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. > > 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. In effect, I think containing setxattr() can only be accomplished with LSM. Thanks, Amir.
