linux-ext4 - [Bug 200933] Divide zero in __ext4_check_dir

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Message-ID: <bug-200933-13602-qOHE3QhxXr@https.bugzilla.kernel.org/>
Date:   Mon, 27 Aug 2018 14:37:45 +0000
From:   bugzilla-daemon@...zilla.kernel.org
To:     linux-ext4@...nel.org
Subject: [Bug 200933] Divide zero in __ext4_check_dir_entry

https://bugzilla.kernel.org/show_bug.cgi?id=200933

--- Comment #3 from Theodore Tso (tytso@....edu) ---
Patch to fix this:

http://patchwork.ozlabs.org/patch/962516/

Note: I will probably be retitling and rewriting the patch description, because
on further reflection, it's not possible for this to become a buffer overrun
attack.   That's because we are checking to make sure the inline directory does
not exceed the bounds of the inline data xattr.  The size == 0 is coming from
the "end of xattr list" marker in the xattr data structure.  That has to be
there, or we would have declared the xattr data structure to be corrupt before
we even tried to parse the inline directory.   So the only way the attacker can
manifest a problem is by causing the divide by zero in
__ext4_check_dir_entry().

So for CVE scoring purposes, this is just a denial of service attack
(triggering a kernel divide by zero trap, which doesn't kill the kernel per se,
but which leaves various locks and refcounts held, which will eventually cause
the system to become wedged).  It is not a buffer overrun.

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