linux-kernel - Re: [PATCH] scsi: megaraid: Fix uninitialized mbox in mega_cmd

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Message-ID: <20230601072352.GA123625@dev-dsk-mheyne-1b-c1362c4d.eu-west-1.amazon.com>
Date:   Thu, 1 Jun 2023 07:23:52 +0000
From:   Maximilian Heyne <mheyne@...zon.de>
To:     "Martin K. Petersen" <martin.petersen@...cle.com>
CC:     Kashyap Desai <kashyap.desai@...adcom.com>,
        Sumit Saxena <sumit.saxena@...adcom.com>,
        Shivasharan S <shivasharan.srikanteshwara@...adcom.com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        James Bottomley <JBottomley@...allels.com>,
        Adam Radford <aradford@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        <megaraidlinux.pdl@...adcom.com>, <linux-scsi@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] scsi: megaraid: Fix uninitialized mbox in mega_cmd_done

On Wed, May 31, 2023 at 06:38:12PM -0400, Martin K. Petersen wrote:
> Hi Maximilian!

Hi Martin,

thanks for your response.

> 
> > This is similar to commit 7a2ae008a53c ("scsi: megaraid: Fix
> > mega_cmd_done() CMDID_INT_CMDS").
> 
> That's supposed to be commit 75cb113cd43f, right?

Sorry, yes I meant this commit. I accidentally got the commit id form the 5.4
backport.

> 
> > When cmdid == CMDID_INT_CMDS and status != 0 then mbox is still NULL
> > but is dereferenced below.
> 
> Is this actually a valid scenario, though? The mbox is only dereferenced
> if there is status returned from an attached device. And I am guessing
> that a controller-internal command does not talk to any attached device.
> Thus status should always be 0 for CMDID_INT_CMDS. I don't have the
> megaraid firmware manual so this is pure guesswork on my part. But you
> would think we would have come across an invalid deref since the
> original 2014 commit made the offending change.
> 

This could indeed be the case. I only found this because it was reported by
static code analysis. However, by reading the code it's not obvious that this is
how it works. Should we therefore skip the whole status checking switch for
internal commands to make it more clear? For example, by a goto to the end of
the loop when the scb gets freed or just adding the scb free, list removal
and a continue?

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