From: Aleksa Sarai <cyphar@cyphar.com> To: Al Viro <viro@zeniv.linux.org.uk> 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>, linux-kernel@vger.kernel.org, David Howells <dhowells@redhat.com>, linux-kselftest@vger.kernel.org, sparclinux@vger.kernel.org, Christian Brauner <christian.brauner@ubuntu.com>, Shuah Khan <shuah@kernel.org>, linux-arch@vger.kernel.org, linux-s390@vger.kernel.org, Tycho Andersen <tycho@tycho.ws>, Aleksa Sarai <asarai@suse.de>, Jiri Olsa <jolsa@redhat.com>, 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>, Jann Horn <jannh@google.com>, linuxppc-dev@lists.ozlabs.org, linux-m68k@lists.linux-m68k.org, 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>, libc-alpha@sourceware.org, linux-parisc@vger.kernel.org, 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@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>, Linus Torvalds <torvalds@linux-foundation.org>, containers@lists.linux-foundation.org Subject: Re: [PATCH v15 3/9] namei: LOOKUP_NO_XDEV: block mountpoint crossing Date: Fri, 15 Nov 2019 00:33:00 +1100 [thread overview] Message-ID: <20191114133300.soxnzmufwbt2ddid@yavin.dot.cyphar.com> (raw) In-Reply-To: <20191114054348.GH26530@ZenIV.linux.org.uk> [-- Attachment #1: Type: text/plain, Size: 3959 bytes --] On 2019-11-14, Al Viro <viro@zeniv.linux.org.uk> wrote: > On Thu, Nov 14, 2019 at 03:49:45PM +1100, Aleksa Sarai wrote: > > On 2019-11-13, Al Viro <viro@zeniv.linux.org.uk> wrote: > > > On Tue, Nov 05, 2019 at 08:05:47PM +1100, Aleksa Sarai wrote: > > > > > > > @@ -862,6 +870,8 @@ static int nd_jump_root(struct nameidata *nd) > > > > void nd_jump_link(struct path *path) > > > > { > > > > struct nameidata *nd = current->nameidata; > > > > + > > > > + nd->last_magiclink.same_mnt = (nd->path.mnt == path->mnt); > > > > path_put(&nd->path); > > > > > > > > nd->path = *path; > > > > @@ -1082,6 +1092,10 @@ const char *get_link(struct nameidata *nd) > > > > if (nd->flags & LOOKUP_MAGICLINK_JUMPED) { > > > > if (unlikely(nd->flags & LOOKUP_NO_MAGICLINKS)) > > > > return ERR_PTR(-ELOOP); > > > > + if (unlikely(nd->flags & LOOKUP_NO_XDEV)) { > > > > + if (!nd->last_magiclink.same_mnt) > > > > + return ERR_PTR(-EXDEV); > > > > + } > > > > } > > > > > > Ugh... Wouldn't it be better to take that logics (some equivalent thereof) > > > into nd_jump_link()? Or just have nd_jump_link() return an error... > > > > This could be done, but the reason for stashing it away in > > last_magiclink is because of the future magic-link re-opening patches > > which can't be implemented like that without putting the open_flags > > inside nameidata (which was decided to be too ugly a while ago). > > > > My point being that I could implement it this way for this series, but > > I'd have to implement something like last_magiclink when I end up > > re-posting the magic-link stuff in a few weeks. > > > > Looking at all the nd_jump_link() users, the other option is to just > > disallow magic-link crossings entirely for LOOKUP_NO_XDEV. The only > > thing allowing them permits is to resolve file descriptors that are > > pointing to the same procfs mount -- and it's unclear to me how useful > > that really is (apparmorfs and nsfs will always give -EXDEV because > > aafs_mnt and nsfs_mnt are internal kernel vfsmounts). > > I would rather keep the entire if (nd->flags & LOOKUP_MAGICLINK_JUMPED) > out of the get_link(). If you want to generate some error if > nd_jump_link() has been called, just do it right there. The fewer > pieces of state need to be carried around, the better... Sure, I can make nd_jump_link() give -ELOOP and drop the current need for LOOKUP_MAGICLINK_JUMPED -- if necessary we can re-add it for the magic-link reopening patches. > And as for opening them... Why would you need full open_flags in there? > Details, please... I was referring to [1] which has been dropped from this series. I misspoke -- you don't need the full open_flags, you just need acc_mode in nameidata -- but from memory you (understandably) weren't in favour of that either because it further muddled the open semantics with namei. So the solution I went with was to stash away the i_mode of the magiclink in nd->last_magiclink.mode (though to avoid a race which Jann found, you actually need to recalculate it when you call nd_jump_link() but that's a different topic) and then check it in