Messages in this thread | | | Date | Wed, 22 May 2019 23:38:29 +0300 | From | Mike Rapoport <> | Subject | Re: [PATCH] mm/gup: continue VM_FAULT_RETRY processing event for pre-faults |
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(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@linux.ibm.com> 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@linux.ibm.com> > >
-- Sincerely yours, Mike.
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