From: Al Viro <firstname.lastname@example.org> To: "Eric W. Biederman" <email@example.com> Cc: Tetsuo Handa <firstname.lastname@example.org>, David Howells <email@example.com>, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com Subject: Re: [PATCH 02/10] vfs: syscall: Add move_mount(2) to move mounts around Date: Mon, 8 Jul 2019 21:21:24 +0100 Message-ID: <20190708202124.GX17978@ZenIV.linux.org.uk> (raw) In-Reply-To: <20190708180132.GU17978@ZenIV.linux.org.uk> On Mon, Jul 08, 2019 at 07:01:32PM +0100, Al Viro wrote: > On Mon, Jul 08, 2019 at 12:12:21PM -0500, Eric W. Biederman wrote: > > > Al you do realize that the TOCTOU you are talking about comes the system > > call API. TOMOYO can only be faulted for not playing in their own > > sandbox and not reaching out and fixing the vfs implementation details. PS: the fact that mount(2) has been overloaded to hell and back (including MS_MOVE, which goes back to v220.127.116.11) predates the introduction of ->sb_mount() and LSM in general (2.5.27). MS_BIND is 2.4.0-test9pre2. In all the years since the introduction of ->sb_mount() I've seen zero questions from LSM folks regarding a sane place for those checks. What I have seen was "we want it immediately upon the syscall entry, let the module figure out what to do" in reply to several times I tried to tell them "folks, it's called in a bad place; you want the checks applied to objects, not to raw string arguments". As it is, we have easily bypassable checks on mount(2) (by way of ->sb_mount(); there are other hooks also in the game for remounts and new mounts). I see no point whatsoever trying to duplicate ->sb_mount() on the top level of move_mount(2). When and if sane checks are agreed upon for that thing, they certainly should be used both for MS_MOVE case of mount(2) and for move_mount(2). And that'll come for free from calling those in do_move_mount(). They won't be the first thing called in mount(2) - we demultiplex first, decide that we have a move and do pathname resolution on source. And that's precisely what we need to avoid the TOCTOU there. I'm sorry, but this "run the hook at the very beginning, the modules know better what they want, just give them as close to syscall arguments as possible before even looking at the flags" model is wrong, plain and simple. As for the MS_MOVE checks, the arguments are clear enough (two struct path *, same as what we pass to do_move_mount()). AFAICS, only tomoyo and apparmor are trying to do anything for MS_MOVE in ->sb_mount(), and both look like it should be easy enough to implement what said checks intend to do (probably - assuming that aa_move_mount() doesn't depend upon having its kern_path() inside the __begin_current_label_crit_section()/ __end_current_label_crit_section() pair; looks like it shouldn't be, but that's for apparmor folks to decide). That's really for LSM folks, though - I've given up on convincing (or even getting a rationale out of) them on anything related to hook semantics years ago.
next prev parent reply index Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-02-19 17:08 [PATCH 00/10] VFS: Provide new mount UAPI David Howells 2019-02-19 17:08 ` [PATCH 01/10] vfs: syscall: Add open_tree(2) to reference or clone a mount David Howells 2019-02-19 17:08 ` [PATCH 02/10] vfs: syscall: Add move_mount(2) to move mounts around David Howells 2019-02-20 12:32 ` Alan Jenkins 2019-02-20 12:41 ` Alan Jenkins 2019-02-20 16:23 ` Jann Horn 2019-07-08 12:02 ` Tetsuo Handa 2019-07-08 13:18 ` Al Viro 2019-07-08 17:12 ` Eric W. Biederman 2019-07-08 18:01 ` Al Viro 2019-07-08 18:13 ` Al Viro 2019-07-08 20:21 ` Al Viro [this message] 2019-07-09 0:13 ` Eric W. Biederman 2019-07-09 10:51 ` Tetsuo Handa 2019-07-22 10:12 ` Tetsuo Handa 2019-07-23 4:16 ` John Johansen 2019-07-23 13:45 ` Tetsuo Handa 2019-08-06 10:43 ` Tetsuo Handa 2019-08-09 15:44 ` [PATCH] LSM: Disable move_mount() syscall when TOMOYO or AppArmor is enabled Tetsuo Handa 2019-08-22 3:51 ` [RFC][PATCH] fix d_absolute_path() interplay with fsmount() Al Viro 2019-08-30 10:11 ` Tetsuo Handa 2019-07-23 21:45 ` [PATCH 02/10] vfs: syscall: Add move_mount(2) to move mounts around James Morris 2019-07-23 23:30 ` Al Viro 2019-02-19 17:08 ` [PATCH 03/10] teach move_mount(2) to work with OPEN_TREE_CLONE David Howells 2019-02-20 18:59 ` Alan Jenkins 2019-02-26 17:45 ` Alan Jenkins 2019-02-19 17:08 ` [PATCH 04/10] Make anon_inodes unconditional David Howells 2019-02-19 17:09 ` [PATCH 05/10] vfs: syscall: Add fsopen() to prepare for superblock creation David Howells 2019-02-19 17:09 ` [PATCH 06/10] vfs: Implement logging through fs_context David Howells 2019-02-19 17:09 ` [PATCH 07/10] vfs: syscall: Add fsconfig() for configuring and managing a context David Howells 2019-02-19 17:09 ` [PATCH 08/10] vfs: syscall: Add fsmount() to create a mount for a superblock David Howells 2019-02-19 17:09 ` [PATCH 09/10] vfs: syscall: Add fspick() to select a superblock for reconfiguration David Howells 2019-02-19 17:09 ` [PATCH 10/10] vfs: Add a sample program for the new mount API David Howells
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