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net: compat: assert the size of cmsg copied in is as expected 

The actual length of cmsg fetched in during the second loop
(i.e., kcmsg - kcmsg_base) could be different from what we
get from the first loop (i.e., kcmlen).

The main reason is that the two get_user() calls in the two
loops (i.e., get_user(ucmlen, &ucmsg->cmsg_len) and
__get_user(ucmlen, &ucmsg->cmsg_len)) could cause ucmlen
to have different values even they fetch from the same userspace
address, as user can race to change the memory content in
&ucmsg->cmsg_len across fetches.

Although in the second loop, the sanity check
if ((char *)kcmsg_base + kcmlen - (char *)kcmsg < CMSG_ALIGN(tmp))
is inplace, it only ensures that the cmsg fetched in during the
second loop does not exceed the length of kcmlen, but not
necessarily equal to kcmlen. But indicated by the assignment
kmsg->msg_controllen = kcmlen, we should enforce that.

This patch adds this additional sanity check and ensures that
what is recorded in kmsg->msg_controllen is the actual cmsg length.

Signed-off-by: Meng Xu <mengxu.gatech@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Meng Xu 2017-09-19 13:19:13 -04:00 committed by David S. Miller
parent ec9dd352d5
commit c2a64bb9fc
1 changed files with 7 additions and 0 deletions
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7
net/compat.c
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@ -185,6 +185,13 @@ int cmsghdr_from_user_compat_to_kern(struct msghdr *kmsg, struct sock *sk,
ucmsg = cmsg_compat_nxthdr(kmsg, ucmsg, ucmlen);
}
/*
* check the length of messages copied in is the same as the
* what we get from the first loop
*/
if ((char *)kcmsg - (char *)kcmsg_base != kcmlen)
goto Einval;
/* Ok, looks like we made it. Hook it up and return success. */
kmsg->msg_control = kcmsg_base;
kmsg->msg_controllen = kcmlen;