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

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

[Wenwen Wang]

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>
---
 net/socket.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/net/socket.c b/net/socket.c
index 01f3f8f..390a8ec 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -2875,9 +2875,14 @@ static int ethtool_ioctl(struct net *net, struct compat_ifreq __user *ifr32)
 		    copy_in_user(&rxnfc->fs.ring_cookie,
 				 &compat_rxnfc->fs.ring_cookie,
 				 (void __user *)(&rxnfc->fs.location + 1) -
-				 (void __user *)&rxnfc->fs.ring_cookie) ||
-		    copy_in_user(&rxnfc->rule_cnt, &compat_rxnfc->rule_cnt,
-				 sizeof(rxnfc->rule_cnt)))
+				 (void __user *)&rxnfc->fs.ring_cookie))
+			return -EFAULT;
+		if (ethcmd == ETHTOOL_GRXCLSRLALL) {
+			if (put_user(rule_cnt, &rxnfc->rule_cnt))
+				return -EFAULT;
+		} else if (copy_in_user(&rxnfc->rule_cnt,
+					&compat_rxnfc->rule_cnt,
+					sizeof(rxnfc->rule_cnt)))
 			return -EFAULT;
 	}
 
-- 
2.7.4

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

[David Miller]

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.

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

[Wenwen Wang]

Hi Florian,

Thanks for your response. The bug is found with the assistance of a
research prototype, which is now not available to the public.

Yes, this is a kind of time-of-check-to-time-of-use (TOCTTOU) bug.

BTW, could you please confirm this bug? Thanks!

Wenwen

On Sat, Oct 20, 2018 at 10:21 PM Florian Fainelli <f.fainelli@gmail.com> wrote:
>
> Hi Wenwen,
>
> On October 20, 2018 8:58:10 AM PDT, Wenwen Wang <wang6495@umn.edu> wrote:
> >In ethtool_ioctl(), the ioctl command is firstly obtained from the
> >user-space buffer 'compat_rxnfc' through get_user() and saved to
> >'ethcmd'.
> >Then, 'ethcmd' is checked to see whether it is necessary to pre-process
> >the
> >ethool structure, because the structure ethtool_rxnfc is defined with
> >padding, as mentioned in the comment. If yes, a user-space buffer
> >'rxnfc'
> >is allocated through compat_alloc_user_space() and then the data in the
> >original buffer 'compat_rxnfc' is copied to 'rxnfc' through
> >copy_in_user(),
> >including the ioctl command. It is worth noting that after this copy,
> >there
> >is no check enforced on the copied ioctl command. That means it is
> >possible
> >that 'rxnfc->cmd' is different from 'ethcmd', because a malicious user
> >can
> >race to modify the ioctl command in 'compat_rxnfc' between these two
> >copies. Eventually, the ioctl command in 'rxnfc' will be used in
> >dev_ethtool(). This can cause undefined behavior of the kernel and
> >introduce potential security risk.
> >
> >This patch avoids the above issue by rewriting 'rxnfc->cmd' using
> >'ethcmd'
> >after copy_in_user().
> >
> >Signed-off-by: Wenwen Wang <wang6495@umn.edu>
>
> Assuming these issues are found with some kind of automated analysis, can you also add in your work flow to provide a Fixes: tag such that this could be backported to stable kernels?
>
> If this is found by a tool is this something that is open source and somehow available? I would also make it clear that these issues are typically named time TOCTOU which might be clearer for people who review those patches.
>
> Thanks!
> --
> Florian

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

[Florian Fainelli]

Hi Wenwen,

On October 20, 2018 8:58:10 AM PDT, Wenwen Wang <wang6495@umn.edu> wrote:
>In ethtool_ioctl(), the ioctl command is firstly obtained from the
>user-space buffer 'compat_rxnfc' through get_user() and saved to
>'ethcmd'.
>Then, 'ethcmd' is checked to see whether it is necessary to pre-process
>the
>ethool structure, because the structure ethtool_rxnfc is defined with
>padding, as mentioned in the comment. If yes, a user-space buffer
>'rxnfc'
>is allocated through compat_alloc_user_space() and then the data in the
>original buffer 'compat_rxnfc' is copied to 'rxnfc' through
>copy_in_user(),
>including the ioctl command. It is worth noting that after this copy,
>there
>is no check enforced on the copied ioctl command. That means it is
>possible
>that 'rxnfc->cmd' is different from 'ethcmd', because a malicious user
>can
>race to modify the ioctl command in 'compat_rxnfc' between these two
>copies. Eventually, the ioctl command in 'rxnfc' will be used in
>dev_ethtool(). This can cause undefined behavior of the kernel and
>introduce potential security risk.
>
>This patch avoids the above issue by rewriting 'rxnfc->cmd' using
>'ethcmd'
>after copy_in_user().
>
>Signed-off-by: Wenwen Wang <wang6495@umn.edu>

Assuming these issues are found with some kind of automated analysis, can you also add in your work flow to provide a Fixes: tag such that this could be backported to stable kernels?

If this is found by a tool is this something that is open source and somehow available? I would also make it clear that these issues are typically named time TOCTOU which might be clearer for people who review those patches.

Thanks!
-- 
Florian

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

[Wenwen Wang]

In ethtool_ioctl(), the ioctl command is firstly obtained from the
user-space buffer 'compat_rxnfc' through get_user() and saved to 'ethcmd'.
Then, 'ethcmd' is checked to see whether it is necessary to pre-process the
ethool structure, because the structure ethtool_rxnfc is defined with
padding, as mentioned in the comment. If yes, a user-space buffer 'rxnfc'
is allocated through compat_alloc_user_space() and then the data in the
original buffer 'compat_rxnfc' is copied to 'rxnfc' through copy_in_user(),
including the ioctl command. It is worth noting that after this copy, there
is no check enforced on the copied ioctl command. That means it is possible
that 'rxnfc->cmd' is different from 'ethcmd', because a malicious user can
race to modify the ioctl command in 'compat_rxnfc' between these two
copies. Eventually, the ioctl command in 'rxnfc' will be used in
dev_ethtool(). This can cause undefined behavior of the kernel and
introduce potential security risk.

This patch avoids the above issue by rewriting 'rxnfc->cmd' using 'ethcmd'
after copy_in_user().

Signed-off-by: Wenwen Wang <wang6495@umn.edu>
---
 net/socket.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/socket.c b/net/socket.c
index 01f3f8f..c5f969c 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -2879,6 +2879,8 @@ static int ethtool_ioctl(struct net *net, struct compat_ifreq __user *ifr32)
 		    copy_in_user(&rxnfc->rule_cnt, &compat_rxnfc->rule_cnt,
 				 sizeof(rxnfc->rule_cnt)))
 			return -EFAULT;
+
+		rxnfc->cmd = ethcmd;
 	}
 
 	ret = dev_ioctl(net, SIOCETHTOOL, &ifr, NULL);
-- 
2.7.4