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Date: Tue, 25 Jan 2005 12:40:56 -0800
From: Matt Mackall <mpm@selenic.com>
To: Zwane Mwaikambo <zwane@arm.linux.org.uk>
Cc: Andrew Morton <akpm@osdl.org>, Linux Kernel <linux-kernel@vger.kernel.org>
Subject: Re: 2.6.11-rc2-mm1 Random related problems
Message-ID: <20050125204056.GL12076@waste.org>
References: <Pine.LNX.4.61.0501242134390.3010@montezuma.fsmlabs.com>
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On Mon, Jan 24, 2005 at 09:36:37PM -0700, Zwane Mwaikambo wrote:
> I'm having trouble booting here, were those random-* patches tested?
> 
> EIP is at __add_entropy_words+0xc5/0x1c0
> eax: 0000002d   ebx: 0000000f   ecx: 0000007b   edx: f5e50000
> esi: c0810980   edi: 0000000f   ebp: f5e4fea4   esp: f5e4fe5c
> ds: 007b   es: 007b   ss: 0068
> Process dd (pid: 2255, threadinfo=f5e4e000 task=ebdd7ac0)
> Stack: 00000010 78000000 c07043e0 00000286 0000007b 0000001f 00000001 00000007
>        0000000e 00000014 0000001a 00000000 0000002d f5e50000 c07043c0 00000080
>        c0704340 c07043c0 f5e4ff40 c037a4e0 00000000 00000010 2cda69d0 f7b9f75e
> Call Trace:
>  [<c010403a>] show_stack+0x7a/0x90
>  [<c01041c6>] show_registers+0x156/0x1c0
>  [<c01043e0>] die+0x100/0x190
>  [<c0116d59>] do_page_fault+0x349/0x63f
>  [<c0103cc7>] error_code+0x2b/0x30
>  [<c037a4e0>] xfer_secondary_pool+0xb0/0xe0

This oops is add_entropy_words running off the top of the stack
reading its input. The overrun is harmless until it causes a page
fault. After much staring:

This should fix Zwane's oops. Looks like I introduced this bug in Aug
2003, but it was hard to trigger until the recent changes. But it
ought to make it to mainline soonish.

Signed-off-by: Matt Mackall <mpm@selenic.com>

Index: rc2mm1/drivers/char/random.c
===================================================================
--- rc2mm1.orig/drivers/char/random.c	2005-01-25 12:27:00.000000000 -0800
+++ rc2mm1/drivers/char/random.c	2005-01-25 12:27:36.000000000 -0800
@@ -701,7 +701,7 @@
 
 		bytes=extract_entropy(r->pull, tmp, bytes,
 				      random_read_wakeup_thresh / 8, rsvd);
-		add_entropy_words(r, tmp, bytes);
+		add_entropy_words(r, tmp, (bytes + 3) / 4);
 		credit_entropy_store(r, bytes*8);
 	}
 }


-- 
Mathematics is the supreme nostalgia of our time.
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