[PATCH 4.19 37/64] um: ubd: Prevent buffer overrun on command completion

[Greg Kroah-Hartman]

From: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx>

[ Upstream commit 6e682d53fc1ef73a169e2a5300326cb23abb32ee ]

On the hypervisor side, when completing commands and the pipe is full,
we retry writing only the entries that failed, by offsetting
io_req_buffer, but we don't reduce the number of bytes written, which
can cause a buffer overrun of io_req_buffer, and write garbage to the
pipe.

Cc: Martyn Welch <martyn.welch@xxxxxxxxxxxxx>
Signed-off-by: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx>
Signed-off-by: Richard Weinberger <richard@xxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 arch/um/drivers/ubd_kern.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/um/drivers/ubd_kern.c b/arch/um/drivers/ubd_kern.c
index 83c470364dfb3..748bd0921dfff 100644
--- a/arch/um/drivers/ubd_kern.c
+++ b/arch/um/drivers/ubd_kern.c
@@ -1574,7 +1574,9 @@ int io_thread(void *arg)
 		written = 0;
 
 		do {
-			res = os_write_file(kernel_fd, ((char *) io_req_buffer) + written, n);
+			res = os_write_file(kernel_fd,
+					    ((char *) io_req_buffer) + written,
+					    n - written);
 			if (res >= 0) {
 				written += res;
 			} else {
-- 
2.20.1