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From: Al Viro <viro@ZenIV.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jann Horn <jannh@google.com>,
	Linux API <linux-api@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	David Drysdale <drysdale@google.com>
Subject: Re: new ...at() flag: AT_NO_JUMPS
Date: Fri, 5 May 2017 05:39:02 +0100	[thread overview]
Message-ID: <20170505043902.GP29622@ZenIV.linux.org.uk> (raw)
In-Reply-To: <CA+55aFy1SokNNUgxBnFLdA1PRyeG13BqyYNg5xVrW-tNGqh2Bg@mail.gmail.com>

On Thu, May 04, 2017 at 08:46:49PM -0700, Linus Torvalds wrote:
> On Thu, May 4, 2017 at 7:47 PM, Jann Horn <jannh@google.com> wrote:
> >
> > Thread 1 starts an AT_BENEATH path walk using an O_PATH fd
> > pointing to /srv/www/example.org/foo; the path given to the syscall is
> > "bar/../../../../etc/passwd". The path walk enters the "bar" directory.
> > Thread 2 moves /srv/www/example.org/foo/bar to
> > /srv/www/example.org/bar.
> > Thread 1 processes the rest of the path ("../../../../etc/passwd"), never
> > hitting /srv/www/example.org/foo in the process.
> >
> > I'm not really familiar with the VFS internals, but from a coarse look
> > at the patch, it seems like it wouldn't block this?
> 
> I think you're right.
> 
> I guess it would be safe for the RCU case due to the sequence number
> check, but not the non-RCU case.

	Yes and no...  FWIW, to exclude that it would suffice to have
mount --rbind /src/www/example.org/foo /srv/www/example.org/foo done first.
Then this kind of race will end up with -ENOENT due to path_connected()
logics in follow_dotdot_rcu()/follow_dotdot().  I'm not sure about the
intended applications, though - is that thing supposed to be used along with
some horror like seccomp, or...?

WARNING: multiple messages have this Message-ID (diff)
From: Al Viro <viro-3bDd1+5oDREiFSDQTTA3OLVCufUGDwFn@public.gmane.org>
To: Linus Torvalds
	<torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
Cc: Jann Horn <jannh-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>,
	Linux API <linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	Linux Kernel Mailing List
	<linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	linux-fsdevel
	<linux-fsdevel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	David Drysdale <drysdale-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
Subject: Re: new ...at() flag: AT_NO_JUMPS
Date: Fri, 5 May 2017 05:39:02 +0100	[thread overview]
Message-ID: <20170505043902.GP29622@ZenIV.linux.org.uk> (raw)
In-Reply-To: <CA+55aFy1SokNNUgxBnFLdA1PRyeG13BqyYNg5xVrW-tNGqh2Bg-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>

On Thu, May 04, 2017 at 08:46:49PM -0700, Linus Torvalds wrote:
> On Thu, May 4, 2017 at 7:47 PM, Jann Horn <jannh-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> wrote:
> >
> > Thread 1 starts an AT_BENEATH path walk using an O_PATH fd
> > pointing to /srv/www/example.org/foo; the path given to the syscall is
> > "bar/../../../../etc/passwd". The path walk enters the "bar" directory.
> > Thread 2 moves /srv/www/example.org/foo/bar to
> > /srv/www/example.org/bar.
> > Thread 1 processes the rest of the path ("../../../../etc/passwd"), never
> > hitting /srv/www/example.org/foo in the process.
> >
> > I'm not really familiar with the VFS internals, but from a coarse look
> > at the patch, it seems like it wouldn't block this?
> 
> I think you're right.
> 
> I guess it would be safe for the RCU case due to the sequence number
> check, but not the non-RCU case.

	Yes and no...  FWIW, to exclude that it would suffice to have
mount --rbind /src/www/example.org/foo /srv/www/example.org/foo done first.
Then this kind of race will end up with -ENOENT due to path_connected()
logics in follow_dotdot_rcu()/follow_dotdot().  I'm not sure about the
intended applications, though - is that thing supposed to be used along with
some horror like seccomp, or...?

  reply	other threads:[~2017-05-05  4:39 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-04-29 22:04 new ...at() flag: AT_NO_JUMPS Al Viro
2017-04-29 22:04 ` Al Viro
2017-04-29 23:17 ` Andy Lutomirski
2017-04-29 23:17   ` Andy Lutomirski
2017-04-29 23:25   ` Al Viro
2017-04-29 23:25     ` Al Viro
2017-04-30  1:13     ` Andy Lutomirski
2017-04-30  4:38     ` Matthew Wilcox
2017-04-30  4:38       ` Matthew Wilcox
2017-04-30 16:10       ` Al Viro
2017-04-30 16:10         ` Al Viro
2017-05-01  4:52         ` Andy Lutomirski
2017-05-01  5:15           ` Al Viro
2017-05-01  5:15             ` Al Viro
2017-05-01 17:36 ` Jann Horn
2017-05-01 19:37   ` Andy Lutomirski
2017-05-05  0:30   ` Al Viro
2017-05-05  0:30     ` Al Viro
2017-05-05  0:44     ` Andy Lutomirski
2017-05-05  1:06       ` Al Viro
2017-05-05  1:27     ` Linus Torvalds
2017-05-05  1:27       ` Linus Torvalds
2017-05-05  3:00       ` Al Viro
2017-05-05  3:00         ` Al Viro
2017-05-05  4:01         ` Linus Torvalds
2017-05-05  4:31           ` Andy Lutomirski
2017-05-05  2:47     ` Jann Horn
2017-05-05  3:46       ` Linus Torvalds
2017-05-05  4:39         ` Al Viro [this message]
2017-05-05  4:39           ` Al Viro
2017-05-05  4:44           ` Andy Lutomirski
2017-05-05 20:04             ` Eric W. Biederman
2017-05-05 20:04               ` Eric W. Biederman
2017-05-05 20:28           ` Eric W. Biederman
2017-05-08 19:34             ` Mickaël Salaün
2017-05-08 19:34               ` Mickaël Salaün
2017-05-18  8:50     ` David Drysdale
2017-09-10 20:26 Jürg Billeter

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