[BUG] Re: [PATCH v3 10/13] mm/khugepaged: collapse_pte_mapped_thp() with mmap_read_lock()

Commit Message

Jann Horn Aug. 14, 2023, 8:36 p.m. UTC

On Wed, Jul 12, 2023 at 6:42 AM Hugh Dickins <[e-mail]> wrote:
> Bring collapse_and_free_pmd() back into collapse_pte_mapped_thp().
> It does need mmap_read_lock(), but it does not need mmap_write_lock(),
> nor vma_start_write() nor i_mmap lock nor anon_vma lock.  All racing
> paths are relying on pte_offset_map_lock() and pmd_lock(), so use those.

We can still have a racing userfaultfd operation at the "/* step 4:
remove page table */" point that installs a new PTE before the page
table is removed.

To reproduce, patch a delay into the kernel like this:


        pml = pmd_lock(mm, pmd);


And then run the attached reproducer against mm/mm-everything. You
should get this in dmesg:

[  206.578096] BUG: Bad rss-counter state mm:000000000942ebea
type:MM_ANONPAGES val:1

Comments

Hugh Dickins Aug. 15, 2023, 6:34 a.m. UTC | #1

On Mon, 14 Aug 2023, Jann Horn wrote:
> On Wed, Jul 12, 2023 at 6:42 AM Hugh Dickins <[e-mail]> wrote:
> > Bring collapse_and_free_pmd() back into collapse_pte_mapped_thp().
> > It does need mmap_read_lock(), but it does not need mmap_write_lock(),
> > nor vma_start_write() nor i_mmap lock nor anon_vma lock.  All racing
> > paths are relying on pte_offset_map_lock() and pmd_lock(), so use those.
> 
> We can still have a racing userfaultfd operation at the "/* step 4:
> remove page table */" point that installs a new PTE before the page
> table is removed.
> 
> To reproduce, patch a delay into the kernel like this:
> 
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 9a6e0d507759..27cc8dfbf3a7 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -20,6 +20,7 @@
>  #include <linux/swapops.h>
>  #include <linux/shmem_fs.h>
>  #include <linux/ksm.h>
> +#include <linux/delay.h>
> 
>  #include <asm/tlb.h>
>  #include <asm/pgalloc.h>
> @@ -1617,6 +1618,11 @@ int collapse_pte_mapped_thp(struct mm_struct
> *mm, unsigned long addr,
>         }
> 
>         /* step 4: remove page table */
> +       if (strcmp(current->comm, "DELAYME") == 0) {
> +               pr_warn("%s: BEGIN DELAY INJECTION\n", __func__);
> +               mdelay(5000);
> +               pr_warn("%s: END DELAY INJECTION\n", __func__);
> +       }
> 
>         /* Huge page lock is still held, so page table must remain empty */
>         pml = pmd_lock(mm, pmd);
> 
> 
> And then run the attached reproducer against mm/mm-everything. You
> should get this in dmesg:
> 
> [  206.578096] BUG: Bad rss-counter state mm:000000000942ebea
> type:MM_ANONPAGES val:1

Thanks a lot, Jann. I haven't thought about it at all yet; and just
tried to reproduce, but haven't yet got the "BUG: Bad rss-counter":
just see "Invalid argument" on the UFFDIO_COPY ioctl.
Will investigate tomorrow.

Hugh

David Hildenbrand Aug. 15, 2023, 7:11 a.m. UTC | #2

On 15.08.23 08:34, Hugh Dickins wrote:
> On Mon, 14 Aug 2023, Jann Horn wrote:
>> On Wed, Jul 12, 2023 at 6:42 AM Hugh Dickins <[e-mail]> wrote:
>>> Bring collapse_and_free_pmd() back into collapse_pte_mapped_thp().
>>> It does need mmap_read_lock(), but it does not need mmap_write_lock(),
>>> nor vma_start_write() nor i_mmap lock nor anon_vma lock.  All racing
>>> paths are relying on pte_offset_map_lock() and pmd_lock(), so use those.
>>
>> We can still have a racing userfaultfd operation at the "/* step 4:
>> remove page table */" point that installs a new PTE before the page
>> table is removed.
>>
>> To reproduce, patch a delay into the kernel like this:
>>
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 9a6e0d507759..27cc8dfbf3a7 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -20,6 +20,7 @@
>>   #include <linux/swapops.h>
>>   #include <linux/shmem_fs.h>
>>   #include <linux/ksm.h>
>> +#include <linux/delay.h>
>>
>>   #include <asm/tlb.h>
>>   #include <asm/pgalloc.h>
>> @@ -1617,6 +1618,11 @@ int collapse_pte_mapped_thp(struct mm_struct
>> *mm, unsigned long addr,
>>          }
>>
>>          /* step 4: remove page table */
>> +       if (strcmp(current->comm, "DELAYME") == 0) {
>> +               pr_warn("%s: BEGIN DELAY INJECTION\n", __func__);
>> +               mdelay(5000);
>> +               pr_warn("%s: END DELAY INJECTION\n", __func__);
>> +       }
>>
>>          /* Huge page lock is still held, so page table must remain empty */
>>          pml = pmd_lock(mm, pmd);
>>
>>
>> And then run the attached reproducer against mm/mm-everything. You
>> should get this in dmesg:
>>
>> [  206.578096] BUG: Bad rss-counter state mm:000000000942ebea
>> type:MM_ANONPAGES val:1
> 
> Thanks a lot, Jann. I haven't thought about it at all yet; and just
> tried to reproduce, but haven't yet got the "BUG: Bad rss-counter":
> just see "Invalid argument" on the UFFDIO_COPY ioctl.
> Will investigate tomorrow.

