Re: scsi: sg: assorted memory corruptions

[Eric Biggers]

On Thu, Feb 01, 2018 at 05:21:12PM +0100, 'Dmitry Vyukov' via syzkaller wrote:
> On Thu, Feb 1, 2018 at 5:17 PM, Ben Hutchings
> <ben.hutchings@xxxxxxxxxxxxxxx> wrote:
> > On Thu, 2018-02-01 at 08:04 +0100, Dmitry Vyukov wrote:
> >> On Thu, Feb 1, 2018 at 7:03 AM, Douglas Gilbert <dgilbert@xxxxxxxxxxxx> wrote:
> >> > On 2018-01-30 07:22 AM, Dmitry Vyukov wrote:
> > [...]
> >> > > [1:0:0:0]    cd/dvd  QEMU     QEMU DVD-ROM     2.0.  /dev/sr0   /dev/sg1
> >> > >
> >> > > # readlink /sys/class/scsi_generic/sg0
> >> > >
> >> > > ../../devices/pci0000:00/0000:00:01.1/ata1/host0/target0:0:0/0:0:0:0/scsi_generic/sg0
> >> > >
> >> > > # cat /sys/class/scsi_generic/sg0/device/vendor
> >> > > ATA
> >> >
> >> >
> >> > ^^^^^
> >> > That subsystem is the culprit IMO, most likely libata.
> >> >
> >> > Until you can show this test failing on something other than an
> >> > ATA disk, then I will treat this issue as closed.
> >>
> >> Hi Doug,
> >>
> >> Why is bug in ATA not a bug? Is it long unused by everybody? I've got
> >> it by running qemu with default flags...
> >
> > If the bug is in libata then it's not on Doug to fix it since he's only
> > maintaining sg.
> 
> 
> Then I think we need to CC ata maintainers rather than treat it as closed.
> +Tejun, linux-ide@, you can see full thread here:
> https://groups.google.com/forum/#!topic/syzkaller/9RNr9Gu0MyY
> 

To get memory corruption it's actually sufficient just to submit "1-byte" reads;
there's no need for the SG_NEXT_CMD_LEN ioctl or anything:

	#include <fcntl.h>
	#include <unistd.h>

	int main()
	{
		int fd = open("/dev/sg0", O_RDWR);
		char buf[43] = { [36] = 0x08 /* READ_6 */ };

		for (;;)
			write(fd, buf, sizeof(buf));
	}

(where /dev/sg0 is the default QEMU disk type, "82371SB PIIX3 IDE")

The SCSI command descriptor block is the 6 bytes at indices 36-41, so index 42
is the only data byte.

Also this is a different bug from the crash in ata_bmdma_fill_sg() which is
fixed by "libata: fix length validation of ATAPI-relayed SCSI commands".

I'm guessing the driver is DMA'ing to somewhere it shouldn't be...

Eric