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

Laurent Dufour
2019-04-23T17:07:57Z

Le 23/04/2019 à 18:04, Dave Hansen a écrit :
> On 4/23/19 4:16 AM, Laurent Dufour wrote:
>> My only concern is the error path.
>> Calling arch_unmap() before handling any error case means that it will
>> have to be undo and there is no way to do so.
>
> Is there a practical scenario where munmap() of the VDSO can split a
> VMA? If the VDSO is guaranteed to be a single page, it would have to be
> a scenario where munmap() was called on a range that included the VDSO
> *and* other VMA that we failed to split.
>
> But, the scenario would have to be that someone tried to munmap() the
> VDSO and something adjacent, the munmap() failed, and they kept on using
> the VDSO and expected the special signal and perf behavior to be maintained.

I've to admit that this should not be a common scenario, and unmapping
the VDSO is not so common anyway.

> BTW, what keeps the VDSO from merging with an adjacent VMA? Is it just
> the vm_ops->close that comes from special_mapping_vmops?

I'd think so.

>> I don't know what is the rational to move arch_unmap() to the beginning
>> of __do_munmap() but the error paths must be managed.
>
> It's in the changelog:
>
> https://patchwork.kernel.org/patch/10909727/
>
> But, the tl;dr version is: x86 is recursively calling __do_unmap() (via
> arch_unmap()) in a spot where the internal rbtree data is inconsistent,
> which causes all kinds of fun. If we move arch_unmap() to before
> __do_munmap() does any data structure manipulation, the recursive call
> doesn't get confused any more.

If only Powerpc is impacted I guess this would be fine but what about
the other architectures?

>> There are 2 assumptions here:
>> 1. 'start' and 'end' are page aligned (this is guaranteed by __do_munmap().
>> 2. the VDSO is 1 page (this is guaranteed by the union vdso_data_store on powerpc)
>
> Are you sure about #2? The 'vdso64_pages' variable seems rather
> unnecessary if the VDSO is only 1 page. ;)

Hum, not so sure now ;)
I got confused, only the header is one page.
The test is working as a best effort, and don't cover the case where
only few pages inside the VDSO are unmmapped (start >
mm->context.vdso_base). This is not what CRIU is doing and so this was
enough for CRIU support.

Michael, do you think there is a need to manage all the possibility
here, since the only user is CRIU and unmapping the VDSO is not a so
good idea for other processes ?