Re: [PATCH] net: socket: fix a missing-check bug

[David Miller <davem@davemloft.net>]

From: Wenwen Wang <wang6495@umn.edu>
Date: Thu, 18 Oct 2018 09:36:46 -0500

> In ethtool_ioctl(), the ioctl command 'ethcmd' is checked through a switch
> statement to see whether it is necessary to pre-process the ethtool
> structure, because, as mentioned in the comment, the structure
> ethtool_rxnfc is defined with padding. If yes, a user-space buffer 'rxnfc'
> is allocated through compat_alloc_user_space(). One thing to note here is
> that, if 'ethcmd' is ETHTOOL_GRXCLSRLALL, the size of the buffer 'rxnfc' is
> partially determined by 'rule_cnt', which is actually acquired from the
> user-space buffer 'compat_rxnfc', i.e., 'compat_rxnfc->rule_cnt', through
> get_user(). After 'rxnfc' is allocated, the data in the original user-space
> buffer 'compat_rxnfc' is then copied to 'rxnfc' through copy_in_user(),
> including the 'rule_cnt' field. However, after this copy, no check is
> re-enforced on 'rxnfc->rule_cnt'. So it is possible that a malicious user
> race to change the value in the 'compat_rxnfc->rule_cnt' between these two
> copies. Through this way, the attacker can bypass the previous check on
> 'rule_cnt' and inject malicious data. This can cause undefined behavior of
> the kernel and introduce potential security risk.
> 
> This patch avoids the above issue via copying the value acquired by
> get_user() to 'rxnfc->rule_cn', if 'ethcmd' is ETHTOOL_GRXCLSRLALL.
> 
> Signed-off-by: Wenwen Wang <wang6495@umn.edu>

This isn't pretty, but I can't come up with a better fix.

Note that we check and validate the rule count value even a third time
when we copy the rules back out to userspace.

Applied and queued up for -stable, thank you.