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