From: Aleksa Sarai <cyphar@cyphar.com> To: Jann Horn <jannh@google.com> Cc: linux-ia64@vger.kernel.org, linux-sh@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>, Rasmus Villemoes <linux@rasmusvillemoes.dk>, Alexei Starovoitov <ast@kernel.org>, kernel list <linux-kernel@vger.kernel.org>, David Howells <dhowells@redhat.com>, "open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@vger.kernel.org>, sparclinux@vger.kernel.org, Jiri Olsa <jolsa@redhat.com>, linux-arch <linux-arch@vger.kernel.org>, linux-s390 <linux-s390@vger.kernel.org>, Tycho Andersen <tycho@tycho.ws>, Aleksa Sarai <asarai@suse.de>, Shuah Khan <shuah@kernel.org>, Alexander Shishkin <alexander.shishkin@linux.intel.com>, Ingo Molnar <mingo@redhat.com>, linux-arm-kernel@lists.infradead.org, linux-mips@vger.kernel.org, linux-xtensa@linux-xtensa.org, Kees Cook <keescook@chromium.org>, Arnd Bergmann <arnd@arndb.de>, linuxppc-dev@lists.ozlabs.org, linux-m68k@lists.linux-m68k.org, Al Viro <viro@zeniv.linux.org.uk>, Andy Lutomirski <luto@kernel.org>, Shuah Khan <skhan@linuxfoundation.org>, Namhyung Kim <namhyung@kernel.org>, David Drysdale <drysdale@google.com>, Christian Brauner <christian@brauner.io>, "J. Bruce Fields" <bfields@fieldses.org>, linux-parisc@vger.kernel.org, Linux API <linux-api@vger.kernel.org>, Chanho Min <chanho.min@lge.com>, Jeff Layton <jlayton@kernel.org>, Oleg Nesterov <oleg@redhat.com>, Eric Biederman <ebiederm@xmission.com>, linux-alpha@vger.kernel.org, linux-fsdevel <linux-fsdevel@vger.kernel.org>, Andrew Morton <akpm@linux-foundation.org>, Linus Torvalds <torvalds@linux-foundation.org>, Linux Containers <containers@lists.linux-foundation.org> Subject: Re: [PATCH v12 05/12] namei: obey trailing magic-link DAC permissions Date: Wed, 18 Sep 2019 15:51:00 +0200 Message-ID: <20190918135100.sdxdmdluq6wlwryv@yavin.microfocus.com> (raw) In-Reply-To: <CAG48ez1_64249RdX6Nj_32YS+jhuXZBAd_ZL9ozggbSQy+cc-A@mail.gmail.com> [-- Attachment #1: Type: text/plain, Size: 3875 bytes --] On 2019-09-17, Jann Horn <jannh@google.com> wrote: > On Wed, Sep 4, 2019 at 10:21 PM Aleksa Sarai <cyphar@cyphar.com> wrote: > > The ability for userspace to "re-open" file descriptors through > > /proc/self/fd has been a very useful tool for all sorts of usecases > > (container runtimes are one common example). However, the current > > interface for doing this has resulted in some pretty subtle security > > holes. Userspace can re-open a file descriptor with more permissions > > than the original, which can result in cases such as /proc/$pid/exe > > being re-opened O_RDWR at a later date even though (by definition) > > /proc/$pid/exe cannot be opened for writing. When combined with O_PATH > > the results can get even more confusing. > [...] > > Instead we have to restrict it in such a way that it doesn't break > > (good) users but does block potential attackers. The solution applied in > > this patch is to restrict *re-opening* (not resolution through) > > magic-links by requiring that mode of the link be obeyed. Normal > > symlinks have modes of a+rwx but magic-links have other modes. These > > magic-link modes were historically ignored during path resolution, but > > they've now been re-purposed for more useful ends. > > Thanks for dealing with this issue! > > [...] > > diff --git a/fs/namei.c b/fs/namei.c > > index 209c51a5226c..54d57dad0f91 100644 > > --- a/fs/namei.c > > +++ b/fs/namei.c > > @@ -872,7 +872,7 @@ void nd_jump_link(struct path *path) > > > > nd->path = *path; > > nd->inode = nd->path.dentry->d_inode; > > - nd->flags |= LOOKUP_JUMPED; > > + nd->flags |= LOOKUP_JUMPED | LOOKUP_MAGICLINK_JUMPED; > > } > [...] > > +static int trailing_magiclink(struct nameidata *nd, int acc_mode, > > + fmode_t *opath_mask) > > +{ > > + struct inode *inode = nd->link_inode; > > + fmode_t upgrade_mask = 0; > > + > > + /* Was the trailing_symlink() a magic-link? */ > > + if (!