NFSv4: Fix buffer overflow checking in __nfs4_get_acl_uncached

Pass the checks made by decode_getacl back to __nfs4_get_acl_uncached
so that it knows if the acl has been truncated.

The current overflow checking is broken, resulting in Oopses on
user-triggered nfs4_getfacl calls, and is opaque to the point
where several attempts at fixing it have failed.
This patch tries to clean up the code in addition to fixing the
Oopses by ensuring that the overflow checks are performed in
a single place (decode_getacl). If the overflow check failed,
we will still be able to report the acl length, but at least
we will no longer attempt to cache the acl or copy the
truncated contents to user space.
Reported-by: Sachin Prabhu <sprabhu@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Tested-by: Sachin Prabhu <sprabhu@redhat.com>

fs/nfs/nfs4proc.c

@@ -3739,7 +3739,7 @@ static void nfs4_write_cached_acl(struct inode *inode, struct page **pages, size
 	struct nfs4_cached_acl *acl;
 	size_t buflen = sizeof(*acl) + acl_len;
 
-	if (pages && buflen <= PAGE_SIZE) {
+	if (buflen <= PAGE_SIZE) {
 		acl = kmalloc(buflen, GFP_KERNEL);
 		if (acl == NULL)
 			goto out;
@@ -3784,7 +3784,6 @@ static ssize_t __nfs4_get_acl_uncached(struct inode *inode, void *buf, size_t bu
 	};
 	unsigned int npages = DIV_ROUND_UP(buflen, PAGE_SIZE);
 	int ret = -ENOMEM, i;
-	size_t acl_len = 0;
 
 	/* As long as we're doing a round trip to the server anyway,
 	 * let's be prepared for a page of acl data. */
@@ -3807,11 +3806,6 @@ static ssize_t __nfs4_get_acl_uncached(struct inode *inode, void *buf, size_t bu
 	args.acl_len = npages * PAGE_SIZE;
 	args.acl_pgbase = 0;
 
-	/* Let decode_getfacl know not to fail if the ACL data is larger than
-	 * the page we send as a guess */
-	if (buf == NULL)
-		res.acl_flags |= NFS4_ACL_LEN_REQUEST;
-
 	dprintk("%s  buf %p buflen %zu npages %d args.acl_len %zu\n",
 		__func__, buf, buflen, npages, args.acl_len);
 	ret = nfs4_call_sync(NFS_SERVER(inode)->client, NFS_SERVER(inode),
@@ -3819,20 +3813,19 @@ static ssize_t __nfs4_get_acl_uncached(struct inode *inode, void *buf, size_t bu
 	if (ret)
 		goto out_free;
 
-	acl_len = res.acl_len;
-	if (acl_len > args.acl_len)
-		nfs4_write_cached_acl(inode, NULL, 0, acl_len);
-	else
-		nfs4_write_cached_acl(inode, pages, res.acl_data_offset,
-				      acl_len);
-	if (buf) {
+	/* Handle the case where the passed-in buffer is too short */
+	if (res.acl_flags & NFS4_ACL_TRUNC) {
+		/* Did the user only issue a request for the acl length? */
+		if (buf == NULL)
+			goto out_ok;
 		ret = -ERANGE;
-		if (acl_len > buflen)
 		goto out_free;
-		_copy_from_pages(buf, pages, res.acl_data_offset,
-				acl_len);
 	}
-	ret = acl_len;
+	nfs4_write_cached_acl(inode, pages, res.acl_data_offset, res.acl_len);
+	if (buf)
+		_copy_from_pages(buf, pages, res.acl_data_offset, res.acl_len);
+out_ok:
+	ret = res.acl_len;
 out_free:
 	for (i = 0; i < npages; i++)
 		if (pages[i])

fs/nfs/nfs4xdr.c

@@ -5072,18 +5072,14 @@ static int decode_getacl(struct xdr_stream *xdr, struct rpc_rqst *req,
 		 * are stored with the acl data to handle the problem of
 		 * variable length bitmaps.*/
 		res->acl_data_offset = xdr_stream_pos(xdr) - pg_offset;
-
-		/* We ignore &savep and don't do consistency checks on
-		 * the attr length.  Let userspace figure it out.... */
 		res->acl_len = attrlen;
-		if (attrlen > (xdr->nwords << 2)) {
-			if (res->acl_flags & NFS4_ACL_LEN_REQUEST) {
-				/* getxattr interface called with a NULL buf */
-				goto out;
-			}
+
+		/* Check for receive buffer overflow */
+		if (res->acl_len > (xdr->nwords << 2) ||
+		    res->acl_len + res->acl_data_offset > xdr->buf->page_len) {
+			res->acl_flags |= NFS4_ACL_TRUNC;
 			dprintk("NFS: acl reply: attrlen %u > page_len %u\n",
 					attrlen, xdr->nwords << 2);
-			return -EINVAL;
 		}
 	} else
 		status = -EOPNOTSUPP;

include/linux/nfs_xdr.h

@@ -652,7 +652,7 @@ struct nfs_getaclargs {
 };
 
 /* getxattr ACL interface flags */
-#define NFS4_ACL_LEN_REQUEST	0x0001	/* zero length getxattr buffer */
+#define NFS4_ACL_TRUNC		0x0001	/* ACL was truncated */
 struct nfs_getaclres {
 	size_t				acl_len;
 	size_t				acl_data_offset;