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SubjectRe: mtd: kernel BUG at arch/x86/mm/pat.c:279!
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On Fri, 2012-09-07 at 16:09 -0700, Linus Torvalds wrote:
> The "u32 len" -> "unsigned long len" thing *might* make a difference, though.

This I believe doesn't fix the reported BUG. I was trying to address
your previous comment about broken types.

>
> I also think your patch is incomplete even on 32-bit, because this:
>
> > if (mtd->type == MTD_RAM || mtd->type == MTD_ROM) {
> > off = vma->vm_pgoff << PAGE_SHIFT;
>
> is still wrong. It probably should be
>
> off = vma->vm_pgoff;
> off <<= PAGE_SHIFT;
>
> because vm_pgoff may be a 32-bit type, while "resource_size_t" may be
> 64-bit. Shifting the 32-bit type without a cast (implicit or explicit)
> isn't going to help.

Agree.

> That said, we have absolutely *tons* of bugs with this particular
> pattern. Just do
>
> git grep 'vm_pgoff.*<<.*PAGE_SHIFT'
>
> and there are distressingly few casts in there (there's a few, mainly
> in fs/proc).
>
> Now, I suspect many of them are fine just because most users probably
> are size-limited anyway, but it's a bit distressing stuff. And I
> suspect it means we might want to introduce a helper function like
>
> static inline u64 vm_offset(struct vm_area_struct *vma)
> {
> return (u64)vma->vm_pgoff << PAGE_SHIFT;
> }
>
> or something. Maybe add the "vm_length()" helper while at it too,
> since the whole "vma->vm_end - vma->vm_start" thing is so common.

Agree.

> Anyway, since Sasha's oops is clearly not 32-bit, the above issues
> don't matter, and it would be interesting to hear if it's the 32-bit
> 'len' thing that triggers this problem. Still, I can't see how it
> would - as far as I can tell, a truncated 'len' would at most result
> in spurious early "return -EINVAL", not any real problem.
>
> What are we missing?
>

On Fri, 2012-09-07 at 15:42 -0700, Suresh Siddha wrote:
> - if ((vma->vm_end - vma->vm_start + off) > len)
> + if (off >= len || (vma->vm_end - vma->vm_start + off) > len)
> return -EINVAL;

This is the relevant portion that I am thinking will address the BUG.

Essentially the user is trying to mmap at a very large offset (from the
oops it appears "vma->vm_pgoff << PAGE_SHIFT + start" ends up to
"0xfffffffffffff000").
So it appears that the condition "(vma->vm_end - vma->vm_start + off) >
len" might be false because of the wraparound? and doesn't return
-EINVAL.

Let's see what Sasha finds. Anyways the patch does indeed require your
above mentioned vm_pgoff fix for the 32-bit case.

thanks,
suresh





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