rpc: fix xdr_truncate_encode to handle buffer ending on page boundary

A struct xdr_stream at a page boundary might point to the end of one
page or the beginning of the next, but xdr_truncate_encode isn't
prepared to handle the former.

This can cause corruption of NFSv4 READDIR replies in the case that a
readdir entry that would have exceeded the client's dircount/maxcount
limit would have ended exactly on a 4k page boundary.  You're more
likely to hit this case on large directories.

Other xdr_truncate_encode callers are probably also affected.

Reported-by: Holger Hoffstätte <holger.hoffstaette@googlemail.com>
Tested-by: Holger Hoffstätte <holger.hoffstaette@googlemail.com>
Fixes: 3e19ce762b53 "rpc: xdr_truncate_encode"
Cc: stable@vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

1 files changed, 3 insertions, 3 deletions

diff --git a/net/sunrpc/xdr.c b/net/sunrpc/xdr.c
index 1cb61242e55e..4439ac4c1b53 100644
--- a/net/sunrpc/xdr.c
+++ b/net/sunrpc/xdr.c
@@ -606,7 +606,7 @@ void xdr_truncate_encode(struct xdr_stream *xdr, size_t len)
         struct kvec *head = buf->head;
         struct kvec *tail = buf->tail;
         int fraglen;
-        int new, old;
+        int new;
 
         if (len > buf->len) {
                 WARN_ON_ONCE(1);
@@ -629,8 +629,8 @@ void xdr_truncate_encode(struct xdr_stream *xdr, size_t len)
         buf->len -= fraglen;
 
         new = buf->page_base + buf->page_len;
-        old = new + fraglen;
-        xdr->page_ptr -= (old >> PAGE_SHIFT) - (new >> PAGE_SHIFT);
+
+        xdr->page_ptr = buf->pages + (new >> PAGE_SHIFT);
 
         if (buf->page_len) {
                 xdr->p = page_address(*xdr->page_ptr);