On 2022-06-08, Amir Goldstein <amir73il@xxxxxxxxx> wrote: > On Wed, Jun 8, 2022 at 3:48 PM Christian Brauner <brauner@xxxxxxxxxx> wrote: > > > > On Wed, Jun 08, 2022 at 03:28:52PM +0300, Amir Goldstein wrote: > > > On Wed, Jun 8, 2022 at 2:57 PM Christian Brauner <brauner@xxxxxxxxxx> wrote: > > > > > > > > On Tue, Jun 07, 2022 at 05:31:39PM +0200, Christian Göttsche wrote: > > > > > From: Miklos Szeredi <mszeredi@xxxxxxxxxx> > > > > > > > > > > Support file descriptors obtained via O_PATH for extended attribute > > > > > operations. > > > > > > > > > > Extended attributes are for example used by SELinux for the security > > > > > context of file objects. To avoid time-of-check-time-of-use issues while > > > > > setting those contexts it is advisable to pin the file in question and > > > > > operate on a file descriptor instead of the path name. This can be > > > > > emulated in userspace via /proc/self/fd/NN [1] but requires a procfs, > > > > > which might not be mounted e.g. inside of chroots, see[2]. > > > > > > > > > > [1]: https://github.com/SELinuxProject/selinux/commit/7e979b56fd2cee28f647376a7233d2ac2d12ca50 > > > > > [2]: https://github.com/SELinuxProject/selinux/commit/de285252a1801397306032e070793889c9466845 > > > > > > > > > > Original patch by Miklos Szeredi <mszeredi@xxxxxxxxxx> > > > > > https://patchwork.kernel.org/project/linux-fsdevel/patch/20200505095915.11275-6-mszeredi@xxxxxxxxxx/ > > > > > > > > > > > While this carries a minute risk of someone relying on the property of > > > > > > xattr syscalls rejecting O_PATH descriptors, it saves the trouble of > > > > > > introducing another set of syscalls. > > > > > > > > > > > > Only file->f_path and file->f_inode are accessed in these functions. > > > > > > > > > > > > Current versions return EBADF, hence easy to detect the presense of > > > > > > this feature and fall back in case it's missing. > > > > > > > > > > CC: linux-api@xxxxxxxxxxxxxxx > > > > > CC: linux-man@xxxxxxxxxxxxxxx > > > > > Signed-off-by: Christian Göttsche <cgzones@xxxxxxxxxxxxxx> > > > > > --- > > > > > > > > I'd be somewhat fine with getxattr and listxattr but I'm worried that > > > > setxattr/removexattr waters down O_PATH semantics even more. I don't > > > > want O_PATH fds to be useable for operations which are semantically > > > > equivalent to a write. > > > > > > It is not really semantically equivalent to a write if it works on a > > > O_RDONLY fd already. > > > > The fact that it works on a O_RDONLY fd has always been weird. And is > > probably a bug. If you look at xattr_permission() you can see that it > > Bug or no bug, this is the UAPI. It is not fixable anymore. > > > checks for MAY_WRITE for set operations... setxattr() writes to disk for > > real filesystems. I don't know how much closer to a write this can get. > > > > In general, one semantic aberration doesn't justify piling another one > > on top. > > > > (And one thing that speaks for O_RDONLY is at least that it actually > > opens the file wheres O_PATH doesn't.) > > Ok. I care mostly about consistent UAPI, so if you want to set the > rule that modify f*() operations are not allowed to use O_PATH fd, > I can live with that, although fcntl(2) may be breaking that rule, but > fine by me. > It's good to have consistent rules and it's good to add a new UAPI for > new behavior. > > However... > > > > > > > > > > > > > > In sensitive environments such as service management/container runtimes > > > > we often send O_PATH fds around precisely because it is restricted what > > > > they can be used for. I'd prefer to not to plug at this string. > > > > > > But unless I am mistaken, path_setxattr() and syscall_fsetxattr() > > > are almost identical w.r.t permission checks and everything else. > > > > > > So this change introduces nothing new that a user in said environment > > > cannot already accomplish with setxattr(). > > > > > > Besides, as the commit message said, doing setxattr() on an O_PATH > > > fd is already possible with setxattr("/proc/self/$fd"), so whatever security > > > hole you are trying to prevent is already wide open. > > > > That is very much a something that we're trying to restrict for this > > exact reason and is one of the main motivator for upgrade mask in > > openat2(). If I want to send a O_PATH around I want it to not be > > upgradable. Aleksa is working on upgrade masks with openat2() (see [1] > > and part of the original patchset in [2]. O_PATH semantics don't need to > > become weird. > > > > [1]: https://lore.kernel.org/all/20220526130355.fo6gzbst455fxywy@senku > > [2]: https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20190728010207.9781-8-cyphar@xxxxxxxxxx > > ... thinking forward, if this patch is going to be rejected, the patch that > will follow is *xattrat() syscalls. > > What will you be able to argue then? > > There are several *at() syscalls that modify metadata. > fchownat(.., AT_EMPTY_PATH) is intentionally designed for this. > > Do you intend to try and block setxattrat()? > Just try and block setxattrat(.., AT_EMPTY_PATH)? > those *at() syscalls have real use cases to avoid TOCTOU races. > Do you propose that applications will have to use fsetxattr() on an open > file to avert races? > > I completely understand the idea behind upgrade masks > for limiting f_mode, but I don't know if trying to retroactively > change semantics of setxattr() in the move to setxattrat() > is going to be a good idea. The goal would be that the semantics of fooat(<fd>, AT_EMPTY_PATH) and foo(/proc/self/fd/<fd>) should always be identical, and the current semantics of /proc/self/fd/<fd> are too leaky so we shouldn't always assume that keeping them makes sense (the most obvious example is being able to do tricks to open /proc/$pid/exe as O_RDWR). I suspect that the long-term solution would be to have more upgrade masks so that userspace can opt-in to not allowing any kind of (metadata) write access through a particular file descriptor. You're quite right that we have several metadata write AT_EMPTY_PATH APIs, and so we can't retroactively block /everything/ but we should try to come up with less leaky rules by default if it won't break userspace. -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/>
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