nfs4: Ensure that ACL pages sent over NFS were not allocated from the slab (v3)

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>

1 files changed, 42 insertions, 2 deletions

diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
index 78936a8f40ab..1ff76acc7e98 100644
--- a/fs/nfs/nfs4proc.c
+++ b/fs/nfs/nfs4proc.c
@@ -51,6 +51,7 @@
 #include <linux/sunrpc/bc_xprt.h>
 #include <linux/xattr.h>
 #include <linux/utsname.h>
+#include <linux/mm.h>
 
 #include "nfs4_fs.h"
 #include "delegation.h"
@@ -3252,6 +3253,35 @@ 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;
@@ -3420,13 +3450,23 @@ 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]);
+
         /*
          * Acl update can result in inode attribute update.
          * so mark the attribute cache invalid.