Re: [PATCH v2] [SCSI] sg: Fix user memory corruption when SG_IO isinterrupted by a signal

From: David Milburn
Date: Wed Aug 07 2013 - 10:38:49 EST

Roland Dreier wrote:
From: Roland Dreier <roland@xxxxxxxxxxxxxxx>

There is a nasty bug in the SCSI SG_IO ioctl that in some circumstances
leads to one process writing data into the address space of some other
random unrelated process if the ioctl is interrupted by a signal.
What happens is the following:

- A process issues an SG_IO ioctl with direction DXFER_FROM_DEV (ie the
underlying SCSI command will transfer data from the SCSI device to
the buffer provided in the ioctl)

- Before the command finishes, a signal is sent to the process waiting
in the ioctl. This will end up waking up the sg_ioctl() code:

result = wait_event_interruptible(sfp->read_wait,
(srp_done(sfp, srp) || sdp->detached));

but neither srp_done() nor sdp->detached is true, so we end up just
setting srp->orphan and returning to userspace:

srp->orphan = 1;
return result; /* -ERESTARTSYS because signal hit process */

At this point the original process is done with the ioctl and
blithely goes ahead handling the signal, reissuing the ioctl, etc.

- Eventually, the SCSI command issued by the first ioctl finishes and
ends up in sg_rq_end_io(). At the end of that function, we run through:

write_lock_irqsave(&sfp->rq_list_lock, iflags);
if (unlikely(srp->orphan)) {
if (sfp->keep_orphan)
srp->sg_io_owned = 0;
done = 0;
srp->done = done;
write_unlock_irqrestore(&sfp->rq_list_lock, iflags);

if (likely(done)) {
/* Now wake up any sg_read() that is waiting for this
* packet.
kill_fasync(&sfp->async_qp, SIGPOLL, POLL_IN);
kref_put(&sfp->f_ref, sg_remove_sfp);
} else {
INIT_WORK(&srp->, sg_rq_end_io_usercontext);

Since srp->orphan *is* set, we set done to 0 (assuming the
userspace app has not set keep_orphan via an SG_SET_KEEP_ORPHAN
ioctl), and therefore we end up scheduling sg_rq_end_io_usercontext()
to run in a workqueue.

- In workqueue context we go through sg_rq_end_io_usercontext() ->
sg_finish_rem_req() -> blk_rq_unmap_user() -> ... ->
bio_uncopy_user() -> __bio_copy_iov() -> copy_to_user().

The key point here is that we are doing copy_to_user() on a
workqueue -- that is, we're on a kernel thread with current->mm
equal to whatever random previous user process was scheduled before
this kernel thread. So we end up copying whatever data the SCSI
command returned to the virtual address of the buffer passed into
the original ioctl, but it's quite likely we do this copying into a
different address space!

As suggested by James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>,
add a check for current->mm (which is NULL if we're on a kernel thread
without a real userspace address space) in bio_uncopy_user(), and skip
the copy if we're on a kernel thread.

There's no reason that I can think of for any caller of bio_uncopy_user()
to want to do copying on a kernel thread with a random active userspace
address space.

Huge thanks to Costa Sapuntzakis <costa@xxxxxxxxxxxxxxx> for the
original pointer to this bug in the sg code.

Signed-off-by: Roland Dreier <roland@xxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
fs/bio.c | 20 +++++++++++++++-----
1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/fs/bio.c b/fs/bio.c
index 94bbc04..c5eae72 100644
--- a/fs/bio.c
+++ b/fs/bio.c
@@ -1045,12 +1045,22 @@ static int __bio_copy_iov(struct bio *bio, struct bio_vec *iovecs,
int bio_uncopy_user(struct bio *bio)
struct bio_map_data *bmd = bio->bi_private;
- int ret = 0;
+ struct bio_vec *bvec;
+ int ret = 0, i;
- if (!bio_flagged(bio, BIO_NULL_MAPPED))
- ret = __bio_copy_iov(bio, bmd->iovecs, bmd->sgvecs,
- bmd->nr_sgvecs, bio_data_dir(bio) == READ,
- 0, bmd->is_our_pages);
+ if (!bio_flagged(bio, BIO_NULL_MAPPED)) {
+ /*
+ * if we're in a workqueue, the request is orphaned, so
+ * don't copy into a random user address space, just free.
+ */
+ if (current->mm)
+ ret = __bio_copy_iov(bio, bmd->iovecs, bmd->sgvecs,
+ bmd->nr_sgvecs, bio_data_dir(bio) == READ,
+ 0, bmd->is_our_pages);
+ else if (bmd->is_our_pages)
+ bio_for_each_segment_all(bvec, bio, i)
+ __free_page(bvec->bv_page);
+ }
return ret;

Hi Roland,

I was able to succesfully test this patch overnight, I had been experimenting with the
sg driver setting the BIO_NULL_MAPPED flag in sg_rq_end_io_usercontext for a orphan process
which prevented the corruption, but your solution seems much better.


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