netdev - Re: [PATCH net-next v4 2/9] net: filter: keep original BPF program around

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Date:	Thu, 11 Sep 2014 20:27:37 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Daniel Borkmann <dborkman@...hat.com>
Cc:	davem@...emloft.net, ast@...mgrid.com, netdev@...r.kernel.org,
	Pavel Emelyanov <xemul@...allels.com>
Subject: Re: [PATCH net-next v4 2/9] net: filter: keep original BPF program
 around

From: Eric Dumazet <edumazet@...gle.com>

On Fri, 2014-03-28 at 18:58 +0100, Daniel Borkmann wrote:
> In order to open up the possibility to internally transform a BPF program
> into an alternative and possibly non-trivial reversible representation, we
> need to keep the original BPF program around, so that it can be passed back
> to user space w/o the need of a complex decoder.
> 
> The reason for that use case resides in commit a8fc92778080 ("sk-filter:
> Add ability to get socket filter program (v2)"), that is, the ability
> to retrieve the currently attached BPF filter from a given socket used
> mainly by the checkpoint-restore project, for example.
> 
> Therefore, we add two helpers sk_{store,release}_orig_filter for taking
> care of that. In the sk_unattached_filter_create() case, there's no such
> possibility/requirement to retrieve a loaded BPF program. Therefore, we
> can spare us the work in that case.
> 
> This approach will simplify and slightly speed up both, sk_get_filter()
> and sock_diag_put_filterinfo() handlers as we won't need to successively
> decode filters anymore through sk_decode_filter(). As we still need
> sk_decode_filter() later on, we're keeping it around.
> 
> Joint work with Alexei Starovoitov.
> 
> Signed-off-by: Alexei Starovoitov <ast@...mgrid.com>
> Signed-off-by: Daniel Borkmann <dborkman@...hat.com>
> Cc: Pavel Emelyanov <xemul@...allels.com>
> ---

Note that this patch added a possible use after free.

Following patch should be sent to stable trees only, as 3.17+
incidentally fixed this with 
commit 278571baca2aecf5fb5cb5c8b002dbfa0a6c524c
    net: filter: simplify socket charging

[PATCH] net: filter: fix possible use after free

If kmemdup() fails, we free fp->orig_prog and return -ENOMEM

sk_attach_filter()
 -> sk_filter_uncharge(sk, fp)
  -> sk_filter_release(fp)
   -> call_rcu(&fp->rcu, sk_filter_release_rcu)
    -> sk_filter_release_rcu()
     -> sk_release_orig_filter()
        fprog = fp->orig_prog; // not NULL, but points to freed memory
	  kfree(fprog->filter); // use after free, potential corruption
          kfree(fprog); // double free or corruption

Note: This was fixed in 3.17+ with commit 278571baca2a
("net: filter: simplify socket charging")

Found by AddressSanitizer
 
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Fixes: a3ea269b8bcdb ("net: filter: keep original BPF program around")
---
diff --git a/net/core/filter.c b/net/core/filter.c
index 1dbf6462f766..3139f966a178 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -1318,6 +1318,7 @@ static int sk_store_orig_filter(struct sk_filter *fp,
 	fkprog->filter = kmemdup(fp->insns, fsize, GFP_KERNEL);
 	if (!fkprog->filter) {
 		kfree(fp->orig_prog);
+		fp->orig_prog = NULL;
 		return -ENOMEM;
 }
 


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