linux-kernel - [PATCH] sg: protect against races between mmap() and SG_SET_RESERVED

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Date:   Tue, 15 Aug 2017 22:41:08 -0700
From:   Todd Poynor <toddpoynor@...gle.com>
To:     Doug Gilbert <dgilbert@...erlog.com>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>
Cc:     linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Hannes Reinecke <hare@...e.de>,
        Todd Poynor <toddpoynor@...gle.com>
Subject: [PATCH] sg: protect against races between mmap() and SG_SET_RESERVED_SIZE

Take f_mutex around mmap() processing to protect against races with
the SG_SET_RESERVED_SIZE ioctl.  Ensure the reserve buffer length
remains consistent during the mapping operation, and set the
"mmap called" flag to prevent further changes to the reserved buffer
size as an atomic operation with the mapping.

Signed-off-by: Todd Poynor <toddpoynor@...gle.com>
---
 drivers/scsi/sg.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
index 3a44b4bc872b..a20718e9f1f4 100644
--- a/drivers/scsi/sg.c
+++ b/drivers/scsi/sg.c
@@ -1233,6 +1233,7 @@ sg_mmap(struct file *filp, struct vm_area_struct *vma)
 	unsigned long req_sz, len, sa;
 	Sg_scatter_hold *rsv_schp;
 	int k, length;
+        int ret = 0;
 
 	if ((!filp) || (!vma) || (!(sfp = (Sg_fd *) filp->private_data)))
 		return -ENXIO;
@@ -1243,8 +1244,11 @@ sg_mmap(struct file *filp, struct vm_area_struct *vma)
 	if (vma->vm_pgoff)
 		return -EINVAL;	/* want no offset */
 	rsv_schp = &sfp->reserve;
-	if (req_sz > rsv_schp->bufflen)
-		return -ENOMEM;	/* cannot map more than reserved buffer */
+	mutex_lock(&sfp->f_mutex);
+	if (req_sz > rsv_schp->bufflen) {
+		ret = -ENOMEM;	/* cannot map more than reserved buffer */
+		goto out;
+	}
 
 	sa = vma->vm_start;
 	length = 1 << (PAGE_SHIFT + rsv_schp->page_order);
@@ -1258,7 +1262,9 @@ sg_mmap(struct file *filp, struct vm_area_struct *vma)
 	vma->vm_flags |= VM_IO | VM_DONTEXPAND | VM_DONTDUMP;
 	vma->vm_private_data = sfp;
 	vma->vm_ops = &sg_mmap_vm_ops;
-	return 0;
+out:
+	mutex_unlock(&sfp->f_mutex);
+	return ret;
 }
 
 static void
-- 
2.14.1.480.gb18f417b89-goog