Re: [PATCH v2 1/2] mm, thp: check page mapping when truncating page cache

[Song Liu]

On Tue, Sep 28, 2021 at 9:20 AM Rongwei Wang
<rongwei.wang@xxxxxxxxxxxxxxxxx> wrote:
>
>
>
> On 9/28/21 6:24 AM, Song Liu wrote:
> > On Fri, Sep 24, 2021 at 12:12 AM Rongwei Wang
> > <rongwei.wang@xxxxxxxxxxxxxxxxx> wrote:
> >>
> >>
> >>
> >> On 9/24/21 10:43 AM, Andrew Morton wrote:
> >>> On Thu, 23 Sep 2021 01:04:54 +0800 Rongwei Wang <rongwei.wang@xxxxxxxxxxxxxxxxx> wrote:
> >>>
> >>>>
> >>>>
> >>>>> On Sep 22, 2021, at 7:37 PM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> >>>>>
> >>>>> On Wed, Sep 22, 2021 at 03:06:44PM +0800, Rongwei Wang wrote:
> >>>>>> Transparent huge page has supported read-only non-shmem files. The file-
> >>>>>> backed THP is collapsed by khugepaged and truncated when written (for
> >>>>>> shared libraries).
> >>>>>>
> >>>>>> However, there is race in two possible places.
> >>>>>>
> >>>>>> 1) multiple writers truncate the same page cache concurrently;
> >>>>>> 2) collapse_file rolls back when writer truncates the page cache;
> >>>>>
> >>>>> As I've said before, the bug here is that somehow there is a writable fd
> >>>>> to a file with THPs.  That's what we need to track down and fix.
> >>>> Hi, Matthew
> >>>> I am not sure get your means. We know “mm, thp: relax the VM_DENYWRITE constraint on file-backed THPs"
> >>>> Introduced file-backed THPs for DSO. It is possible {very rarely} for DSO to be opened in writeable way.
> >>>>
> >>>> ...
> >>>>
> >>>>> https://lore.kernel.org/linux-mm/YUdL3lFLFHzC80Wt@xxxxxxxxxxxxxxxxxxxx/
> >>>> All in all, what you mean is that we should solve this race at the source?
> >>>
> >>> Matthew is being pretty clear here: we shouldn't be permitting
> >>> userspace to get a writeable fd for a thp-backed file.
> >>>
> >>> Why are we permitting the DSO to be opened writeably?  If there's a
> >>> legitimate case for doing this then presumably "mm, thp: relax the
> >> There is a use case to stress file-backed THP within attachment.
> >> I test this case in a system which has enabled CONFIG_READ_ONLY_THP_FOR_FS:
> >>
> >> $ gcc -Wall -g -o stress_madvise_dso stress_madvise_dso.c
> >> $ ulimit -s unlimited
> >> $ ./stress_madvise_dso 10000 <libtest.so>
> >>
> >> the meaning of above parameters:
> >> 10000: the max test time;
> >> <libtest.so>: the DSO that will been mapped into file-backed THP by
> >> madvise. It recommended that the text segment of DSO to be tested is
> >> greater than 2M.
> >>
> >> The crash will been triggered at once in the latest kernel. And this
> >> case also can used to trigger the bug that mentioned in our another patch.
> >
> > Hmm.. I am not able to use the repro program to crash the system. Not
> > sure what I did wrong.
> >
> Hi
> I have tried to check my test case again. Can you make sure the DSO that
> you test have THP mapping?
>
> If you are willing to try again, I can send my libtest.c which is used
> to test by myself (actually, it shouldn't be target DSO problem).
>
> Thanks very much!
> > OTOH, does it make sense to block writes within khugepaged, like:
> >
> > diff --git i/mm/khugepaged.c w/mm/khugepaged.c
> > index 045cc579f724e..ad7c41ec15027 100644
> > --- i/mm/khugepaged.c
> > +++ w/mm/khugepaged.c
> > @@ -51,6 +51,7 @@ enum scan_result {
> >          SCAN_CGROUP_CHARGE_FAIL,
> >          SCAN_TRUNCATED,
> >          SCAN_PAGE_HAS_PRIVATE,
> > +       SCAN_BUSY_WRITE,
> >   };
> >
> >   #define CREATE_TRACE_POINTS
> > @@ -1652,6 +1653,11 @@ static void collapse_file(struct mm_struct *mm,
> >          /* Only allocate from the target node */
> >          gfp = alloc_hugepage_khugepaged_gfpmask() | __GFP_THISNODE;
> >
> > +       if (deny_write_access(file)) {
> > +               result = SCAN_BUSY_WRITE;
> > +               return;
> > +       }
> > +
> This can indeed avoid some possible races from source.
>
> But, I am thinking about whether this will lead to DDoS attack?
> I remember the reason of DSO has ignored MAP_DENYWRITE in kernel
> is that DDoS attack. In addition, 'deny_write_access' will change
> the behavior, such as user will get 'Text file busy' during
> collapse_file. I am not sure whether the behavior changing is acceptable
> in user space.
>
> If it is acceptable, I am very willing to fix the races like your way.

I guess we should not let the write get ETXTBUSY for khugepaged work.

I am getting some segfault on stress_madvise_dso. And it doesn't really
generate the bug stack in my vm (qemu-system-x86_64). Is there an newer
version of it?

Thanks,
Song