linux-kernel - Re: [PATCH] RDMA/ocrdma: Fix an off-by-one issue in 'ocrdma_add

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Date:   Fri, 17 Apr 2020 16:09:55 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     g@...pe.ca, Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        selvin.xavier@...adcom.com, devesh.sharma@...adcom.com,
        dledford@...hat.com, leon@...nel.org, colin.king@...onical.com,
        linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] RDMA/ocrdma: Fix an off-by-one issue in 'ocrdma_add_stat'

On Fri, Apr 17, 2020 at 09:25:42AM -0300, Jason Gunthorpe wrote:
> On Fri, Apr 17, 2020 at 02:26:24PM +0300, Dan Carpenter wrote:
> > On Thu, Apr 16, 2020 at 03:47:54PM -0300, Jason Gunthorpe wrote:
> > > On Thu, Apr 16, 2020 at 04:08:47PM +0300, Dan Carpenter wrote:
> > > > On Tue, Apr 14, 2020 at 03:34:41PM -0300, Jason Gunthorpe wrote:
> > > > > The memcpy is still kind of silly right? What about this:
> > > > > 
> > > > > static int ocrdma_add_stat(char *start, char *pcur, char *name, u64 count)
> > > > > {
> > > > > 	size_t len = (start + OCRDMA_MAX_DBGFS_MEM) - pcur;
> > > > > 	int cpy_len;
> > > > > 
> > > > > 	cpy_len = snprintf(pcur, len, "%s: %llu\n", name, count);
> > > > > 	if (cpy_len >= len || cpy_len < 0) {
> > > > 
> > > > The kernel version of snprintf() doesn't and will never return
> > > > negatives.  It would cause a huge security headache if it started
> > > > returning negatives.
> > > 
> > > Begs the question why it returns an int then :)
> > 
> > People should use "int" as their default type.  "int i;".  It means
> > "This is a normal number.  Nothing special about it.  It's not too high.
> > It's not defined by hardware requirements."  Other types call attention
> > to themselves, but int is the humble datatype.
> 
> No, I strongly disagree with this, it is one of my pet peeves to see
> 'int' being used for data which is known to be only ever be positive
> just to save typing 'unsigned'.
> 
> Not only is it confusing, but allowing signed values has caused tricky
> security bugs, unfortuntely.

I have the opposite pet peeve.

I complain about it a lot.  It pains me every time I see a "u32 i;".  I
think there is a static analysis warning for using signed which
encourages people to write code like that.  That warning really upsets
me for two reasons 1) The static checker should know the range of values
but it doesn't so it makes me sad to see inferior technology being used
when it should deleted instead.  2)  I have never seen this warning
prevent a real life bug.  You would need to hit a series of fairly rare
events for this warning to be useful and I have never seen that happen
yet.

The most common bug caused by unsigned variables is that it breaks the
kernel error handling but there are other problems as well.  There was
an example a little while back where someone "fixed" a security problem
by making things unsigned.

	for (i = 0; i < user_value; i++) {

Originally if user_value was an int then the loop would have been a
harmless no-op but now it was a large positive value so it lead to
memory corruption.  Another example is:

	for (i = 0; i < user_value - 1; i++) {

If "user_value" is zero the subtraction becomes UINT_MAX.  Or some
people use a "u16 i;" but then the limit increases so the loop doesn't
work any more.

>From my experience with static analysis and security audits, making
things unsigned en mass causes more security bugs.  There are definitely
times where making variables unsigned is correct for security reasons
like when you are taking a size from userspace.

Complicated types call attention to themselves and they hurt
readability.  You sometimes *need* other datatypes and you want those to
stand out but if everything is special then nothing is special.

regards,
dan carpenter