[PATCH] fib_trie: remove potential out of bound access

[PATCH] fib_trie: remove potential out of bound access

[Eric Dumazet]

From: Eric Dumazet <edumazet@google.com>

AddressSanitizer [1] dynamic checker pointed a potential
out of bound access in leaf_walk_rcu()

We could allocate one more slot in tnode_new() to leave the prefetch()
in-place but it looks not worth the pain.

Bug added in commit 82cfbb008572b ("[IPV4] fib_trie: iterator recode")

[1] :
https://code.google.com/p/address-sanitizer/wiki/AddressSanitizerForKernel

Reported-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
---
 net/ipv4/fib_trie.c |    5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
index 108a1e9c..3df6d3e 100644
--- a/net/ipv4/fib_trie.c
+++ b/net/ipv4/fib_trie.c
@@ -71,7 +71,6 @@
 #include <linux/init.h>
 #include <linux/list.h>
 #include <linux/slab.h>
-#include <linux/prefetch.h>
 #include <linux/export.h>
 #include <net/net_namespace.h>
 #include <net/ip.h>
@@ -1761,10 +1760,8 @@ static struct leaf *leaf_walk_rcu(struct tnode *p, struct rt_trie_node *c)
 			if (!c)
 				continue;
 
-			if (IS_LEAF(c)) {
-				prefetch(rcu_dereference_rtnl(p->child[idx]));
+			if (IS_LEAF(c))
 				return (struct leaf *) c;
-			}
 
 			/* Rescan start scanning in new node */
 			p = (struct tnode *) c;

Re: [PATCH] fib_trie: remove potential out of bound access

[David Miller]

From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Mon, 05 Aug 2013 11:18:49 -0700

> From: Eric Dumazet <edumazet@google.com>
> 
> AddressSanitizer [1] dynamic checker pointed a potential
> out of bound access in leaf_walk_rcu()
> 
> We could allocate one more slot in tnode_new() to leave the prefetch()
> in-place but it looks not worth the pain.
> 
> Bug added in commit 82cfbb008572b ("[IPV4] fib_trie: iterator recode")
> 
> [1] :
> https://code.google.com/p/address-sanitizer/wiki/AddressSanitizerForKernel
> 
> Reported-by: Andrey Konovalov <andreyknvl@google.com>
> Signed-off-by: Eric Dumazet <edumazet@google.com>

I question the validity of the prefetch anyways, even without the
out-of-bounds concerns.

Applied and queued up for -stable, thanks Eric.

Re: [PATCH] fib_trie: remove potential out of bound access

[Stephen Hemminger]

On Mon, 05 Aug 2013 11:18:49 -0700
Eric Dumazet <eric.dumazet@gmail.com> wrote:

> From: Eric Dumazet <edumazet@google.com>
> 
> AddressSanitizer [1] dynamic checker pointed a potential
> out of bound access in leaf_walk_rcu()
> 
> We could allocate one more slot in tnode_new() to leave the prefetch()
> in-place but it looks not worth the pain.
> 
> Bug added in commit 82cfbb008572b ("[IPV4] fib_trie: iterator recode")
> 
> [1] :
> https://code.google.com/p/address-sanitizer/wiki/AddressSanitizerForKernel
> 
> Reported-by: Andrey Konovalov <andreyknvl@google.com>
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>

Isn't prefetch supposed to always be safe, even out of bounds; even prefetch(NULL).
Although I really doubt prefetch helps in in this code anyway.

Re: [PATCH] fib_trie: remove potential out of bound access

[Eric Dumazet]

On Mon, 2013-08-05 at 15:41 -0700, Stephen Hemminger wrote:
> On Mon, 05 Aug 2013 11:18:49 -0700
> Eric Dumazet <eric.dumazet@gmail.com> wrote:
> 
> > From: Eric Dumazet <edumazet@google.com>
> > 
> > AddressSanitizer [1] dynamic checker pointed a potential
> > out of bound access in leaf_walk_rcu()
> > 
> > We could allocate one more slot in tnode_new() to leave the prefetch()
> > in-place but it looks not worth the pain.
> > 
> > Bug added in commit 82cfbb008572b ("[IPV4] fib_trie: iterator recode")
> > 
> > [1] :
> > https://code.google.com/p/address-sanitizer/wiki/AddressSanitizerForKernel
> > 
> > Reported-by: Andrey Konovalov <andreyknvl@google.com>
> > Signed-off-by: Eric Dumazet <edumazet@google.com>
> > Cc: Dmitry Vyukov <dvyukov@google.com>
> 
> Isn't prefetch supposed to always be safe, even out of bounds; even prefetch(NULL).
> Although I really doubt prefetch helps in in this code anyway.


prefetch(...) was not the problem here.

The problem was X = array[N]   with N being >= size(array)