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Subject[PATCH 5/5] net: Block MSG_CMSG_COMPAT in send(m)msg and recv(m)msg
Date
MSG_CMSG_COMPAT is (AFAIK) not intended to be part of the API --
it's a hack that steals a bit to indicate to other networking code
that a compat entry was used. So don't allow it from a non-compat
syscall.

This prevents an oops when running this code:

int main()
{
int s;
struct sockaddr_in addr;
struct msghdr *hdr;

char *highpage = mmap((void*)(TASK_SIZE_MAX - 4096), 4096,
PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED, -1, 0);
if (highpage == MAP_FAILED)
err(1, "mmap");

s = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
if (s == -1)
err(1, "socket");

addr.sin_family = AF_INET;
addr.sin_port = htons(1);
addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
if (connect(s, (struct sockaddr*)&addr, sizeof(addr)) != 0)
err(1, "connect");

void *evil = highpage + 4096 - COMPAT_MSGHDR_SIZE;
printf("Evil address is %p\n", evil);

if (syscall(__NR_sendmmsg, s, evil, 1, MSG_CMSG_COMPAT) < 0)
err(1, "sendmmsg");

return 0;
}

Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---
net/socket.c | 33 +++++++++++++++++++++++++++++++--
1 file changed, 31 insertions(+), 2 deletions(-)

diff --git a/net/socket.c b/net/socket.c
index 88f759a..0e16888 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -2097,8 +2097,12 @@ SYSCALL_DEFINE3(sendmsg, int, fd, struct msghdr __user *, msg, unsigned int, fla
{
int fput_needed, err;
struct msghdr msg_sys;
- struct socket *sock = sockfd_lookup_light(fd, &err, &fput_needed);
+ struct socket *sock;
+
+ if (flags & MSG_CMSG_COMPAT)
+ return -EINVAL;

+ sock = sockfd_lookup_light(fd, &err, &fput_needed);
if (!sock)
goto out;

@@ -2171,6 +2175,8 @@ int __sys_sendmmsg(int fd, struct mmsghdr __user *mmsg, unsigned int vlen,
SYSCALL_DEFINE4(sendmmsg, int, fd, struct mmsghdr __user *, mmsg,
unsigned int, vlen, unsigned int, flags)
{
+ if (flags & MSG_CMSG_COMPAT)
+ return -EINVAL;
return __sys_sendmmsg(fd, mmsg, vlen, flags);
}

@@ -2271,8 +2277,12 @@ SYSCALL_DEFINE3(recvmsg, int, fd, struct msghdr __user *, msg,
{
int fput_needed, err;
struct msghdr msg_sys;
- struct socket *sock = sockfd_lookup_light(fd, &err, &fput_needed);
+ struct socket *sock;
+
+ if (flags & MSG_CMSG_COMPAT)
+ return -EINVAL;

+ sock = sockfd_lookup_light(fd, &err, &fput_needed);
if (!sock)
goto out;

@@ -2397,6 +2407,9 @@ SYSCALL_DEFINE5(recvmmsg, int, fd, struct mmsghdr __user *, mmsg,
int datagrams;
struct timespec timeout_sys;

+ if (flags & MSG_CMSG_COMPAT)
+ return -EINVAL;
+
if (!timeout)
return __sys_recvmmsg(fd, mmsg, vlen, flags, NULL);

@@ -2512,15 +2525,31 @@ SYSCALL_DEFINE2(socketcall, int, call, unsigned long __user *, args)
(int __user *)a[4]);
break;
case SYS_SENDMSG:
+ if (a[2] & MSG_CMSG_COMPAT) {
+ err = -EINVAL;
+ break;
+ }
err = sys_sendmsg(a0, (struct msghdr __user *)a1, a[2]);
break;
case SYS_SENDMMSG:
+ if (a[3] & MSG_CMSG_COMPAT) {
+ err = -EINVAL;
+ break;
+ }
err = sys_sendmmsg(a0, (struct mmsghdr __user *)a1, a[2], a[3]);
break;
case SYS_RECVMSG:
+ if (a[2] & MSG_CMSG_COMPAT) {
+ err = -EINVAL;
+ break;
+ }
err = sys_recvmsg(a0, (struct msghdr __user *)a1, a[2]);
break;
case SYS_RECVMMSG:
+ if (a[3] & MSG_CMSG_COMPAT) {
+ err = -EINVAL;
+ break;
+ }
err = sys_recvmmsg(a0, (struct mmsghdr __user *)a1, a[2], a[3],
(struct timespec __user *)a[4]);
break;
--
1.8.1.4


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