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Date: Fri, 16 May 2014 22:18:40 -0400
From: "Theodore Ts'o" <tytso@mit.edu>
To: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
        "H. Peter Anvin" <hpa@linux.intel.com>,
        Sasha Levin <sasha.levin@oracle.com>, Ingo Molnar <mingo@kernel.org>,
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        Thomas Gleixner <tglx@linutronix.de>, Dave Jones <davej@redhat.com>,
        price@mit.edu
Subject: Re: BUG_ON drivers/char/random.c:986 (Was: perf: use after free in
 perf_remove_from_context)
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On Fri, May 16, 2014 at 05:46:22PM -0700, Hannes Frederic Sowa wrote:
> This should do the trick:
> dd if=/dev/urandom of=/dev/zero bs=67108707
> 
> I suspect ee1de406ba6eb1 ("random: simplify accounting logic") as the
> culprit.

Yep, that it's it.  Thanks for noticing this so quickly!  I'll push
the following patch to Linus.

    	      	    			- Ted

commit 29fb0ca5b3922288fba3f4c975a55032a51df0f0
Author: Theodore Ts'o <tytso@mit.edu>
Date:   Fri May 16 21:40:41 2014 -0400

    random: fix BUG_ON caused by accounting simplification
    
    Commit ee1de406ba6eb1 ("random: simplify accounting logic") simplified
    things too much, in that it allows the following to trigger an
    overflow that results in a BUG_ON crash:
    
    dd if=/dev/urandom of=/dev/zero bs=67108707
    
    Thanks to Peter Zihlstra for discovering the crash, and Hannes
    Frederic for analyizing the root cause.
    
    Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
    Reported-by: Peter Zijlstra <peterz@infradead.org>
    Reported-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
    Cc: Greg Price <price@mit.edu>

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 6b75713..102c50d 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -995,8 +995,11 @@ retry:
 		ibytes = min_t(size_t, ibytes, have_bytes - reserved);
 	if (ibytes < min)
 		ibytes = 0;
-	entropy_count = max_t(int, 0,
-			      entropy_count - (ibytes << (ENTROPY_SHIFT + 3)));
+	if (have_bytes >= ibytes + reserved)
+		entropy_count -= ibytes << (ENTROPY_SHIFT + 3);
+	else
+		entropy_count = reserved << (ENTROPY_SHIFT + 3);
+
 	if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig)
 		goto retry;