[PATCH] vfio: hisi_acc_vfio_pci: fix integer overflow check in hisi_acc_vf_resume_write()

[Dan Carpenter <dan.carpenter@oracle.com>]

The casting on this makes the integer overflow check slightly wrong.
"len" is an unsigned long. "*pos" and "requested_length" are signed
long longs.  Imagine "len" is ULONG_MAX and "*pos" is 2.
"ULONG_MAX + 2 = 1".  That's an integer overflow.  However, if we cast
the ULONG_MAX to long long then "-1 + 2 = 1".  That's not an integer
overflow.

It's simpler if "requested_length" length is an unsigned value so we
don't have to worry about negatives.

I believe that the checks in the VFS layer and the check for "*pos < 0"
probably prevent this bug in real life, but it's safer to just be sure.

Fixes: b0eed085903e ("hisi_acc_vfio_pci: Add support for VFIO live migration")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
---
It is strange that we are doing:

	pos = &filp->f_pos;

instead of using the passed in value of pos.  The VFS layer ensures
that the passed in value of "*pos + len" cannot overflow in
rw_verify_area() so normally this check could have been removed.

 drivers/vfio/pci/hisilicon/hisi_acc_vfio_pci.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/vfio/pci/hisilicon/hisi_acc_vfio_pci.c b/drivers/vfio/pci/hisilicon/hisi_acc_vfio_pci.c
index ea762e28c1cc..dcc34488b0c0 100644
--- a/drivers/vfio/pci/hisilicon/hisi_acc_vfio_pci.c
+++ b/drivers/vfio/pci/hisilicon/hisi_acc_vfio_pci.c
@@ -701,7 +701,7 @@ static ssize_t hisi_acc_vf_resume_write(struct file *filp, const char __user *bu
 					size_t len, loff_t *pos)
 {
 	struct hisi_acc_vf_migration_file *migf = filp->private_data;
-	loff_t requested_length;
+	unsigned long requested_length;
 	ssize_t done = 0;
 	int ret;
 
@@ -709,8 +709,8 @@ static ssize_t hisi_acc_vf_resume_write(struct file *filp, const char __user *bu
 		return -ESPIPE;
 	pos = &filp->f_pos;
 
-	if (*pos < 0 ||
-	    check_add_overflow((loff_t)len, *pos, &requested_length))
+	if (*pos < 0 || *pos > ULONG_MAX ||
+	    check_add_overflow(len, (unsigned long)*pos, &requested_length))
 		return -EINVAL;
 
 	if (requested_length > sizeof(struct acc_vf_data))
-- 
2.35.1