linux-kernel - Re: use-after-free in irtty

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Date:	Thu, 26 Nov 2015 09:12:05 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	Dmitry Vyukov <dvyukov@...gle.com>,
	Samuel Ortiz <samuel@...tiz.org>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.com>,
	LKML <linux-kernel@...r.kernel.org>,
	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	David Miller <davem@...emloft.net>
Subject: Re: use-after-free in irtty_open

[ + David Miller ]

On 11/25/2015 10:37 AM, Dmitry Vyukov wrote:
> Hello,
> 
> The following program causes a use-after-free in irtty_open:
> 
> // autogenerated by syzkaller (http://github.com/google/syzkaller)
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <fcntl.h>
> #include <sys/ioctl.h>
> 
> int main()
> {
>         int fd = open("/dev/ptmx", O_RDONLY|O_NOCTTY);
>         int val = 9;
>         ioctl(fd, TIOCSETD, &val);
>         val = 11;
>         ioctl(fd, TIOCSETD, &val);
>         return 0;
> }
> 
> 
> ==================================================================
> BUG: KASAN: use-after-free in irtty_open+0x422/0x550 at addr ffff8800331dd068
> Read of size 4 by task a.out/13960
> =============================================================================
> BUG kmalloc-512 (Tainted: G    B          ): kasan: bad access detected
> -----------------------------------------------------------------------------

Thanks for the report, Dmitry.
Would you please test the patch below?

As I wrote to Sasha a couple of days ago, the ldisc api should really prevent
these kinds of errors. I have a patch for the ldisc core that eliminates
these errors (reduces them to dead code).

Seems like this was a common pattern at some point; probably need to challenge
my coccinelle-fu to remove this pattern tree-wide.

Regards,
Peter Hurley

--- >% ---
Subject: [PATCH] net: irda: Fix use-after-free in irtty_open()

The N_IRDA line discipline may access the previous line discipline's closed
and already-fre private data on open [1].

The tty->disc_data field _never_ refers to valid data on entry to the
line discipline's open() method. Rather, the ldisc is expected to
initialize that field for its own use for the lifetime of the instance
(ie. from open() to close() only).

[1] Report from Dmitry Vyukov <dvyukov@...gle.com>
    ==================================================================
    BUG: KASAN: use-after-free in irtty_open+0x422/0x550 at addr ffff8800331dd068
    Read of size 4 by task a.out/13960
    =============================================================================
    BUG kmalloc-512 (Tainted: G    B          ): kasan: bad access detected
    -----------------------------------------------------------------------------
    ...
    Call Trace:
     [<ffffffff815fa2ae>] __asan_report_load4_noabort+0x3e/0x40 mm/kasan/report.c:279
     [<ffffffff836938a2>] irtty_open+0x422/0x550 drivers/net/irda/irtty-sir.c:436
     [<ffffffff829f1b80>] tty_ldisc_open.isra.2+0x60/0xa0 drivers/tty/tty_ldisc.c:447
     [<ffffffff829f21c0>] tty_set_ldisc+0x1a0/0x940 drivers/tty/tty_ldisc.c:567
     [<     inline     >] tiocsetd drivers/tty/tty_io.c:2650
     [<ffffffff829da49e>] tty_ioctl+0xace/0x1fd0 drivers/tty/tty_io.c:2883
     [<     inline     >] vfs_ioctl fs/ioctl.c:43
     [<ffffffff816708ac>] do_vfs_ioctl+0x57c/0xe60 fs/ioctl.c:607
     [<     inline     >] SYSC_ioctl fs/ioctl.c:622
     [<ffffffff81671204>] SyS_ioctl+0x74/0x80 fs/ioctl.c:613
     [<ffffffff852a7876>] entry_SYSCALL_64_fastpath+0x16/0x7a

Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
Signed-off-by: Peter Hurley <peter@...leysoftware.com>
---
 drivers/net/irda/irtty-sir.c | 10 ----------
 1 file changed, 10 deletions(-)

diff --git a/drivers/net/irda/irtty-sir.c b/drivers/net/irda/irtty-sir.c
index 696852e..7a3f990 100644
--- a/drivers/net/irda/irtty-sir.c
+++ b/drivers/net/irda/irtty-sir.c
@@ -430,16 +430,6 @@ static int irtty_open(struct tty_struct *tty)
 
 	/* Module stuff handled via irda_ldisc.owner - Jean II */
 
-	/* First make sure we're not already connected. */
-	if (tty->disc_data != NULL) {
-		priv = tty->disc_data;
-		if (priv && priv->magic == IRTTY_MAGIC) {
-			ret = -EEXIST;
-			goto out;
-		}
-		tty->disc_data = NULL;		/* ### */
-	}
-
 	/* stop the underlying  driver */
 	irtty_stop_receiver(tty, TRUE);
 	if (tty->ops->stop)
-- 
2.6.3


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