linux-kernel - RE: [PATCH V2 for-6.10/block 1/2] loop: Fix a race between loop detach and loop open

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Date: Wed, 22 May 2024 19:11:02 +0000
From: Gulam Mohamed <gulam.mohamed@...cle.com>
To: Jens Axboe <axboe@...nel.dk>,
        "linux-block@...r.kernel.org"
	<linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
CC: "shinichiro.kawasaki@....com" <shinichiro.kawasaki@....com>,
        "chaitanyak@...dia.com" <chaitanyak@...dia.com>,
        "hch@....de" <hch@....de>
Subject: RE: [PATCH V2 for-6.10/block 1/2] loop: Fix a race between loop
 detach and loop open

Hi Jens,

> -----Original Message-----
> From: Jens Axboe <axboe@...nel.dk>
> Sent: Wednesday, May 22, 2024 7:10 AM
> To: Gulam Mohamed <gulam.mohamed@...cle.com>; linux-
> block@...r.kernel.org; linux-kernel@...r.kernel.org
> Cc: shinichiro.kawasaki@....com; chaitanyak@...dia.com; hch@....de
> Subject: Re: [PATCH V2 for-6.10/block 1/2] loop: Fix a race between loop
> detach and loop open
> 
> On 5/21/24 4:42 PM, Gulam Mohamed wrote:
> > Description
> > ===========
> >
> > 1. Userspace sends the command "losetup -d" which uses the open() call
> >    to open the device
> > 2. Kernel receives the ioctl command "LOOP_CLR_FD" which calls the
> >    function loop_clr_fd()
> > 3. If LOOP_CLR_FD is the first command received at the time, then the
> >    AUTOCLEAR flag is not set and deletion of the
> >    loop device proceeds ahead and scans the partitions (drop/add
> >    partitions)
> >
> > 	if (disk_openers(lo->lo_disk) > 1) {
> > 		lo->lo_flags |= LO_FLAGS_AUTOCLEAR;
> > 		loop_global_unlock(lo, true);
> > 		return 0;
> > 	}
> >
> >  4. Before scanning partitions, it will check to see if any partition of
> >     the loop device is currently opened  5. If any partition is
> > opened, then it will return EBUSY:
> >
> >     if (disk->open_partitions)
> > 		return -EBUSY;
> >  6. So, after receiving the "LOOP_CLR_FD" command and just before the
> above
> >     check for open_partitions, if any other command
> >     (like blkid) opens any partition of the loop device, then the partition
> >     scan will not proceed and EBUSY is returned as shown in above code
> > 7. But in "__loop_clr_fd()", this EBUSY error is not propagated  8. We
> > have noticed that this is causing the partitions of the loop to
> >     remain stale even after the loop device is detached resulting in the
> >     IO errors on the partitions
> >
> > Fix
> > ---
> > Re-introduce the lo_open() call to restrict any process to open the
> > loop device when its being detached
> >
> > Test case
> > =========
> > Test case involves the following two scripts:
> >
> > script1.sh
> > ----------
> > while [ 1 ];
> > do
> > 	losetup -P -f /home/opt/looptest/test10.img
> > 	blkid /dev/loop0p1
> > done
> >
> > script2.sh
> > ----------
> > while [ 1 ];
> > do
> > 	losetup -d /dev/loop0
> > done
> >
> > Without fix, the following IO errors have been observed:
> >
> > kernel: __loop_clr_fd: partition scan of loop0 failed (rc=-16)
> > kernel: I/O error, dev loop0, sector 20971392 op 0x0:(READ) flags 0x80700
> >         phys_seg 1 prio class 0
> > kernel: I/O error, dev loop0, sector 108868 op 0x0:(READ) flags 0x0
> >         phys_seg 1 prio class 0
> > kernel: Buffer I/O error on dev loop0p1, logical block 27201, async page
> >         read
> >
> > V1->V2:
> > 	Added a test case, 010, in blktests in tests/loop/
> > Signed-off-by: Gulam Mohamed <gulam.mohamed@...cle.com>
> > ---
> >  drivers/block/loop.c | 19 +++++++++++++++++++
> >  1 file changed, 19 insertions(+)
> >
> > diff --git a/drivers/block/loop.c b/drivers/block/loop.c index
> > 28a95fd366fe..9a235d8c062d 100644
> > --- a/drivers/block/loop.c
> > +++ b/drivers/block/loop.c
> > @@ -1717,6 +1717,24 @@ static int lo_compat_ioctl(struct block_device
> > *bdev, blk_mode_t mode,  }  #endif
> >
> > +static int lo_open(struct gendisk *disk, blk_mode_t mode) {
> > +        struct loop_device *lo = disk->private_data;
> > +        int err;
> > +
> > +        if (!lo)
> > +                return -ENXIO;
> > +
> > +        err = mutex_lock_killable(&lo->lo_mutex);
> > +        if (err)
> > +                return err;
> > +
> > +        if (lo->lo_state == Lo_rundown)
> > +                err = -ENXIO;
> > +        mutex_unlock(&lo->lo_mutex);
> > +	return err;
> > +}
> 
> Most of this function uses spaces rather than tabs.
I am sorry I should have checked that. Will address the issue in next version
> 
> --
> Jens Axboe