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Date: Sat, 5 Aug 2017 07:04:17 -0400
From: Brad Spengler <spender@...ecurity.net>
To: oss-security@...ts.openwall.com
Cc: fulldisclosure@...lists.org, bugtraq@...urityfocus.com
Subject: Re: [CVE-2017-7533] kernel: inotify: a race between
 inotify_handle_event() and sys_rename()

This bothers me, and since nobody else has bothered to ask, I'll chime in:
Can someone please clear up this timeline for me?

We have:
(nearly a month ago) 7/7/2017 Silent upstream fix for vulnerability, no 
stable cc, no backports performed, no distro fixes.

Possibly before this, possibly after (please clarify) there is some 
academic research regarding the vulnerability.  Did these researchers
report the vulnerability to security@...nel.org?

Possibly before this, possibly after (please clarify) there is an exploit 
"in the wild" but which I have not been able to find publicly easily.  
Does "in the wild" here mean it's being actively exploited?  When was the 
existence of this exploit first known?  Who discovered it?

Was this vulnerability embargoed?  Did the researchers request a one month
embargo?  For what reason?  Why are we hearing about a vulnerability
with an exploit "in the wild" a month after it was public knowledge to
attackers?

Thanks,
-Brad


On Thu, Aug 03, 2017 at 10:00:02AM -0400, Vladis Dronov wrote:
> Hello,
> 
> A race condition was found in Linux kernel present since v3.14-rc1 upto v4.12
> including. The race happens between threads of inotify_handle_event() and
> vfs_rename() while running the rename operation against the same file. The next
> slab data or the slab's free list pointer can be corrupted with attacker-controlled
> data as a result of the race.
> 
> The researchers of this flaw are Fan Wu and Shixiong Zhao from a research group
> supervised by Dr. Heming Cui of the Department of Computer Science, The University
> of Hong Kong. Thanks to Rui Gu and Prof. Junfeng Yang from Columbia University for
> tools and suggestions.
> 
> References:
> 
> https://bugzilla.redhat.com/show_bug.cgi?id=1468283
> 
> https://access.redhat.com/security/vulnerabilities/3112931
> 
> https://patchwork.kernel.org/patch/9755753/
> 
> https://patchwork.kernel.org/patch/9755757/
> 
> An upstream patch:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=49d31c2f389acfe83417083e1208422b4091cd9
> 
> So as for the flaw itself.
> 
> There is quite easily reached race condition between inotify_handle_event() and
> sys_rename(). A rename thread can change the dentry name before an fsnotify
> thread is copying the dentry name but after a memory for this is allocated:
> 
> CPU 1                          CPU 2
> 
> fsnotify()
>   inotify_handle_event(.., file_name)
>     strlen(file_name)  // file_name is "foobar"
>     alloc_len += len + 1;
>     event = kmalloc(alloc_len, GFP_KERNEL); // 7 bytes for the file_name
> 
>                                sys_rename()
>                                  __d_move() [in fs/dcache.c]
>                                    copy_name()
>                                    // rename to "foobar_lol_kek_u_pwned"
> 
>     strcpy(event->name, file_name);
>     // now file_name points to "foobar_lol_kek_u_pwned"
>     // but there is a space only for "foobar\0"
>     // the next slab or slab's *freelist is corrupted with user controlled data
> 
> There is a working exploit allowing privileges escalation in the wild for 32 bit
> kernels. We are unaware of such exploit for 64 bit kernels, but these are affected
> by this race too and we believe such an exploit could exist.
> 
> The result of exploiting the flaw is modified data after the slab, which can be
> the next slab data, freelist pointer or something else (if the slab is the last
