Re: [PATCH] x86/mpx: fix recursive munmap() corruption

[Michael Ellerman]

Thomas Gleixner <tglx@xxxxxxxxxxxxx> writes:
> On Mon, 1 Apr 2019, Dave Hansen wrote:
>> diff -puN mm/mmap.c~mpx-rss-pass-no-vma mm/mmap.c
>> --- a/mm/mmap.c~mpx-rss-pass-no-vma	2019-04-01 06:56:53.409411123 -0700
>> +++ b/mm/mmap.c	2019-04-01 06:56:53.423411123 -0700
>> @@ -2731,9 +2731,17 @@ int __do_munmap(struct mm_struct *mm, un
>>  		return -EINVAL;
>>  
>>  	len = PAGE_ALIGN(len);
>> +	end = start + len;
>>  	if (len == 0)
>>  		return -EINVAL;
>>  
>> +	/*
>> +	 * arch_unmap() might do unmaps itself.  It must be called
>> +	 * and finish any rbtree manipulation before this code
>> +	 * runs and also starts to manipulate the rbtree.
>> +	 */
>> +	arch_unmap(mm, start, end);
>
> ...
>   
>> -static inline void arch_unmap(struct mm_struct *mm, struct vm_area_struct *vma,
>> -			      unsigned long start, unsigned long end)
>> +static inline void arch_unmap(struct mm_struct *mm, unsigned long start,
>> +			      unsigned long end)
>
> While you fixed up the asm-generic thing, this breaks arch/um and
> arch/unicorn32. For those the fixup is trivial by removing the vma
> argument.
>
> But itt also breaks powerpc and there I'm not sure whether moving
> arch_unmap() to the beginning of __do_munmap() is safe. Micheal???

I don't know for sure but I think it should be fine. That code is just
there to handle CRIU unmapping/remapping the VDSO. So that either needs
to happen while the process is stopped or it needs to handle races
anyway, so I don't see how the placement within the unmap path should
matter.

> Aside of that the powerpc variant looks suspicious:
>
> static inline void arch_unmap(struct mm_struct *mm,
>                               unsigned long start, unsigned long end)
> {
>  	if (start <= mm->context.vdso_base && mm->context.vdso_base < end)
>                 mm->context.vdso_base = 0;
> }
>
> Shouldn't that be: 
>
>  	if (start >= mm->context.vdso_base && mm->context.vdso_base < end)
>
> Hmm?

Yeah looks pretty suspicious. I'll follow-up with Laurent who wrote it.
Thanks for spotting it!

cheers