Date:   Wed, 29 Sep 2021 09:59:38 -0700
From:   Song Liu <song@...nel.org>
To:     Rongwei Wang <rongwei.wang@...ux.alibaba.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        William Kucharski <william.kucharski@...cle.com>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH v2 1/2] mm, thp: check page mapping when truncating page cache

On Wed, Sep 29, 2021 at 12:50 AM Rongwei Wang
<rongwei.wang@...ux.alibaba.com> wrote:
>
>
>
> On 9/29/21 3:14 PM, Song Liu wrote:
> > On Tue, Sep 28, 2021 at 9:20 AM Rongwei Wang
> > <rongwei.wang@...ux.alibaba.com> wrote:
> >>
> >>
> >>
> >> On 9/28/21 6:24 AM, Song Liu wrote:
> >>> On Fri, Sep 24, 2021 at 12:12 AM Rongwei Wang
> >>> <rongwei.wang@...ux.alibaba.com> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On 9/24/21 10:43 AM, Andrew Morton wrote:
> >>>>> On Thu, 23 Sep 2021 01:04:54 +0800 Rongwei Wang <rongwei.wang@...ux.alibaba.com> wrote:
> >>>>>
> >>>>>>
> >>>>>>
> >>>>>>> On Sep 22, 2021, at 7:37 PM, Matthew Wilcox <willy@...radead.org> 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@casper.infradead.org/
> >>>>>> 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
> Hi, I can sure I am not update the stress_madvise_dso.c.
>
> My test environment is vm (qemu-system-aarch64, 32 cores). And I can
> think of the following possibilities:
>
> (1) in thread_read()
>
> printf("read %s\n", dso_path);
> d = open(dso_path, O_RDONLY);
> /* The start addr must be alignment with 2M */
> void *p = mmap((void *)0x40000dc00000UL, 0x800000, PROT_READ |
> PROT_EXEC,MAP_PRIVATE, fd, 0);
> if (p == MAP_FAILED) {
>         perror("mmap");
>         goto out;
> }
>
> 0x40000dc00000 is random setting by myself. I am not sure this address
> is available in your vm.
>
> (2) in thread_write()
> int fd = open(dso_path, O_RDWR);
> p = mmap(NULL, 0x800000, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
> if (p == MAP_FAILED) {
>         perror("mmap");
>          goto out; /* fail */
> }
>
> because of I am sure the DSO is bigger than 0x800000, so directly map
> the DSO using 0x800000. Maybe I had use '-z max-page-size=0x200000' to
> compile the DSO? likes:
> $ gcc -z max-page-size=0x200000 -o libtest.so -shared libtest.o
>
> If you don't mind, you can send the segment fault log to me. And I will
> find x86 environment to test.

I fixed the segfault with
1. malloc buf (as it is too big for stack) in thread_read
2. reduce memcpy() size in thread_read.

Now, I am able to crash the system on
    find_lock_entries () {
     ...
       VM_BUG_ON_PAGE(page->index != xas.xa_index, page);
    }
I guess it is related. I will test more.

Thanks,
Song

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