Memory corruption in nfs3_xdr_setaclargs()

["Sachin S. Prabhu" <sprabhu@redhat.com>]

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A mistake in calculating the space left in the header in nfs3_xdr_setaclargs()
can cause memory corruption when setting a large number of acls.

Reproducer:

On Server:
1) Create directory /test and set mode 777.
mkdir /test; chmod 777 /test
2) Add 200 users and set default acl for user on /test
for i in {1..200}; do echo $i; useradd user$i; setfacl -m d:u:user$i:rwx
/test;done
3) Add export /test  in /etc/exports
/test *(rw)

On client
1) Mount server:/test
mount server:/test /mnt
2) Create large number of directories on the the share.
cd/mnt; for i in {1..1000}; do mkdir $i; done
At this point, the client should crash.

A change in call_header changes the value req->rq_snd_buf->head[0]->iov_len to
reflect the exact size of the header.
[PATCH] RPC: Ensure XDR iovec length is initialized correctly in call_header
334ccfd545bba9690515f2c5c167d5adb161989b

The iov_len is set to the size of the header in  call_header().
req->rq_slen = xdr_adjust_iovec(&req->rq_svec[0], p);

nfs3_xdr_setaclargs() depends on the older behavior and uses this value when
calculating the number of ACLs it can fit into the header.

        /* put as much of the acls into head as possible. */
        len_in_head = min_t(unsigned int, buf->head->iov_len - base, len);
        len -= len_in_head;
        req->rq_slen = xdr_adjust_iovec(req->rq_svec, p + (len_in_head >> 2));

Since at this stage, iov_len < base, len_in_head will always be set to len. For
a large number of ACLs, this will end up over-writing other parts of memory on
the nfs client.

The following patch which set len_in_head to 0 was tested with the reproducer
and was found to fix the problem.

--- fs/nfs/nfs3xdr.c.orig	2009-01-20 15:18:12.000000000 +0000
+++ fs/nfs/nfs3xdr.c	2009-01-20 15:33:45.000000000 +0000
@@ -691,7 +691,10 @@ nfs3_xdr_setaclargs(struct rpc_rqst *req
 	*p++ = htonl(args->mask);
 	base = (char *)p - (char *)buf->head->iov_base;
 	/* put as much of the acls into head as possible. */
-	len_in_head = min_t(unsigned int, buf->head->iov_len - base, len);
+	if ( buf->head->iov_len > base )
+		len_in_head = min_t(unsigned int, buf->head->iov_len - base, len);
+	else
+		len_in_head = 0;
 	len -= len_in_head;
 	req->rq_slen = xdr_adjust_iovec(req->rq_svec, p + (len_in_head >> 2));

Thanks to Kevin Rudd who did the major legwork here to figure the problem and
create the patch.

Sachin Prabhu


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