Follow @Openwall on Twitter for new release announcements and other news
[<prev] [next>] [thread-next>] [day] [month] [year] [list]
Date: Mon, 07 Mar 2011 13:47:50 +0800
From: Eugene Teo <eugene@...hat.com>
To: oss-security@...ts.openwall.com
CC: "Steven M. Christey" <coley@...us.mitre.org>
Subject: CVE request - kernel: nfs4: Ensure that ACL pages sent over NFS were
 not allocated from the slab

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=e9e3d724e2145f5039b423c290ce2b2c3d8f94bc

The "bad_page()" page allocator sanity check was reported recently (call
chain as follows):

   bad_page+0x69/0x91
   free_hot_cold_page+0x81/0x144
   skb_release_data+0x5f/0x98
   __kfree_skb+0x11/0x1a
   tcp_ack+0x6a3/0x1868
   tcp_rcv_established+0x7a6/0x8b9
   tcp_v4_do_rcv+0x2a/0x2fa
   tcp_v4_rcv+0x9a2/0x9f6
   do_timer+0x2df/0x52c
   ip_local_deliver+0x19d/0x263
   ip_rcv+0x539/0x57c
   netif_receive_skb+0x470/0x49f
   :virtio_net:virtnet_poll+0x46b/0x5c5
   net_rx_action+0xac/0x1b3
   __do_softirq+0x89/0x133
   call_softirq+0x1c/0x28
   do_softirq+0x2c/0x7d
   do_IRQ+0xec/0xf5
   default_idle+0x0/0x50
   ret_from_intr+0x0/0xa
   default_idle+0x29/0x50
   cpu_idle+0x95/0xb8
   start_kernel+0x220/0x225
   _sinittext+0x22f/0x236

It occurs because an skb with a fraglist was freed from the tcp
retransmit queue when it was acked, but a page on that fraglist had
PG_Slab set (indicating it was allocated from the Slab allocator (which
means the free path above can't safely free it via put_page.

We tracked this back to an nfsv4 setacl operation, in which the nfs code
attempted to fill convert the passed in buffer to an array of pages in
__nfs4_proc_set_acl, which gets used by the skb->frags list in
xs_sendpages.  __nfs4_proc_set_acl just converts each page in the buffer
to a page struct via virt_to_page, but the vfs allocates the buffer via
kmalloc, meaning the PG_slab bit is set.  We can't create a buffer with
kmalloc and free it later in the tcp ack path with put_page, so we need
to either:

1) ensure that when we create the list of pages, no page struct has
    PG_Slab set

  or

2) not use a page list to send this data

Given that these buffers can be multiple pages and arbitrarily sized, I
think (1) is the right way to go.  I've written the below patch to
allocate a page from the buddy allocator directly and copy the data over
to it.  This ensures that we have a put_page free-able page for every
entry that winds up on an skb frag list, so it can be safely freed when
the frame is acked.  We do a put page on each entry after the
rpc_call_sync call so as to drop our own reference count to the page,
leaving only the ref count taken by tcp_sendpages.  This way the data
will be properly freed when the ack comes in

Successfully tested by [Neil Horman] to solve the above oops.

Note, as this is the result of a setacl operation that exceeded a page
of data, I think this amounts to a local DOS trigger-able by an
privileged user, so [Neil Horman] CCing security on this as well.

Thanks, Eugene
-- 
Eugene Teo / Red Hat Security Response Team

Powered by blists - more mailing lists

Please check out the Open Source Software Security Wiki, which is counterpart to this mailing list.

Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.