Re: net: socket: NULL ptr deref in sendmsg

From: Andrey Ryabinin
Date: Sat Jul 26 2014 - 11:49:04 EST


2014-07-26 2:15 GMT+04:00 Hannes Frederic Sowa <hannes@xxxxxxxxxxxxxxxxxxx>:
>
> Thanks for looking at this! I certainly have overlooked this case.
>
> I wonder, if we allow sendto with valid NULL pointer and positive
> msg_namelen to work, why don't we do the same for recvmsg, as in
> replacing the VERIFY_WRITE case non-null check with an
> access_ok(VERIFY_WRITE, ...) check? We can even get rid of the check
> because recvmsg will do it nonetheless when copying the sockaddrs to
> user space. So kern_msg->msg_name can always be set to kern_address.
>

recvmsg's man page are pretty clear about that:

"If the application does not need to
know the source address, msg_name can be specified as NULL"

I think we shouldn't change current recvmsg behavior, this could break
some crappy
userpace that doesn't set msg_namelen to zero.


> Otherwise I would just set msg_namelen = 0, too, and just not handle
> passed in NULL pointers to sockaddrs.
>

I like that, how about such chage:

diff --git a/net/compat.c b/net/compat.c
index 9a76eaf..bc8aeef 100644
--- a/net/compat.c
+++ b/net/compat.c
@@ -85,7 +85,7 @@ int verify_compat_iovec(struct msghdr *kern_msg,
struct iovec *kern_iov,
{
int tot_len;

- if (kern_msg->msg_namelen) {
+ if (kern_msg->msg_name && kern_msg->msg_namelen) {
if (mode == VERIFY_READ) {
int err = move_addr_to_kernel(kern_msg->msg_name,
kern_msg->msg_namelen,
@@ -93,10 +93,11 @@ int verify_compat_iovec(struct msghdr *kern_msg,
struct iovec *kern_iov,
if (err < 0)
return err;
}
- if (kern_msg->msg_name)
- kern_msg->msg_name = kern_address;
- } else
+ kern_msg->msg_name = kern_address;
+ } else {
kern_msg->msg_name = NULL;
+ kern_msg->msg_namelen = 0;
+ }

tot_len = iov_from_user_compat_to_kern(kern_iov,
(struct compat_iovec __user
*)kern_msg->msg_iov,
diff --git a/net/core/iovec.c b/net/core/iovec.c
index 827dd6b..e1ec45a 100644
--- a/net/core/iovec.c
+++ b/net/core/iovec.c
@@ -39,7 +39,7 @@ int verify_iovec(struct msghdr *m, struct iovec
*iov, struct sockaddr_storage *a
{
int size, ct, err;

- if (m->msg_namelen) {
+ if (m->msg_name && m->msg_namelen) {
if (mode == VERIFY_READ) {
void __user *namep;
namep = (void __user __force *) m->msg_name;
@@ -48,10 +48,10 @@ int verify_iovec(struct msghdr *m, struct iovec
*iov, struct sockaddr_storage *a
if (err < 0)
return err;
}
- if (m->msg_name)
- m->msg_name = address;
+ m->msg_name = address;
} else {
m->msg_name = NULL;
+ m->msg_namelen = 0;
}

size = m->msg_iovlen * sizeof(struct iovec);


--
Best regards,
Andrey Ryabinin
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/