(nd->flags & LOOKUP_MAGICLINK_JUMPED)) > > + return 0; > > + > > + /* > > + * Figure out the upgrade-mask of the link_inode. Since these aren't > > + * strictly POSIX semantics we don't do an acl_permission_check() here, > > + * so we only care that at least one bit is set for each upgrade-mode. > > + */ > > + if (inode->i_mode & S_IRUGO) > > + upgrade_mask |= FMODE_PATH_READ; > > + if (inode->i_mode & S_IWUGO) > > + upgrade_mask |= FMODE_PATH_WRITE; > > + /* Restrict the O_PATH upgrade-mask of the caller. */ > > + if (opath_mask) > > + *opath_mask &= upgrade_mask; > > + return may_open_magiclink(upgrade_mask, acc_mode); > > } > > This looks racy because entries in the file descriptor table can be > switched out as long as task->files->file_lock isn't held. Unless I'm > missing something, something like the following (untested) would > bypass this restriction: You're absolutely right -- good catch! > Perhaps you could change nd_jump_link() to "void nd_jump_link(struct > path *path, umode_t link_mode)", and let proc_pid_get_link() pass the > link_mode through from an out-argument of .proc_get_link()? Then > proc_fd_link() could grab the proper mode in a race-free manner. And > nd_jump_link() could stash the mode in the nameidata. This indeed does appear to be the simplest solution -- I'm currently testing a variation of the patch you proposed (with a few extra bits to deal with nd_jump_link and proc_get_link being used elsewhere). I'll include this change (assuming it fixes the flaw you found) in the v13 series I'll send around next week. Thanks, Jann! -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/> [-- Attachment #2: signature.asc --] [-- Type: application/pgp-signature, Size: 228 bytes --]
next prev parent reply index Thread overview: 65+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-09-04 20:19 [PATCH v12 00/12] namei: openat2(2) path resolution restrictions Aleksa Sarai 2019-09-04 20:19 ` [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers Aleksa Sarai 2019-09-04 20:48 ` [PATCH v12 01/12] lib: introduce copy_struct_{to, from}_user helpers Linus Torvalds 2019-09-04 21:00 ` [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers Randy Dunlap 2019-09-05 7:32 ` Peter Zijlstra 2019-09-05 9:26 ` Aleksa Sarai 2019-09-05 9:43 ` Peter Zijlstra 2019-09-05 10:57 ` Peter Zijlstra 2019-09-11 10:37 ` Aleksa Sarai 2019-09-05 13:35 ` Aleksa Sarai 2019-09-05 17:01 ` Aleksa Sarai 2019-09-05 8:43 ` Rasmus Villemoes 2019-09-05 9:50 ` Aleksa Sarai 2019-09-05 10:45 ` Christian Brauner 2019-09-05 9:09 ` [PATCH v12 01/12] lib: introduce copy_struct_{to, from}_user helpers Andreas Schwab 2019-09-05 10:13 ` Gabriel Paubert 2019-09-05 11:05 ` [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers Christian Brauner 2019-09-05 11:17 ` Rasmus Villemoes 2019-09-05 11:29 ` Christian Brauner 2019-09-05 13:40 ` Aleksa Sarai 2019-09-05 11:09 ` Christian Brauner 2019-09-05 11:27 ` Aleksa Sarai 2019-09-05 11:40 ` Christian Brauner 2019-09-05 18:07 ` Al Viro 2019-09-05 18:23 ` Christian Brauner 2019-09-05 18:28 ` Al Viro 2019-09-05 18:35 ` Christian Brauner 2019-09-05 19:56 ` Aleksa Sarai 2019-09-05 22:31 ` Al Viro 2019-09-06 7:00 ` Christian Brauner 2019-09-05 23:00 ` Aleksa Sarai 2019-09-05 23:49 ` Al Viro 2019-09-06 0:09 ` Aleksa Sarai 2019-09-06 0:14 ` Al Viro 2019-09-04 20:19 ` [PATCH v12 02/12] clone3: switch to copy_struct_from_user() Aleksa Sarai 2019-09-04 20:19 ` [PATCH v12 03/12] sched_setattr: switch to copy_struct_{to, from}_user() Aleksa Sarai 2019-09-04 20:19 ` [PATCH v12 04/12] perf_event_open: switch to copy_struct_from_user() Aleksa Sarai 2019-09-04 20:19 ` [PATCH v12 05/12] namei: obey trailing magic-link DAC permissions Aleksa Sarai 2019-09-17 21:30 ` Jann Horn 2019-09-18 13:51 ` Aleksa Sarai [this message] 2019-09-18 15:46 ` Aleksa Sarai 2019-09-04 20:19 ` [PATCH v12 06/12] procfs: switch magic-link modes to be more sane Aleksa Sarai 2019-09-04 20:19 ` [PATCH v12 07/12] open: O_EMPTYPATH: procfs-less file descriptor re-opening Aleksa Sarai 2019-09-04 20:19 ` [PATCH v12 08/12] namei: O_BENEATH-style path resolution flags Aleksa Sarai 2019-09-04 20:19 ` [PATCH v12 09/12] namei: LOOKUP_IN_ROOT: chroot-like path resolution Aleksa Sarai 2019-09-04 20:19 ` [PATCH v12 10/12] namei: aggressively check for nd->root escape on ".." resolution Aleksa Sarai 2019-09-04 21:09 ` Linus Torvalds 2019-09-04 21:35 ` Linus Torvalds 2019-09-04 21:36 ` Linus Torvalds 2019-09-04 21:48 ` Aleksa Sarai 2019-09-04 22:16 ` Linus Torvalds 2019-09-04 22:31 ` David Howells 2019-09-04 22:38 ` Linus Torvalds 2019-09-04 23:29 ` Al Viro 2019-09-04 23:44 ` Linus Torvalds 2019-09-04 20:19 ` [PATCH v12 11/12] open: openat2(2) syscall Aleksa Sarai 2019-09-04 21:00 ` Randy Dunlap 2019-09-07 12:40 ` Jeff Layton 2019-09-07 16:58 ` Linus Torvalds 2019-09-07 17:42 ` Andy Lutomirski 2019-09-07 17:45 ` Linus Torvalds 2019-09-07 18:15 ` Andy Lutomirski 2019-09-10 6:35 ` Ingo Molnar 2019-09-08 16:24 ` Aleksa Sarai 2019-09-04 20:19 ` [PATCH v12 12/12] selftests: add openat2(2) selftests Aleksa Sarai
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