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[Qemu-devel] [PATCH] Zero out the host's `msg_control` buffer

From: jonasschievink
Subject: [Qemu-devel] [PATCH] Zero out the host's `msg_control` buffer
Date: Wed, 11 Jul 2018 00:20:27 +0200

From: Jonas Schievink <address@hidden>

If this is not done, qemu would drop any control message after the first

This is because glibc's `CMSG_NXTHDR` macro accesses the uninitialized
cmsghdr's length field in order to find out if the message fits into the
`msg_control` buffer. This is fine for completed messages we receive
from the kernel, but is - as far as I know - not needed since the
kernel won't return such an invalid cmsghdr in the first place.

It's probably also a good idea to bail with an error if `CMSG_NXTHDR`
returns NULL but `TARGET_CMSG_NXTHDR` doesn't (ie. we still expect
 linux-user/syscall.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index e4b1b7d7da..77ce173b27 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -3845,6 +3845,8 @@ static abi_long do_sendrecvmsg_locked(int fd, struct 
target_msghdr *msgp,
     msg.msg_control = alloca(msg.msg_controllen);
     msg.msg_flags = tswap32(msgp->msg_flags);
+    memset(msg.msg_control, 0, msg.msg_controllen);
     count = tswapal(msgp->msg_iovlen);
     target_vec = tswapal(msgp->msg_iov);

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