Re: [PATCH mm] VFS: seq_file: ensure ->from is valid.

From: Jann Horn <jannh@google.com>
To: Kees Cook <keescook@chromium.org>
Cc: neilb@suse.com, Andrew Morton <akpm@linux-foundation.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-doc@vger.kernel.org,
	kernel list <linux-kernel@vger.kernel.org>,
	linux-fsdevel@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>
Subject: Re: [PATCH mm] VFS: seq_file: ensure ->from is valid.
Date: Mon, 9 Jul 2018 21:40:54 +0200	[thread overview]
Message-ID: <CAG48ez36DjH__USsW4ViFXqUj3y8GJf1Bz7tyje0Wpub6zgXcQ@mail.gmail.com> (raw)
In-Reply-To: <CAGXu5j+F3zUZ6+8cKuurq2fV8L5Piy5dvYrYXeFVG0kVmpZMRw@mail.gmail.com>

On Mon, Jul 9, 2018 at 8:16 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Fri, Jul 6, 2018 at 8:29 PM, NeilBrown <neilb@suse.com> wrote:
> >
> > Previous patch ("VFS: simplify seq_file iteration code and interface")
> > removed code to set ->from to zero when ->count is zero, as ->from is
> > dead at that time.  However it didn't ensure ->from was set properly
> > whenever ->count becomes non-zero.
> > This can only happen when ->show() is called.  Of the three places it
> > is called one already has ->from set to zero.  The other two are
> > fixed by setting from to zero after fully flushing the buffer (at which
> > point ->count will also be zero).
> >
> > Reported-by: Jann Horn <jannh@google.com>
> > Signed-off-by: NeilBrown <neilb@suse.com>
>
> I *think* this solves this report, which looks very much like Jann's reproducer:
>
> https://syzkaller.appspot.com/bug?extid=4b712dce5cbce6700f27

I don't think the path I found and what syzcaller reported match
precisely. The syz reproducer linked on that page is:

r0 = memfd_create(&(0x7f00000000c0)='md5sum\x00', 0x0)
mmap(&(0x7f0000001000/0x1000)=nil, 0x1000, 0x0, 0x51, r0, 0x0)
mkdir(&(0x7f0000000040)='./control\x00', 0x0)
r1 = socket$inet_sctp(0x2, 0x1, 0x84)
getsockopt$inet_sctp_SCTP_ADAPTATION_LAYER(r1, 0x84, 0x7,
&(0x7f0000000000), &(0x7f0000000080)=0x4)
r2 = syz_open_procfs(0x0, &(0x7f0000000040)='smaps\x00')
readv(r2, &(0x7f00000021c0)=[{&(0x7f0000000140)=""/79, 0x432},
{&(0x7f00000001c0)=""/4096, 0x1000}], 0x2)

If we assume that this is the same bug, only the last two lines are
interesting. They should cause two invocations of seq_read(). The
first invocation should start with ->from==0, so the bug shouldn't
trigger there. So it has to happen on the second invocation. So the
first invocation has to leave ->from at some high value and ->count
nonzero. But the only place in seq_read() that could set ->from on the
first read (if the traverse case doesn't happen) is the one at the end
of seq_read() that properly adjusts ->count down by the same number in
the line above. This doesn't add up.