linux-kernel - Re: [PATCH] hugetlbfs: Fix integer overflow check in hugetlbfs_file

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Date:   Wed, 12 Jul 2023 16:58:13 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Linke Li <lilinke99@...mail.com>, linux-mm@...ck.org,
        llvm@...ts.linux.dev, linux-kernel@...r.kernel.org,
        trix@...hat.com, ndesaulniers@...gle.com, nathan@...nel.org,
        muchun.song@...ux.dev, Linke Li <lilinke99@...il.com>
Subject: Re: [PATCH] hugetlbfs: Fix integer overflow check in
 hugetlbfs_file_mmap()

On 07/11/23 13:49, David Hildenbrand wrote:
> On 10.07.23 10:32, Linke Li wrote:
> > From: Linke Li <lilinke99@...il.com>
> > 
> > 	vma_len = (loff_t)(vma->vm_end - vma->vm_start);
> > 	len = vma_len + ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
> > 	/* check for overflow */
> > 	if (len < vma_len)
> > 		return -EINVAL;
> > 
> > The existing code includes an integer overflow check, which indicates
> > that the variable len has the potential to overflow, leading to undefined
> >   behavior according to the C standard. However, both GCC and Clang
> > compilers may eliminate this overflow check based on the assumption
> > that there will be no undefined behavior. Although the Linux kernel
> > disables these optimizations by using the -fno-strict-overflow option,
> > there is still a risk if the compilers make mistakes in the future.
> 
> So we're adding code to handle eventual future compiler bugs? That sounds
> wrong, but maybe I misunderstood the problem you are trying to solve?

Like David, adding a fix for a potential future compiler bug sounds wrong.

I have no problem with restructuring code to make it more immune to
potential issues.  However, it appears there are several places throughout
the kernel that perform similar checks.  For example:

do_mmap()

	/* offset overflow? */
	if ((pgoff + (len >> PAGE_SHIFT)) < pgoff)
		return -EOVERFLOW;

expand_upwards()

	/* Enforce stack_guard_gap */
	gap_addr = address + stack_guard_gap;

	/* Guard against overflow */
	if (gap_addr < address || gap_addr > TASK_SIZE)
		gap_addr = TASK_SIZE;

do_madvise()

	end = start + len;
	if (end < start)
		return -EINVAL;

I am not suggesting that these all be changed.  The question of a real
issue still remains.  However, if this is a real issue it would make more
sense to look for and change all such checks rather than one single occurrence.
-- 
Mike Kravetz