linux-kernel - Re: [syzbot] [usb-storage?] divide error in isd200_ata

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Message-ID: <d5328312-4cd0-426f-8db4-29d67790c213@rowland.harvard.edu>
Date: Wed, 28 Feb 2024 14:23:25 -0500
From: Alan Stern <stern@...land.harvard.edu>
To: Oliver Neukum <oneukum@...e.com>
Cc: syzbot <syzbot+28748250ab47a8f04100@...kaller.appspotmail.com>,
  bvanassche@....org, emilne@...hat.com, gregkh@...uxfoundation.org,
  linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
  martin.petersen@...cle.com, syzkaller-bugs@...glegroups.com,
  tasos@...ossah.com, usb-storage@...ts.one-eyed-alien.net
Subject: Re: [syzbot] [usb-storage?] divide error in isd200_ata_command

On Wed, Feb 28, 2024 at 05:20:29PM +0100, Oliver Neukum wrote:
> On 26.02.24 19:13, Alan Stern wrote:
> 
> > > It oopses here:
> > > 
> > > 		} else {
> > > 			if (!id[ATA_ID_SECTORS] || !id[ATA_ID_HEADS])
> > > 				goto too_early;
> > 
> > Those two lines are debugging code you added, right?
> 
> Yes, sorry about that.
> 
> > 
> > > 			sectnum = (u8)((lba % id[ATA_ID_SECTORS]) + 1);
> > > 			cylinder = (u16)(lba / (id[ATA_ID_SECTORS] *
> > > 					id[ATA_ID_HEADS]));
> > > 
> > > in isd200_scsi_to_ata() because it must not be called before isd200_get_inquiry_data()
> > > has completed.
> > 
> > It can't be; isd200_get_inquiry_data is called by isd200_Initialization
> > during probe before any SCSI commands are transmitted.
> 
> So, you are concluding that the bisection is spurious because
> without that patch the SCSI layer would see a capacity of zero
> and not even try to read anything?

I don't know.  My guess is that without this patch, the test would fail 
for some reason before the SCSI layer has a chance to issue a READ command.  
Maybe because of a zero capacity, like you said, or maybe something 
else.

Whatever the reason, it looks like Martin's commit merely exposed a 
problem which has existed all along.

Alan Stern