > > > > > > > > > --b. > > > > > > > > > > > > > > > > After nfsd re-parses the > > > > > > configuration file /etc/exports, the attacker can remount it to > > achieve > > > > > > real file system escape. > > > > > > > > > > > > It seems that nfsd should strengthen the restrictions on symbolic > > link > > > > > > resolution in configuration files. > > > > > > > > > > > > Best regards, > > > > > > > > > > > > J. Bruce Fields <bfields@xxxxxxxxxxxx> 于2021年1月8日周五 下午11:20写道： > > > > > > > > > > > > > On Fri, Jan 08, 2021 at 12:37:43PM +0100, Greg KH wrote: > > > > > > > > On Tue, Jan 05, 2021 at 11:56:33AM -0500, J. Bruce Fields > > wrote: > > > > > > > > > On Tue, Jan 05, 2021 at 11:37:26PM +0800, 吴异 wrote: > > > > > > > > > > It is not safe to export only the root of the file system, > > > > > > > > > > Attacker can use lookup ".." to get the file handle to the > > ".." > > > > > > > directory， > > > > > > > > > > > > > > > > > > You have claimed the ability to read and write any file on > > the file > > > > > > > > > server. But if you put /fs/nfs/ on a separate filesystem > > then > > > > > that is > > > > > > > > > no longer possible. Perhaps you can learn the value of the > > > > > filehandle > > > > > > > > > /fs/, but you can't use that filehandle to gain additional > > access > > > > > > > unless > > > > > > > > > something on that filesystem is already exported. > > > > > > > > > > > > > > > > > > So, there may be some information disclosure here, I agree, > > but > > > > > it's > > > > > > > > > relatively mild, given that filehandles are not well-kept > > secrets > > > > > > > > > anyway. > > > > > > > > > > > > > > > > Ok, to clarify, you are saying that this really isn't an issue > > at all > > > > > > > > with the kernel code, so there's nothing to do here? > > > > > > > > > > > > > > > > Just trying to figure out if there's anything to actually work > > on > > > > > here > > > > > > > > or not. > > > > > > > > > > > > > > I think there's one possibly interesting issue, but it doesn't > > require > > > > > > > special handling. Reporter has raised a few issues: > > > > > > > > > > > > > > - The one that might be interesting: clients may be able > > to > > > > > > > discover filehandles that they shouldn't be able to > > using > > > > > > > readdir_plus. On review of the code, I agree it's odd > > that > > > > > > > the logic in compose_entry_fh() is different from that > > in > > > > > > > nfsd_lookup_dentry(). I'd also prefer to take that > > > > > discussion > > > > > > > to linux-nfs@xxxxxxxxxxxxxxx, as this seems at most a > > mild > > > > > > > information disclosure. > > > > > > > > > > > > > > - If only part of a filesystem is exported, an attacker > > can > > > > > gain > > > > > > > access to files outside the exported directory by > > guessing > > > > > > > filehandles. This issue has been known for decades. > > If we > > > > > > > want to discuss mitigation strategies, I believe the > > correct > > > > > > > forum is again linux-nfs@xxxxxxxxxxxxxxx, not > > > > > > > security@xxxxxxxxxx. > > > > > > > > > > > > > > - An attacker might replace an exported mountpoint by a > > symlink > > > > > > > pointing elsewhere. That seems to assume the attacker > > > > > already > > > > > > > has a lot of access, so I'm not clear what advantage > > they > > > > > gain > > > > > > > from this. > > > > > > > > > > > > > > --b. > > > > > > > > > > > > > > > > > > > >
