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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Wed, 22 May 2019 21:43:24 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Mike Rapoport <rppt@...ux.ibm.com>,
        Andrea Arcangeli <aarcange@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH] mm/gup: continue VM_FAULT_RETRY processing event for
 pre-faults

On 2019-05-22 12:21:13 [-0700], Andrew Morton wrote:
> On Tue, 14 May 2019 17:29:55 +0300 Mike Rapoport <rppt@...ux.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?

buf_fx is user stack pointer and it should not be 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.

exactly, this is not expected.

> 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.

It looks, like fault_in_pages_writeable() would work. If this is the
recommendation from the MM department than I can switch to that.

> > 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?

Sebastian