Maybe you're missing a fixup:

https://lkml.kernel.org/r/[e-mail]

When the src address is not page aligned, UFFDIO_COPY in mm-unstable 
would erroneously fail.

Hugh Dickins Aug. 15, 2023, 3:41 p.m. UTC | #3

On Tue, 15 Aug 2023, David Hildenbrand wrote:
> On 15.08.23 08:34, Hugh Dickins wrote:
> > On Mon, 14 Aug 2023, Jann Horn wrote:
> >>
> >>          /* step 4: remove page table */
> >> +       if (strcmp(current->comm, "DELAYME") == 0) {
> >> +               pr_warn("%s: BEGIN DELAY INJECTION\n", __func__);
> >> +               mdelay(5000);
> >> +               pr_warn("%s: END DELAY INJECTION\n", __func__);
> >> +       }
> >>
> >>          /* Huge page lock is still held, so page table must remain empty
> >>          */
> >>          pml = pmd_lock(mm, pmd);
> >>
> >>
> >> And then run the attached reproducer against mm/mm-everything. You
> >> should get this in dmesg:
> >>
> >> [  206.578096] BUG: Bad rss-counter state mm:000000000942ebea
> >> type:MM_ANONPAGES val:1
> > 
> > Thanks a lot, Jann. I haven't thought about it at all yet; and just
> > tried to reproduce, but haven't yet got the "BUG: Bad rss-counter":
> > just see "Invalid argument" on the UFFDIO_COPY ioctl.
> > Will investigate tomorrow.
> 
> Maybe you're missing a fixup:
> 
> https://lkml.kernel.org/r/[e-mail]
> 
> When the src address is not page aligned, UFFDIO_COPY in mm-unstable would
> erroneously fail.

You got it, many thanks David: I had assumed that my next-20230808 tree
would be up-to-date enough, but it wasn't.  Reproduced now.

Hugh

Hugh Dickins Aug. 21, 2023, 7:48 p.m. UTC | #4

On Mon, 14 Aug 2023, Jann Horn wrote:
> On Wed, Jul 12, 2023 at 6:42 AM Hugh Dickins <[e-mail]> wrote:
> > Bring collapse_and_free_pmd() back into collapse_pte_mapped_thp().
> > It does need mmap_read_lock(), but it does not need mmap_write_lock(),
> > nor vma_start_write() nor i_mmap lock nor anon_vma lock.  All racing
> > paths are relying on pte_offset_map_lock() and pmd_lock(), so use those.
> 
> We can still have a racing userfaultfd operation at the "/* step 4:
> remove page table */" point that installs a new PTE before the page
> table is removed.

And you've been very polite not to remind me that this is exactly
what you warned me about, in connection with retract_page_tables(),
nearly three months ago:

https://lore.kernel.org/linux-mm/[e-mail]/

> 
> To reproduce, patch a delay into the kernel like this:
> 
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 9a6e0d507759..27cc8dfbf3a7 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -20,6 +20,7 @@
>  #include <linux/swapops.h>
>  #include <linux/shmem_fs.h>
>  #include <linux/ksm.h>
> +#include <linux/delay.h>
> 
>  #include <asm/tlb.h>
>  #include <asm/pgalloc.h>
> @@ -1617,6 +1618,11 @@ int collapse_pte_mapped_thp(struct mm_struct
> *mm, unsigned long addr,
>         }
> 
>         /* step 4: remove page table */
> +       if (strcmp(current->comm, "DELAYME") == 0) {
> +               pr_warn("%s: BEGIN DELAY INJECTION\n", __func__);
> +               mdelay(5000);
> +               pr_warn("%s: END DELAY INJECTION\n", __func__);
> +       }
> 
>         /* Huge page lock is still held, so page table must remain empty */
>         pml = pmd_lock(mm, pmd);
> 
> 
> And then run the attached reproducer against mm/mm-everything. You
> should get this in dmesg:
> 
> [  206.578096] BUG: Bad rss-counter state mm:000000000942ebea
> type:MM_ANONPAGES val:1

Very helpful, thank you Jann.

I got a bit distracted when I then found mm's recent addition of
UFFDIO_POISON: thought I needed to change both collapse_pte_mapped_thp()
and retract_page_tables() now to cope with mfill_atomic_pte_poison()
inserting into even a userfaultfd_armed shared VMA.

But eventually, on second thoughts, realized that's only inserting a pte
marker, invalid, so won't cause any actual trouble.  A little untidy,
to leave that behind in a supposedly empty page table about to be freed,
but not worth refactoring these functions to avoid a non-bug.

And though syzbot and JH may find some fun with it, I don't think any
real application would be insertng a PTE_MARKER_POISONED where a huge
page collapse is almost complete.

So I scaled back to a more proportionate fix, following.  Sorry, I've
slightly messed up applying the "DELAY INJECTION" patch above: not
intentional, honest!  (mdelay while holding the locks is still good.)

Hugh

Patch

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 9a6e0d507759..27cc8dfbf3a7 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -20,6 +20,7 @@ 
 #include <linux/swapops.h>
 #include <linux/shmem_fs.h>
 #include <linux/ksm.h>
+#include <linux/delay.h>

 #include <asm/tlb.h>
 #include <asm/pgalloc.h>
@@ -1617,6 +1618,11 @@  int collapse_pte_mapped_thp(struct mm_struct
*mm, unsigned long addr,
        }

        /* step 4: remove page table */
+       if (strcmp(current->comm, "DELAYME") == 0) {
+               pr_warn("%s: BEGIN DELAY INJECTION\n", __func__);
+               mdelay(5000);
+               pr_warn("%s: END DELAY INJECTION\n", __func__);
+       }

        /* Huge page lock is still held, so page table must remain empty */