Re: [PATCH] hugetlbfs: Fix integer overflow check in hugetlbfs_file_mmap()

[Dan Carpenter <dan.carpenter@linaro.org>]

On Thu, Jul 13, 2023 at 03:57:00PM +0800, linke li wrote:
> > 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.
> 
> Hi, Mike. I have checked the example code you provided, and the
> difference between
> those codes and the patched code is that those checks are checks for
> unsigned integer
>  overflow, which is well-defined. Only undefined behavior poses a
> security risk. So they
>  don't need any modifications. I have only found one occurrence of
> signed number
> overflow so far.

I used to have a similar check to that but I eventually deleted it
because I decided that the -fno-strict-overflow option works.  It didn't
produce a lot of warnings.

Historically we have done a bad job at open coding integer overflow
checks.  Some that I wrote turned out to be incorrect.  And even when
I write them correctly a couple times people have "fixed" them even
harder without CCing me or asking me why I wrote them the way I did.

What about using the check_add_overflow() macro?

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 7b17ccfa039d..c512165736e0 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -155,9 +155,8 @@ static int hugetlbfs_file_mmap(struct file *file, struct vm_area_struct *vma)
 		return -EINVAL;
 
 	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)
+	if (check_add_overflow(vma_len, (loff_t)vma->vm_pgoff << PAGE_SHIFT,
+			       &len))
 		return -EINVAL;
 
 	inode_lock(inode);