linux-ext4 - [PATCH] ext4: fix null pointer deref on mount

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Date:	Mon, 05 Jan 2009 02:19:55 +0100
From:	Thiemo Nagel <thiemo.nagel@...tum.de>
To:	Ext4 Developers List <linux-ext4@...r.kernel.org>
CC:	tytso@....edu
Subject: [PATCH] ext4: fix null pointer deref on mount

I came across a null pointer dereference when mounting an intentionally 
corrupted filesystem (cf. debug.dmesg).  In my opinion, the problem lies 
in ext4_fill_super(), where truncation may occur on setting the integer 
db_count, which results in too little memory being allocated for 
sbi->s_group_desc.  The attached patch (against 2.6.28) fixes this by 
changing the type of db_count to unsigned long.  I also took the 
opportunity to make the check against sign extension in calculation of 
db_count more strict, so that it now excludes cases in which db_count 
comes out as zero.

Comments are welcome!

Signed-off-by: Thiemo Nagel <thiemo.nagel@...tum.de>

View attachment "null_deref.patch" of type "text/x-diff" (927 bytes)

View attachment "debug.dmesg" of type "text/plain" (5485 bytes)