Re: [PATCH] iommu/omap: fix buffer overflow in debugfs

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

On Thu, Aug 04, 2022 at 05:31:39PM +0100, Robin Murphy wrote:
> On 04/08/2022 3:32 pm, Dan Carpenter wrote:
> > There are two issues here:
> > 
> > 1) The "len" variable needs to be checked before the very first write.
> >     Otherwise if omap2_iommu_dump_ctx() with "bytes" less than 32 it is a
> >     buffer overflow.
> > 2) The snprintf() function returns the number of bytes that *would* have
> >     been copied if there were enough space.  But we want to know the
> >     number of bytes which were *actually* copied so use scnprintf()
> >     instead.
> > 
> > Fixes: bd4396f09a4a ("iommu/omap: Consolidate OMAP IOMMU modules")
> 
> FWIW I think this has actually been broken since day one back in
> 14e0e6796a0d, but I'm also not inclined to care that much - 2014 is already
> plenty long ago.
> 

I don't know how I didn't see that.  It's like my eyeballs are broken
sometimes.  Possibly it should be:

Fixes: 14e0e6796a0d ("OMAP: iommu: add initial debugfs support")

Althought that's debatable as well...


> > Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
> > ---
> >   drivers/iommu/omap-iommu-debug.c | 6 +++---
> >   1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/iommu/omap-iommu-debug.c b/drivers/iommu/omap-iommu-debug.c
> > index a99afb5d9011..259f65291d90 100644
> > --- a/drivers/iommu/omap-iommu-debug.c
> > +++ b/drivers/iommu/omap-iommu-debug.c
> > @@ -32,12 +32,12 @@ static inline bool is_omap_iommu_detached(struct omap_iommu *obj)
> >   		ssize_t bytes;						\
> >   		const char *str = "%20s: %08x\n";			\
> >   		const int maxcol = 32;					\
> > -		bytes = snprintf(p, maxcol, str, __stringify(name),	\
> > +		if (len < maxcol)					\
> > +			goto out;					\
> > +		bytes = scnprintf(p, maxcol, str, __stringify(name),	\
> >   				 iommu_read_reg(obj, MMU_##name));	\
> 
> I suppose snprintf is OK in practice since none of the names are anywhere
> near 20 characters long anyway, but I agree it's better to be obviously
> foolproof.

The issue with scnprintf() vs snprintf() is not the 32 character limit.
The issue is the "p - buf" math in omap2_iommu_dump_ctx().  The "p"
variable is past the end of the buffer.  The user decides the size of
the buffer so it's a real bug.

regards,
dan carpenter