[PATCH nf] xtables: zero padding in data_to_user

[PATCH nf] xtables: zero padding in data_to_user

[Willem de Bruijn]

From: Willem de Bruijn <willemb@google.com>

When looking up an iptables rule, the iptables binary compares the
aligned match and target data (XT_ALIGN). In some cases this can
exceed the actual data size to include padding bytes.

Before commit f77bc5b23fb1 ("iptables: use match, target and data
copy_to_user helpers") the malloc()ed bytes were overwritten by the
kernel with kzalloced contents, zeroing the padding and making the
comparison succeed. After this patch, the kernel copies and clears
only data, leaving the padding bytes undefined.

Extend the clear operation from data size to aligned data size to
include the padding bytes, if any.

Padding bytes can be observed in both match and target, and the bug
triggered, by issuing a rule with match icmp and target ACCEPT:

  iptables -t mangle -A INPUT -i lo -p icmp --icmp-type 1 -j ACCEPT
  iptables -t mangle -D INPUT -i lo -p icmp --icmp-type 1 -j ACCEPT

Fixes: f77bc5b23fb1 ("iptables: use match, target and data copy_to_user helpers")
Reported-by: Paul Moore <pmoore@redhat.com>
Reported-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
---
 include/linux/netfilter/x_tables.h | 2 +-
 net/bridge/netfilter/ebtables.c    | 9 ++++++---
 net/netfilter/x_tables.c           | 9 ++++++---
 3 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/include/linux/netfilter/x_tables.h b/include/linux/netfilter/x_tables.h
index be378cf47fcc..b3044c2c62cb 100644
--- a/include/linux/netfilter/x_tables.h
+++ b/include/linux/netfilter/x_tables.h
@@ -294,7 +294,7 @@ int xt_match_to_user(const struct xt_entry_match *m,
 int xt_target_to_user(const struct xt_entry_target *t,
 		      struct xt_entry_target __user *u);
 int xt_data_to_user(void __user *dst, const void *src,
-		    int usersize, int size);
+		    int usersize, int size, int aligned_size);
 
 void *xt_copy_counters_from_user(const void __user *user, unsigned int len,
 				 struct xt_counters_info *info, bool compat);
diff --git a/net/bridge/netfilter/ebtables.c b/net/bridge/netfilter/ebtables.c
index 9ec0c9f908fa..9c6e619f452b 100644
--- a/net/bridge/netfilter/ebtables.c
+++ b/net/bridge/netfilter/ebtables.c
@@ -1373,7 +1373,8 @@ static inline int ebt_obj_to_user(char __user *um, const char *_name,
 	strlcpy(name, _name, sizeof(name));
 	if (copy_to_user(um, name, EBT_FUNCTION_MAXNAMELEN) ||
 	    put_user(datasize, (int __user *)(um + EBT_FUNCTION_MAXNAMELEN)) ||
-	    xt_data_to_user(um + entrysize, data, usersize, datasize))
+	    xt_data_to_user(um + entrysize, data, usersize, datasize,
+			    XT_ALIGN(datasize)))
 		return -EFAULT;
 
 	return 0;
@@ -1658,7 +1659,8 @@ static int compat_match_to_user(struct ebt_entry_match *m, void __user **dstptr,
 		if (match->compat_to_user(cm->data, m->data))
 			return -EFAULT;
 	} else {
-		if (xt_data_to_user(cm->data, m->data, match->usersize, msize))
+		if (xt_data_to_user(cm->data, m->data, match->usersize, msize,
+				    COMPAT_XT_ALIGN(msize)))
 			return -EFAULT;
 	}
 
@@ -1687,7 +1689,8 @@ static int compat_target_to_user(struct ebt_entry_target *t,
 		if (target->compat_to_user(cm->data, t->data))
 			return -EFAULT;
 	} else {
-		if (xt_data_to_user(cm->data, t->data, target->usersize, tsize))
+		if (xt_data_to_user(cm->data, t->data, target->usersize, tsize,
+				    COMPAT_XT_ALIGN(tsize)))
 			return -EFAULT;
 	}
 
diff --git a/net/netfilter/x_tables.c b/net/netfilter/x_tables.c
index f134d384852f..c64716a735b0 100644
--- a/net/netfilter/x_tables.c
+++ b/net/netfilter/x_tables.c
@@ -283,12 +283,13 @@ static int xt_obj_to_user(u16 __user *psize, u16 size,
 		       &U->u.user.revision, K->u.kernel.TYPE->revision)
 
 int xt_data_to_user(void __user *dst, const void *src,
-		    int usersize, int size)
+		    int usersize, int size, int aligned_size)
 {
 	usersize = usersize ? : size;
 	if (copy_to_user(dst, src, usersize))
 		return -EFAULT;
-	if (usersize != size && clear_user(dst + usersize, size - usersize))
+	if (usersize != aligned_size &&
+	    clear_user(dst + usersize, aligned_size - usersize))
 		return -EFAULT;
 
 	return 0;
@@ -298,7 +299,9 @@ EXPORT_SYMBOL_GPL(xt_data_to_user);
 #define XT_DATA_TO_USER(U, K, TYPE, C_SIZE)				\
 	xt_data_to_user(U->data, K->data,				\
 			K->u.kernel.TYPE->usersize,			\
-			C_SIZE ? : K->u.kernel.TYPE->TYPE##size)
+			C_SIZE ? : K->u.kernel.TYPE->TYPE##size,	\
+			C_SIZE ? COMPAT_XT_ALIGN(C_SIZE) :		\
+				 XT_ALIGN(K->u.kernel.TYPE->TYPE##size))
 
 int xt_match_to_user(const struct xt_entry_match *m,
 		     struct xt_entry_match __user *u)
-- 
2.13.0.rc2.291.g57267f2277-goog

Re: [PATCH nf] xtables: zero padding in data_to_user

[Pablo Neira Ayuso]

On Tue, May 09, 2017 at 04:17:37PM -0400, Willem de Bruijn wrote:
> From: Willem de Bruijn <willemb@google.com>
> 
> When looking up an iptables rule, the iptables binary compares the
> aligned match and target data (XT_ALIGN). In some cases this can
> exceed the actual data size to include padding bytes.
> 
> Before commit f77bc5b23fb1 ("iptables: use match, target and data
> copy_to_user helpers") the malloc()ed bytes were overwritten by the
> kernel with kzalloced contents, zeroing the padding and making the
> comparison succeed. After this patch, the kernel copies and clears
> only data, leaving the padding bytes undefined.
> 
> Extend the clear operation from data size to aligned data size to
> include the padding bytes, if any.
> 
> Padding bytes can be observed in both match and target, and the bug
> triggered, by issuing a rule with match icmp and target ACCEPT:
> 
>   iptables -t mangle -A INPUT -i lo -p icmp --icmp-type 1 -j ACCEPT
>   iptables -t mangle -D INPUT -i lo -p icmp --icmp-type 1 -j ACCEPT

Applied, thanks.