trailing_magiclink(). However, I've since figured out that we need to restrict things like bind-mounts and truncate() because they can be used to get around the restrictions. I dropped that patch from this series so that I could work on implementing the restrictions for the other relevant VFS syscalls separately from openat2 (upgrade_mask will be re-added to open_how with those patches). My point was that AFAICS we will either have to have nd->acc_mode (or something similar) or have nd->last_magiclink in order to implement the magic-link reopening hardening. [1]: https://lore.kernel.org/lkml/20190930183316.10190-2-cyphar@cyphar.com/ -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/> [-- Attachment #2: signature.asc --] [-- Type: application/pgp-signature, Size: 228 bytes --]
WARNING: multiple messages have this Message-ID (diff)
From: Aleksa Sarai <cyphar@cyphar.com> To: Al Viro <viro@zeniv.linux.org.uk> Cc: Jeff Layton <jlayton@kernel.org>, "J. Bruce Fields" <bfields@fieldses.org>, Arnd Bergmann <arnd@arndb.de>, David Howells <dhowells@redhat.com>, Shuah Khan <shuah@kernel.org>, Shuah Khan <skhan@linuxfoundation.org>, Ingo Molnar <mingo@redhat.com>, Peter Zijlstra <peterz@infradead.org>, Christian Brauner <christian.brauner@ubuntu.com>, David Drysdale <drysdale@google.com>, Andy Lutomirski <luto@kernel.org>, Linus Torvalds <torvalds@linux-foundation.org>, Eric Biederman <ebiederm@xmission.com>, Andrew Morton <akpm@linux-foundation.org>, Alexei Starovoitov <ast@kernel.org>, Kees Cook <keescook@chromium.org>, Jann Horn <jannh@google.com>, Tycho Andersen <tycho@tycho.ws>, Chanho Min <chanho.min@lge.com>, Oleg Nesterov <oleg@redhat.com>, Rasmus Villemoes <linux@rasmusvillemoes.dk>, Alexander Shishkin <alexander.shishkin@linux.intel.com>, Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>, Christian Brauner <christian@brauner.io>, Aleksa Sarai <asarai@suse.de>, containers@lists.linux-foundation.org, linux-alpha@vger.kernel.org, linux-api@vger.kernel.org, libc-alpha@sourceware.org, linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-ia64@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, linux-xtensa@linux-xtensa.org, sparclinux@vger.kernel.org Subject: Re: [PATCH v15 3/9] namei: LOOKUP_NO_XDEV: block mountpoint crossing Date: Fri, 15 Nov 2019 00:33:00 +1100 [thread overview] Message-ID: <20191114133300.soxnzmufwbt2ddid@yavin.dot.cyphar.com> (raw) Message-ID: <20191114133300.6wbSt74OpHFi8iXtrJTyX92NLvv4-aOZaqZsz0dqoaQ@z> (raw) In-Reply-To: <20191114054348.GH26530@ZenIV.linux.org.uk> [-- Attachment #1: Type: text/plain, Size: 3959 bytes --] On 2019-11-14, Al Viro <viro@zeniv.linux.org.uk> wrote: > On Thu, Nov 14, 2019 at 03:49:45PM +1100, Aleksa Sarai wrote: > > On 2019-11-13, Al Viro <viro@zeniv.linux.org.uk> wrote: > > > On Tue, Nov 05, 2019 at 08:05:47PM +1100, Aleksa Sarai wrote: > > > > > > > @@ -862,6 +870,8 @@ static int nd_jump_root(struct nameidata *nd) > > > > void nd_jump_link(struct path *path) > > > > { > > > > struct nameidata *nd = current->nameidata; > > > > + > > > > + nd->last_magiclink.same_mnt = (nd->path.mnt == path->mnt); > > > > path_put(&nd->path); > > > > > > > > nd->path = *path; > > > > @@ -1082,6 +1092,10 @@ const char *get_link(struct nameidata *nd) > > > > if (nd->flags & LOOKUP_MAGICLINK_JUMPED) { > > > > if (unlikely(nd->flags & LOOKUP_NO_MAGICLINKS)) > > > > return ERR_PTR(-ELOOP); > > > > + if (unlikely(nd->flags & LOOKUP_NO_XDEV)) { > > > > + if (!nd->last_magiclink.same_mnt) > > > > + return ERR_PTR(-EXDEV); > > > > + } > > > > } > > > > > > Ugh... Wouldn't it be better to take that logics (some equivalent thereof) > > > into nd_jump_link()? Or just have nd_jump_link() return an error... > > > > This could be done, but the reason for stashing it away in > > last_magiclink is because of the future magic-link re-opening patches > > which can't be implemented like that without putting the open_flags > > inside nameidata (which was decided to be too ugly a while ago). > > > > My point being that I could implement it this way for this series, but > > I'd have to implement something like last_magiclink when I end up > > re-posting the magic-link stuff in a few weeks. > > > > Looking at all the nd_jump_link() users, the other option is to just > > disallow magic-link crossings entirely for LOOKUP_NO_XDEV. The only > > thing allowing them permits is to resolve file