[PATCH 5.15 148/530] SUNRPC: Fix svcxdr_init_encodes buflen calculation

[Greg Kroah-Hartman]

From: Chuck Lever <chuck.lever@xxxxxxxxxx>

[ Upstream commit 1242a87da0d8cd2a428e96ca68e7ea899b0f4624 ]

Commit 2825a7f90753 ("nfsd4: allow encoding across page boundaries")
added an explicit computation of the remaining length in the rq_res
XDR buffer.

The computation appears to suffer from an "off-by-one" bug. Because
buflen is too large by one page, XDR encoding can run off the end of
the send buffer by eventually trying to use the struct page address
in rq_page_end, which always contains NULL.

Fixes: bddfdbcddbe2 ("NFSD: Extract the svcxdr_init_encode() helper")
Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 include/linux/sunrpc/svc.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
index 6be55d0e73fd..045f34add206 100644
--- a/include/linux/sunrpc/svc.h
+++ b/include/linux/sunrpc/svc.h
@@ -609,7 +609,7 @@ static inline void svcxdr_init_encode(struct svc_rqst *rqstp)
 	xdr->end = resv->iov_base + PAGE_SIZE - rqstp->rq_auth_slack;
 	buf->len = resv->iov_len;
 	xdr->page_ptr = buf->pages - 1;
-	buf->buflen = PAGE_SIZE * (1 + rqstp->rq_page_end - buf->pages);
+	buf->buflen = PAGE_SIZE * (rqstp->rq_page_end - buf->pages);
 	buf->buflen -= rqstp->rq_auth_slack;
 	xdr->rqst = NULL;
 }
-- 
2.35.1