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Subject[34-longterm 084/196] nfs4: Ensure that ACL pages sent over NFS were not allocated from the slab (v3)
Date
From: Neil Horman <nhorman@tuxdriver.com>

-------------------
This is a commit scheduled for the next v2.6.34 longterm release.
If you see a problem with using this for longterm, please comment.
-------------------
commit e9e3d724e2145f5039b423c290ce2b2c3d8f94bc upstream.

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 myself 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 triggerable by an
uprivlidged user, so I'm CCing security on this as well.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
CC: Trond Myklebust <Trond.Myklebust@netapp.com>
CC: security@kernel.org
CC: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
---
fs/nfs/nfs4proc.c | 44 ++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 42 insertions(+), 2 deletions(-)
diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
index 071fced..5b33856 100644
--- a/fs/nfs/nfs4proc.c
+++ b/fs/nfs/nfs4proc.c
@@ -49,6 +49,7 @@
#include <linux/mount.h>
#include <linux/module.h>
#include <linux/sunrpc/bc_xprt.h>
+#include <linux/mm.h>

#include "nfs4_fs.h"
#include "delegation.h"
@@ -3241,6 +3242,35 @@ static void buf_to_pages(const void *buf, size_t buflen,
}
}

+static int buf_to_pages_noslab(const void *buf, size_t buflen,
+ struct page **pages, unsigned int *pgbase)
+{
+ struct page *newpage, **spages;
+ int rc = 0;
+ size_t len;
+ spages = pages;
+
+ do {
+ len = min(PAGE_CACHE_SIZE, buflen);
+ newpage = alloc_page(GFP_KERNEL);
+
+ if (newpage == NULL)
+ goto unwind;
+ memcpy(page_address(newpage), buf, len);
+ buf += len;
+ buflen -= len;
+ *pages++ = newpage;
+ rc++;
+ } while (buflen != 0);
+
+ return rc;
+
+unwind:
+ for(; rc > 0; rc--)
+ __free_page(spages[rc-1]);
+ return -ENOMEM;
+}
+
struct nfs4_cached_acl {
int cached;
size_t len;
@@ -3407,13 +3437,23 @@ static int __nfs4_proc_set_acl(struct inode *inode, const void *buf, size_t bufl
.rpc_argp = &arg,
.rpc_resp = &res,
};
- int ret;
+ int ret, i;

if (!nfs4_server_supports_acls(server))
return -EOPNOTSUPP;
+ i = buf_to_pages_noslab(buf, buflen, arg.acl_pages, &arg.acl_pgbase);
+ if (i < 0)
+ return i;
nfs_inode_return_delegation(inode);
- buf_to_pages(buf, buflen, arg.acl_pages, &arg.acl_pgbase);
ret = nfs4_call_sync(server, &msg, &arg, &res, 1);
+
+ /*
+ * Free each page after tx, so the only ref left is
+ * held by the network stack
+ */
+ for (; i > 0; i--)
+ put_page(pages[i-1]);
+
nfs_access_zap_cache(inode);
nfs_zap_acl_cache(inode);
return ret;
--
1.7.9.3


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