descriptors that are > > pointing to the same procfs mount -- and it's unclear to me how useful > > that really is (apparmorfs and nsfs will always give -EXDEV because > > aafs_mnt and nsfs_mnt are internal kernel vfsmounts). > > I would rather keep the entire if (nd->flags & LOOKUP_MAGICLINK_JUMPED) > out of the get_link(). If you want to generate some error if > nd_jump_link() has been called, just do it right there. The fewer > pieces of state need to be carried around, the better... Sure, I can make nd_jump_link() give -ELOOP and drop the current need for LOOKUP_MAGICLINK_JUMPED -- if necessary we can re-add it for the magic-link reopening patches. > And as for opening them... Why would you need full open_flags in there? > Details, please... I was referring to [1] which has been dropped from this series. I misspoke -- you don't need the full open_flags, you just need acc_mode in nameidata -- but from memory you (understandably) weren't in favour of that either because it further muddled the open semantics with namei. So the solution I went with was to stash away the i_mode of the magiclink in nd->last_magiclink.mode (though to avoid a race which Jann found, you actually need to recalculate it when you call nd_jump_link() but that's a different topic) and then check it in trailing_magiclink(). However, I've since figured out that we need to restrict things like bind-mounts and truncate() because they can be used to get around the restrictions. I dropped that patch from this series so that I could work on implementing the restrictions for the other relevant VFS syscalls separately from openat2 (upgrade_mask will be re-added to open_how with those patches). My point was that AFAICS we will either have to have nd->acc_mode (or something similar) or have nd->last_magiclink in order to implement the magic-link reopening hardening. [1]: https://lore.kernel.org/lkml/20190930183316.10190-2-cyphar@cyphar.com/ -- 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 other threads:[~2019-11-14 13:44 UTC|newest] Thread overview: 49+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-11-05 9:05 [PATCH v15 0/9] open: introduce openat2(2) syscall Aleksa Sarai 2019-11-05 9:05 ` [PATCH v15 1/9] namei: LOOKUP_NO_SYMLINKS: block symlink resolution Aleksa Sarai 2019-11-05 9:05 ` [PATCH v15 2/9] namei: LOOKUP_NO_MAGICLINKS: block magic-link resolution Aleksa Sarai 2019-11-13 1:24 ` Al Viro 2019-11-13 1:24 ` Al Viro 2019-11-05 9:05 ` [PATCH v15 3/9] namei: LOOKUP_NO_XDEV: block mountpoint crossing Aleksa Sarai 2019-11-13 1:36 ` Al Viro 2019-11-13 1:36 ` Al Viro 2019-11-14 4:49 ` Aleksa Sarai 2019-11-14 4:49 ` Aleksa Sarai 2019-11-14 5:43 ` Al Viro 2019-11-14 5:43 ` Al Viro 2019-11-14 13:33 ` Aleksa Sarai [this message] 2019-11-14 13:33 ` Aleksa Sarai 2019-11-05 9:05 ` [PATCH v15 4/9] namei: LOOKUP_BENEATH: O_BENEATH-like scoped resolution Aleksa Sarai 2019-11-13 1:55 ` Al Viro 2019-11-13 1:55 ` Al Viro 2019-11-13 7:47 ` Aleksa Sarai 2019-11-13 7:47 ` Aleksa Sarai 2019-11-14 4:57 ` Aleksa Sarai 2019-11-14 4:57 ` Aleksa Sarai 2019-11-05 9:05 ` [PATCH v15 5/9] namei: LOOKUP_IN_ROOT: chroot-like " Aleksa Sarai 2019-11-13 2:03 ` Al Viro 2019-11-13 2:03 ` Al Viro 2019-11-13 2:44 ` Aleksa Sarai 2019-11-13 2:44 ` Aleksa Sarai 2019-11-13 2:59 ` Al Viro 2019-11-13 2:59 ` Al Viro 2019-11-13 3:55 ` Aleksa Sarai 2019-11-13 3:55 ` Aleksa Sarai 2019-11-05 9:05 ` [PATCH v15 6/9] namei: LOOKUP_{IN_ROOT, BENEATH}: permit limited ".." resolution Aleksa Sarai 2019-11-13 2:09 ` [PATCH v15 6/9] namei: LOOKUP_{IN_ROOT,BENEATH}: " Al Viro 2019-11-13 2:09 ` Al Viro 2019-11-13 7:52 ` Aleksa Sarai 2019-11-13 7:52 ` Aleksa Sarai 2019-11-05 9:05 ` [PATCH v15 7/9] open: introduce openat2(2) syscall Aleksa Sarai 2019-11-13 2:29 ` Al Viro 2019-11-13 2:29 ` Al Viro 2019-11-13 2:35 ` Aleksa Sarai 2019-11-13 2:35 ` Aleksa Sarai 2019-11-05 9:05 ` [PATCH v15 8/9] selftests: add openat2(2) selftests Aleksa Sarai 2019-11-05 9:05 ` [PATCH v15 9/9] Documentation: path-lookup: mention LOOKUP_MAGICLINK_JUMPED Aleksa Sarai 2019-11-11 13:24 ` [PATCH v15 0/9] open: introduce openat2(2) syscall Aleksa Sarai 2019-11-12 23:01 ` Kees Cook 2019-11-12 23:01 ` Kees Cook 2019-11-12 23:06 ` Christian Brauner 2019-11-12 23:06 ` Christian Brauner 2019-11-13 0:46 ` Aleksa Sarai 2019-11-13 0:46 ` Aleksa Sarai
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