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From: Andrew Morton <akpm@linux-foundation.org>
To: Mike Rapoport <rppt@linux.ibm.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Borislav Petkov <bp@suse.de>
Subject: Re: [PATCH] mm/gup: continue VM_FAULT_RETRY processing event for pre-faults
Date: Wed, 22 May 2019 12:21:13 -0700	[thread overview]
Message-ID: <20190522122113.a2edc8aba32f0fad189bae21@linux-foundation.org> (raw)
In-Reply-To: <1557844195-18882-1-git-send-email-rppt@linux.ibm.com>

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?

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.

> 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?

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



  parent reply	other threads:[~2019-05-22 19:21 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-26 17:33 [PATCH] x86/fpu: Use fault_in_pages_writeable() for pre-faulting Sebastian Andrzej Siewior
2019-05-26 17:35 ` Sebastian Andrzej Siewior
2019-05-26 19:25   ` Hugh Dickins
2019-05-28 11:54     ` My emacs problem -- was " Pavel Machek
2019-05-29  4:18 ` Andrew Morton
2019-05-29  7:25   ` [PATCH v2] " Sebastian Andrzej Siewior
2019-05-14 14:29     ` [PATCH] mm/gup: continue VM_FAULT_RETRY processing event for pre-faults Mike Rapoport
2019-05-16 16:25       ` Andrei Vagin
2019-05-21 15:53       ` Mike Rapoport
2019-05-22 19:21       ` Andrew Morton [this message]
2019-05-22 19:43         ` Sebastian Andrzej Siewior
2019-05-24 22:22           ` Hugh Dickins
2019-05-25  8:45             ` Sebastian Andrzej Siewior
2019-05-25 18:09               ` Hugh Dickins
2019-05-26 19:36                 ` Sebastian Andrzej Siewior
2019-05-26 20:17                   ` Hugh Dickins
2019-05-26 17:25             ` Pavel Machek
2019-05-22 20:38         ` Mike Rapoport
2019-05-22 21:18           ` Andrew Morton
2019-06-22 17:51             ` Andrea Arcangeli
2019-06-06 17:25       ` [tip:x86/urgent] x86/fpu: Use fault_in_pages_writeable() for pre-faulting tip-bot for Hugh Dickins
2019-05-29 21:29     ` [PATCH v2] " Chris Wilson

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