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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Dmitry Vyukov <dvyukov@google.com>,
	syzbot 
	<bot+6a5269ce759a7bb12754ed9622076dc93f65a1f6@syzkaller.appspotmail.com>,
	Jan Beulich <JBeulich@suse.com>, "H. Peter Anvin" <hpa@zytor.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Laurent Dufour <ldufour@linux.vnet.ibm.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	syzkaller-bugs@googlegroups.com,
	Thomas Gleixner <tglx@linutronix.de>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Michal Hocko <mhocko@suse.com>, Hugh Dickins <hughd@google.com>,
	David Rientjes <rientjes@google.com>,
	linux-mm <linux-mm@kvack.org>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Thorsten Leemhuis <regressions@leemhuis.info>
Subject: Re: KASAN: use-after-free Read in __do_page_fault
Date: Tue, 31 Oct 2017 08:37:47 -0700	[thread overview]
Message-ID: <CA+55aFwWJyArZMEuo1-4+VaiP95e__cRHkVvrfiQ+NUVJ15DNQ@mail.gmail.com> (raw)
In-Reply-To: <4bc852be-7ef3-0b60-6dbb-81139d25a817@suse.cz>

On Tue, Oct 31, 2017 at 6:57 AM, Vlastimil Babka <vbabka@suse.cz> wrote:
>
> However, __do_page_fault() only expects that mmap_sem to be released
> when handle_mm_fault() returns with VM_FAULT_RETRY. It doesn't expect it
> to be released and then acquired again, because then vma can be indeed
> gone.

Yes. Accessing "vma" after calling "handle_mm_fault()" is a bug. An
unfortunate issue with userfaultfd.

The suggested fix to simply look up pkey beforehand seems sane and simple.

But sadly, from a quick check, it looks like arch/um/ has the same
bug, but even worse. It will do

 (a) handle_mm_fault() in a loop without re-calculating vma. Don't ask me why.

 (b) flush_tlb_page(vma, address); afterwards

but much more importantly, I think __get_user_pages() is broken in two ways:

 - faultin_page() does:

        ret = handle_mm_fault(vma, address, fault_flags);
        ...
        if ((ret & VM_FAULT_WRITE) && !(vma->vm_flags & VM_WRITE))

   (easily fixed the same way)

 - more annoyingly and harder to fix: the retry case in
__get_user_pages(), and the VMA saving there.

Ho humm.

Andrea, looking at that get_user_pages() case, I really think it's
userfaultfd that is broken.

Could we perhaps limit userfaultfd to _only_ do the VM_FAULT_RETRY,
and simply fail for non-retry faults?

             Linus

  parent reply	other threads:[~2017-10-31 15:37 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-10-30 19:12 syzbot
2017-10-30 19:15 ` Dmitry Vyukov
2017-10-31 12:00   ` Vlastimil Babka
2017-10-31 12:42     ` Dmitry Vyukov
2017-10-31 13:20       ` Vlastimil Babka
2017-10-31 13:57         ` Vlastimil Babka
2017-10-31 14:11           ` Kirill A. Shutemov
2017-10-31 14:28             ` Vlastimil Babka
2017-10-31 19:15               ` Andrea Arcangeli
2017-11-01  7:42                 ` Vlastimil Babka
2017-11-01 10:17                   ` Andrea Arcangeli
2017-11-01 12:14                     ` Vlastimil Babka
2017-10-31 15:37           ` Linus Torvalds [this message]
2017-10-31 19:13             ` Andrea Arcangeli
2017-11-01 15:26               ` Linus Torvalds
2017-11-02 19:36                 ` Andrea Arcangeli
2017-11-02 10:00           ` Laurent Dufour

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