From: Greg KH

Sent: 13 September 2020 07:14
On Sun, Sep 13, 2020 at 11:26:39AM +0530, Anant Thazhemadam wrote:

quoted

The crash report showed that there was a local variable;

----iovstack.i@__sys_sendmmsg created at:
 ___sys_sendmsg net/socket.c:2388 [inline]
 __sys_sendmmsg+0x6db/0xc90 net/socket.c:2480

 that was left uninitialized.

The contents of iovstack are of interest, since the respective pointer
is passed down as an argument to sendmsg_copy_msghdr as well.
Initializing this contents of this stack prevents this bug from happening.

Since the memory that was initialized is freed at the end of the function
call, memory leaks are not likely to be an issue.

syzbot seems to have triggered this error by passing an array of 0's as
a parameter while making the initial system call.

Reported-by: syzbot+09a5d591c1f98cf5efcb@syzkaller.appspotmail.com
Tested-by: syzbot+09a5d591c1f98cf5efcb@syzkaller.appspotmail.com
Signed-off-by: Anant Thazhemadam <redacted>
---
 net/socket.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/socket.c b/net/socket.c
index 0c0144604f81..d74443dfd73b 100644
--- a/net/socket.c
+++ b/net/socket.c

@@ -2396,6 +2396,7 @@ static int ___sys_sendmsg(struct socket *sock, struct user_msghdr __user *msg,
 {
 	struct sockaddr_storage address;
 	struct iovec iovstack[UIO_FASTIOV], *iov = iovstack;
+	memset(iov, 0, UIO_FASTIOV);
 	ssize_t err;

 	msg_sys->msg_name = &address;

I don't think you built this code change, otherwise you would have seen
that it adds a build warning to the system, right?

Also it can't be the right 'fix' for whatever sysbot found.
(I can't find the sysbot report.)

Zeroing iov[] just slows down a path that is already too slow because
of the contorted functions used to read in iov[].

If it does need to be zerod then it would be needed in a lot
of other code paths that read in iov[].

If a zero length iov[] needs converting into a single entity
with a zero length - then that needs to be done elsewhere.

I've a patch series I might redo that changes the code that
reads in iov[] to return the address of any buffer that
needed to be malloced (more than UIV_FASTIO buffers) rather
than using the iov parameter to pass in the cache and
return the buffer to free.
It would be less confusing and error prone.

	David

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