[PATCH RESEND] sg: allow waiting for commands to complete on removed device

[Tony Battersby <tonyb@cybernetics.com>]

When a SCSI device is removed while in active use, currently sg will
immediately return -ENODEV on any attempt to wait for active commands
that were sent before the removal.  This is problematic for commands
that use SG_FLAG_DIRECT_IO since the data buffer may still be in use by
the kernel when userspace frees or reuses it after getting ENODEV,
leading to corrupted userspace memory (in the case of READ-type
commands) or corrupted data being sent to the device (in the case of
WRITE-type commands).  This has been seen in practice when logging out
of a iscsi_tcp session, where the iSCSI driver may still be processing
commands after the device has been marked for removal.

So change the policy to allow userspace to wait for active sg commands
even when the device is being removed.  Return -ENODEV only when there
are no more responses to read.

Cc: <stable@vger.kernel.org>
Signed-off-by: Tony Battersby <tonyb@cybernetics.com>
---
 drivers/scsi/sg.c | 53 +++++++++++++++++++++++++++++------------------
 1 file changed, 33 insertions(+), 20 deletions(-)

diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
index 118c7b4a8af2..340b050ad28d 100644
--- a/drivers/scsi/sg.c
+++ b/drivers/scsi/sg.c
@@ -195,7 +195,7 @@ static void sg_link_reserve(Sg_fd * sfp, Sg_request * srp, int size);
 static void sg_unlink_reserve(Sg_fd * sfp, Sg_request * srp);
 static Sg_fd *sg_add_sfp(Sg_device * sdp);
 static void sg_remove_sfp(struct kref *);
-static Sg_request *sg_get_rq_mark(Sg_fd * sfp, int pack_id);
+static Sg_request *sg_get_rq_mark(Sg_fd * sfp, int pack_id, bool *busy);
 static Sg_request *sg_add_request(Sg_fd * sfp);
 static int sg_remove_request(Sg_fd * sfp, Sg_request * srp);
 static Sg_device *sg_get_dev(int dev);
@@ -444,6 +444,7 @@ sg_read(struct file *filp, char __user *buf, size_t count, loff_t * ppos)
 	Sg_fd *sfp;
 	Sg_request *srp;
 	int req_pack_id = -1;
+	bool busy;
 	sg_io_hdr_t *hp;
 	struct sg_header *old_hdr;
 	int retval;
@@ -466,20 +467,16 @@ sg_read(struct file *filp, char __user *buf, size_t count, loff_t * ppos)
 	if (retval)
 		return retval;
 
-	srp = sg_get_rq_mark(sfp, req_pack_id);
+	srp = sg_get_rq_mark(sfp, req_pack_id, &busy);
 	if (!srp) {		/* now wait on packet to arrive */
-		if (atomic_read(&sdp->detaching))
-			return -ENODEV;
 		if (filp->f_flags & O_NONBLOCK)
 			return -EAGAIN;
 		retval = wait_event_interruptible(sfp->read_wait,
-			(atomic_read(&sdp->detaching) ||
-			(srp = sg_get_rq_mark(sfp, req_pack_id))));
-		if (atomic_read(&sdp->detaching))
-			return -ENODEV;
-		if (retval)
-			/* -ERESTARTSYS as signal hit process */
-			return retval;
+			((srp = sg_get_rq_mark(sfp, req_pack_id, &busy)) ||
+			(!busy && atomic_read(&sdp->detaching))));
+		if (!srp)
+			/* signal or detaching */
+			return retval ? retval : -ENODEV;
 	}
 	if (srp->header.interface_id != '\0')
 		return sg_new_read(sfp, buf, count, srp);
@@ -940,9 +937,7 @@ sg_ioctl_common(struct file *filp, Sg_device *sdp, Sg_fd *sfp,
 		if (result < 0)
 			return result;
 		result = wait_event_interruptible(sfp->read_wait,
-			(srp_done(sfp, srp) || atomic_read(&sdp->detaching)));
-		if (atomic_read(&sdp->detaching))
-			return -ENODEV;
+			srp_done(sfp, srp));
 		write_lock_irq(&sfp->rq_list_lock);
 		if (srp->done) {
 			srp->done = 2;
@@ -2079,19 +2074,28 @@ sg_unlink_reserve(Sg_fd * sfp, Sg_request * srp)
 }
 
 static Sg_request *
-sg_get_rq_mark(Sg_fd * sfp, int pack_id)
+sg_get_rq_mark(Sg_fd * sfp, int pack_id, bool *busy)
 {
 	Sg_request *resp;
 	unsigned long iflags;
 
+	*busy = false;
 	write_lock_irqsave(&sfp->rq_list_lock, iflags);
 	list_for_each_entry(resp, &sfp->rq_list, entry) {
-		/* look for requests that are ready + not SG_IO owned */
-		if ((1 == resp->done) && (!resp->sg_io_owned) &&
+		/* look for requests that are not SG_IO owned */
+		if ((!resp->sg_io_owned) &&
 		    ((-1 == pack_id) || (resp->header.pack_id == pack_id))) {
-			resp->done = 2;	/* guard against other readers */
-			write_unlock_irqrestore(&sfp->rq_list_lock, iflags);
-			return resp;
+			switch (resp->done) {
+			case 0: /* request active */
+				*busy = true;
+				break;
+			case 1: /* request done; response ready to return */
+				resp->done = 2;	/* guard against other readers */
+				write_unlock_irqrestore(&sfp->rq_list_lock, iflags);
+				return resp;
+			case 2: /* response already being returned */
+				break;
+			}
 		}
 	}
 	write_unlock_irqrestore(&sfp->rq_list_lock, iflags);
@@ -2145,6 +2149,15 @@ sg_remove_request(Sg_fd * sfp, Sg_request * srp)
 		res = 1;
 	}
 	write_unlock_irqrestore(&sfp->rq_list_lock, iflags);
+
+	/*
+	 * If the device is detaching, wakeup any readers in case we just
+	 * removed the last response, which would leave nothing for them to
+	 * return other than -ENODEV.
+	 */
+	if (unlikely(atomic_read(&sfp->parentdp->detaching)))
+		wake_up_interruptible_all(&sfp->read_wait);
+
 	return res;
 }
 
-- 2.25.1