Re: [PATCH] mm/gup: continue VM_FAULT_RETRY processing event for pre-faults

[Mike Rapoport]

(added kvm)

On Wed, May 22, 2019 at 12:21:13PM -0700, Andrew Morton wrote:
> On Tue, 14 May 2019 17:29:55 +0300 Mike Rapoport <rppt@xxxxxxxxxxxxx> wrote:
> 
> > When get_user_pages*() is called with pages = NULL, the processing of
> > VM_FAULT_RETRY terminates early without actually retrying to fault-in all
> > the pages.
> > 
> > If the pages in the requested range belong to a VMA that has userfaultfd
> > registered, handle_userfault() returns VM_FAULT_RETRY *after* user space
> > has populated the page, but for the gup pre-fault case there's no actual
> > retry and the caller will get no pages although they are present.
> > 
> > This issue was uncovered when running post-copy memory restore in CRIU
> > after commit d9c9ce34ed5c ("x86/fpu: Fault-in user stack if
> > copy_fpstate_to_sigframe() fails").
> > 
> > After this change, the copying of FPU state to the sigframe switched from
> > copy_to_user() variants which caused a real page fault to get_user_pages()
> > with pages parameter set to NULL.
> 
> You're saying that argument buf_fx in copy_fpstate_to_sigframe() is NULL?

Apparently I haven't explained well. The 'pages' parameter in the call to
get_user_pages_unlocked() is NULL.
 
> If so was that expected by the (now cc'ed) developers of
> d9c9ce34ed5c8923 ("x86/fpu: Fault-in user stack if
> copy_fpstate_to_sigframe() fails")?
> 
> It seems rather odd.  copy_fpregs_to_sigframe() doesn't look like it's
> expecting a NULL argument.
> 
> Also, I wonder if copy_fpstate_to_sigframe() would be better using
> fault_in_pages_writeable() rather than get_user_pages_unlocked().  That
> seems like it operates at a more suitable level and I guess it will fix
> this issue also.

If I understand correctly, one of the points of d9c9ce34ed5c8923 ("x86/fpu:
Fault-in user stack if copy_fpstate_to_sigframe() fails") was to to avoid
page faults, hence the use of get_user_pages().

With fault_in_pages_writeable() there might be a page fault, unless I've
completely mistaken.

Unrelated to copy_fpstate_to_sigframe(), the issue could happen if any call
to get_user_pages() with pages parameter set to NULL tries to access
userfaultfd-managed memory. Currently, there are 4 in tree users:

arch/x86/kernel/fpu/signal.c:198:8-31:  -> gup with !pages
arch/x86/mm/mpx.c:423:11-25:  -> gup with !pages
virt/kvm/async_pf.c:90:1-22:  -> gup with !pages
virt/kvm/kvm_main.c:1437:6-20:  -> gup with !pages

I don't know if anybody is using mpx with uffd and anyway mpx seems to go
away.

As for KVM, I think that post-copy live migration of L2 guest might trigger
that as well. Not sure though, I'm not really familiar with KVM code.
 
> > In post-copy mode of CRIU, the destination memory is managed with
> > userfaultfd and lack of the retry for pre-fault case in get_user_pages()
> > causes a crash of the restored process.
> > 
> > Making the pre-fault behavior of get_user_pages() the same as the "normal"
> > one fixes the issue.
> 
> Should this be backported into -stable trees?

I think that it depends on whether KVM affected by this or not.

> > Fixes: d9c9ce34ed5c ("x86/fpu: Fault-in user stack if copy_fpstate_to_sigframe() fails")
> > Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> 
> 

-- 
Sincerely yours,
Mike.