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

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

[Dan Carpenter]

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

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

[Jason Gunthorpe]

On Tue, Jul 05, 2022 at 12:05:28PM +0300, Dan Carpenter wrote:
> 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". 

I wonder if this can happen, len is a kernel controlled value bounded
by a memory allocation..

> Fixes: b0eed085903e ("hisi_acc_vfio_pci: Add support for VFIO live migration")
> Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
> ---

This code was copy and pasted from drivers/vfio/pci/mlx5/main.c, so it
should be fixed too

> It is strange that we are doing:
> 
> 	pos = &filp->f_pos;
>
> instead of using the passed in value of pos.  

IIRC the way we have the struct file configured the pos argument is
NULL.

Jason

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

[Dan Carpenter]

On Tue, Jul 05, 2022 at 03:06:49PM -0300, Jason Gunthorpe wrote:
> On Tue, Jul 05, 2022 at 12:05:28PM +0300, Dan Carpenter wrote:
> > 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". 
> 
> I wonder if this can happen, len is a kernel controlled value bounded
> by a memory allocation..
> 

Oh.  Smatch uses a model which says that all read/writes come from
vfs_write().  The problem with tracking kernel read/writes is that
recursion is tricky.  So Smatch just deletes those from the DB.

> > Fixes: b0eed085903e ("hisi_acc_vfio_pci: Add support for VFIO live migration")
> > Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
> > ---
> 
> This code was copy and pasted from drivers/vfio/pci/mlx5/main.c, so it
> should be fixed too

Sure.

I created a static checker warning for this type of thing but it didn't
catch the issue in drivers/vfio/pci/mlx5/main.c because Smatch says that
the bug is impossible.  Which is true.

Smatch doesn't really parse rw_verify_area() accurately.  I just hard
coded that function as accepting values 0-1000000000 for both *ppos and
count.

regards,
dan carpenter

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

[Jason Gunthorpe]

On Wed, Jul 06, 2022 at 08:51:24AM +0300, Dan Carpenter wrote:
> On Tue, Jul 05, 2022 at 03:06:49PM -0300, Jason Gunthorpe wrote:
> > On Tue, Jul 05, 2022 at 12:05:28PM +0300, Dan Carpenter wrote:
> > > 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". 
> > 
> > I wonder if this can happen, len is a kernel controlled value bounded
> > by a memory allocation..
> > 
> 
> Oh.  Smatch uses a model which says that all read/writes come from
> vfs_write().  The problem with tracking kernel read/writes is that
> recursion is tricky.  So Smatch just deletes those from the DB.

Oh, maybe I got it wrong, len is the user input, so yes that does look
bad

> > This code was copy and pasted from drivers/vfio/pci/mlx5/main.c, so it
> > should be fixed too
> 
> Sure.
> 
> I created a static checker warning for this type of thing but it didn't
> catch the issue in drivers/vfio/pci/mlx5/main.c because Smatch says that
> the bug is impossible.  Which is true.

How come it is different?

Jason

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

[Dan Carpenter]

On Wed, Jul 06, 2022 at 01:18:12PM -0300, Jason Gunthorpe wrote:
> On Wed, Jul 06, 2022 at 08:51:24AM +0300, Dan Carpenter wrote:
>
> > > This code was copy and pasted from drivers/vfio/pci/mlx5/main.c, so it
> > > should be fixed too
> > 
> > Sure.
> > 
> > I created a static checker warning for this type of thing but it didn't
> > catch the issue in drivers/vfio/pci/mlx5/main.c because Smatch says that
> > the bug is impossible.  Which is true.
> 
> How come it is different?

No, it doesn't find either one.  I don't think it's a real bug because
of the rw_verify_area() thing.  But I wrote the check based on noticing
it during review just to see if there were similar issues and didn't
find anything.

regards,
dan carpenter