> one in the cache).
> 
> The slab corruption caused by the exploit or the reproducer cat be easily seen
> with "slub_debug=FZ" kernel parameter. The following log indicates a write beyond
> the allocated slab, in this case a write to the slab's red zone:
> 
> [  144.109993] =============================================================================
> [  144.110011] BUG kmalloc-64 (Not tainted): Redzone overwritten
> [  144.110011] -----------------------------------------------------------------------------
> [  144.110011] Disabling lock debugging due to kernel taint
> [  144.110011] INFO: 0xffff8800bbb544f0-0xffff8800bbb544f7. First byte 0x33 instead of 0xcc
> [  144.110011] INFO: Slab 0xffffea0002eed500 objects=51 used=23 fp=0xffff8800bbb54d70 flags=0x5fffff00000081
> [  144.110011] INFO: Object 0xffff8800bbb544b0 @offset=1200 fp=0xffff8800bbb544b0
> [  144.110011]
> [  144.110011] Bytes b4 ffff8800bbb544a0: cc cc cc cc cc cc cc cc 00 00 00 00 00 00 00 00  ................
> [  144.110011] Object ffff8800bbb544b0: b0 44 b5 bb 00 88 ff ff b0 44 b5 bb 00 88 ff ff  .D.......D......
> [  144.110011] Object ffff8800bbb544c0: b8 78 62 bb 00 88 ff ff 20 00 00 08 00 00 00 00  .xb..... .......
> [  144.110011] Object ffff8800bbb544d0: 01 00 00 00 00 00 00 00 01 00 00 00 61 61 61 61  ............aaaa
> [  144.110011] Object ffff8800bbb544e0: 33 32 31 30 33 32 31 30 33 32 31 30 33 32 31 30  3210321032103210
> [  144.110011] Redzone ffff8800bbb544f0: 33 32 31 30 33 32 31 30                          32103210
> [  144.110011] Padding ffff8800bbb544f8: 00 00 00 00 00 00 00 00                          ........
> [  144.110011] CPU: 2 PID: 1016 Comm: inotify Tainted: G    B          ------------   3.10.0-514.16.1.el7.x86_64 #1
> [  144.110011] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.9.3-1.fc25 04/01/2014
> [  144.110011]  ffff88007d801d00 0000000070c5e4c2 ffff88013bad7c08 ffffffff816869c3
> [  144.110011]  ffff88013bad7c48 ffffffff811d9cad 0000000000000008 ffff880000000001
> [  144.110011]  ffff8800bbb544f8 ffff88007d801d00 00000000000000cc ffff8800bbb544b0
> [  144.110011] Call Trace:
> [  144.110011]  [<ffffffff816869c3>] dump_stack+0x19/0x1b
> [  144.110011]  [<ffffffff811d9cad>] print_trailer+0x14d/0x200
> [  144.110011]  [<ffffffff811d9e9f>] check_bytes_and_report+0xcf/0x110
> [  144.110011]  [<ffffffff811dab33>] check_object+0x193/0x250
> [  144.110011]  [<ffffffff8168380f>] free_debug_processing+0xcc/0x259
> [  144.110011]  [<ffffffff81213130>] ? poll_select_copy_remaining+0x150/0x150
> [  144.110011]  [<ffffffff81244f9e>] ? inotify_free_event+0xe/0x10
> [  144.110011]  [<ffffffff81244f9e>] ? inotify_free_event+0xe/0x10
> [  144.110011]  [<ffffffff811dca30>] __slab_free+0x250/0x2f0
> [  144.110011]  [<ffffffff81213130>] ? poll_select_copy_remaining+0x150/0x150
> [  144.110011]  [<ffffffff8168ba60>] ? __schedule+0x3b0/0x990
> [  144.110011]  [<ffffffff81244f9e>] ? inotify_free_event+0xe/0x10
> [  144.110011]  [<ffffffff811dd173>] kfree+0x103/0x140
> [  144.110011]  [<ffffffff81244f9e>] inotify_free_event+0xe/0x10
> [  144.110011]  [<ffffffff81242b30>] fsnotify_destroy_event+0x30/0x50
> [  144.110011]  [<ffffffff81245424>] inotify_read+0x224/0x3e0
> [  144.110011]  [<ffffffff810b1b20>] ? wake_up_atomic_t+0x30/0x30
> [  144.110011]  [<ffffffff811fe61e>] vfs_read+0x9e/0x170
> [  144.110011]  [<ffffffff811ff1ef>] SyS_read+0x7f/0xe0
> [  144.110011]  [<ffffffff81214954>] ? SyS_poll+0x74/0x110
> [  144.110011]  [<ffffffff81697089>] system_call_fastpath+0x16/0x1b
> [  144.110011] FIX kmalloc-64: Restoring 0xffff8800bbb544f0-0xffff8800bbb544f7=0xcc
> 
> Best regards,
> Vladis Dronov | Red Hat, Inc. | Product Security Engineer

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