linux-kernel - Re: [rfc][possible solution] RCU vfsmounts

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CA+55aFzrjiM6bgqNgdckJGAZks2fR8ZD+R_5AqxvrnS64DD3ow@mail.gmail.com>
Date:	Sat, 28 Sep 2013 13:43:49 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [rfc][possible solution] RCU vfsmounts

On Sat, Sep 28, 2013 at 1:27 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>         FWIW, I think I have a kinda-sorta solution for that and I'd like
> to hear your comments on that.  I want to replace vfsmount_lock with seqlock
> and store additional seq number in nameidata, set to vfsmount_seq in the
> beginning and rechecked in unlazy_walk/complete_walk.

Yes, that would be lovely.

>         The obvious variant would be to have unlazy_walk/complete_walk to
> grab refcount, check vfsmount_seq and mntput on mismatch.  The trouble
> with that is race with what would've been the final mntput() done by
> umount(2); complete_walk() would drop that temporary reference and
> fail, all right, but... we would get a umount(2) returning without having
> actually shut the filesystem down.  Said shutdown would happen in whoever
> had been doing pathname resolution that stepped into the race.

Sounds reasonable to to me.

Side note: I really wish there was some way to avoid having to
finalize the vfsmount entirely for some common things. For example,
"[l]stat[at]()" really doesn't need it for the common cases (network
filesystems may need to revalidate), and is a very critical operation,
and we *could* just look up the inode under RCU and never finalize the
dentry _or_ the vfsmount. However, very annoyingly, the security layer
wants the vfsmount, and we don't know if that is RCU-safe...

                Linus
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/