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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

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