Re: [RESEND PATCH] mm: page_alloc: validate buddy before check the migratetype

Xianting Tian <xianting.tian@xxxxxxxxxxxxxxxxx> · Thu, 16 Jun 2022 09:51:58 +0800

在 2022/6/16 上午12:15, Xianting Tian 写道:

在 2022/6/15 下午9:55, Zi Yan 写道:
On 15 Jun 2022, at 2:47, Xianting Tian wrote:

在 2022/6/14 上午8:14, Zi Yan 写道:
On 13 Jun 2022, at 19:47, Guo Ren wrote:

On Tue, Jun 14, 2022 at 3:49 AM Zi Yan <ziy@xxxxxxxxxx> wrote:
On 13 Jun 2022, at 12:32, Guo Ren wrote:

On Mon, Jun 13, 2022 at 11:23 PM Zi Yan <ziy@xxxxxxxxxx> wrote:
Hi Xianting,

Thanks for your patch.

On 13 Jun 2022, at 9:10, Xianting Tian wrote:

Commit 787af64d05cd ("mm: page_alloc: validate buddy before 
check its migratetype.")
added buddy check code. But unfortunately, this fix isn't 
backported to
linux-5.17.y and the former stable branches. The reason is it 
added wrong
fixes message:
       Fixes: 1dd214b8f21c ("mm: page_alloc: avoid merging 
non-fallbackable
                           pageblocks with others")
No, the Fixes tag is right. The commit above does need to 
validate buddy.
I think Xianting is right. The “Fixes:" tag is not accurate and the
page_is_buddy() is necessary here.

This patch could be applied to the early version of the stable tree
(eg: Linux-5.10.y, not the master tree)
This is quite misleading. Commit 787af64d05cd applies does not 
mean it is
intended to fix the preexisting bug. Also it does not apply cleanly
to commit d9dddbf55667, there is a clear indentation mismatch. At 
best,
you can say the way of 787af64d05cd fixing 1dd214b8f21c also 
fixes d9dddbf55667.
There is no way you can apply 787af64d05cd to earlier trees and 
call it a day.

You can mention 787af64d05cd that it fixes a bug in 1dd214b8f21c 
and there is
a similar bug in d9dddbf55667 that can be fixed in a similar way 
too. Saying
the fixes message is wrong just misleads people, making them 
think there is
no bug in 1dd214b8f21c. We need to be clear about this.
First, d9dddbf55667 is earlier than 1dd214b8f21c in Linus tree. The
origin fixes could cover the Linux-5.0.y tree if they give the
accurate commit number and that is the cause we want to point out.
Yes, I got that d9dddbf55667 is earlier and commit 787af64d05cd fixes
the issue introduced by d9dddbf55667. But my point is that 
787af64d05cd
is not intended to fix d9dddbf55667 and saying it has a wrong fixes
message is misleading. This is the point I want to make.

Second, if the patch is for d9dddbf55667 then it could cover any tree
in the stable repo. Actually, we only know Linux-5.10.y has the
problem.
But it is not and does not apply to d9dddbf55667 cleanly.

Maybe, Gregkh could help to direct us on how to deal with the issue:
(Fixup a bug which only belongs to the former stable branch.)

I think you just need to send this patch without saying “commit
787af64d05cd fixes message is wrong” would be a good start. You also
need extra fix to mm/page_isolation.c for kernels between 5.15 and 
5.17
(inclusive). So there will need to be two patches:

1) your patch to stable tree prior to 5.15 and

2) your patch with an additional mm/page_isolation.c fix to stable 
tree
between 5.15 and 5.17.

Also, you will need to fix the mm/page_isolation.c code too to 
make this patch
complete, unless you can show that PFN=0x1000 is never going to 
be encountered
in the mm/page_isolation.c code I mentioned below.
No, we needn't fix mm/page_isolation.c in linux-5.10.y, because it 
had
pfn_valid_within(buddy_pfn) check after __find_buddy_pfn() to prevent
buddy_pfn=0.
The root cause comes from __find_buddy_pfn():
return page_pfn ^ (1 << order);
Right. But pfn_valid_within() was removed since 5.15. So your fix is
required for kernels between 5.15 and 5.17 (inclusive).

When page_pfn is the same as the order size, it will return the
previous buddy not the next. That is the only exception for this
algorithm, right?




In fact, the bug is a very long time to reproduce and is not easy to
debug, so we want to contribute it to the community to prevent other
guys from wasting time. Although there is no new patch at all.
Thanks for your reporting and sending out the patch. I really
appreciate it. We definitely need your inputs. Throughout the email
thread, I am trying to help you clarify the bug and how to fix it
properly:

1. The commit 787af64d05cd does not apply cleanly to commits
d9dddbf55667, meaning you cannot just cherry-pick that commit to
fix the issue. That is why we need your patch to fix the issue.
And saying it has a wrong fixes message in this patch’s git log is
misleading.

2. For kernels between 5.15 and 5.17 (inclusive), an additional fix
to mm/page_isolation.c is also needed, since pfn_valid_within() was
removed since 5.15 and the issue can appear during page isolation.

3. For kernels before 5.15, this patch will apply.
Zi Yan, Guo Ren,

I think we still need some imporvemnt for MASTER branch, as we 
discussed above, we will get an illegal buddy page if buddy_pfn is 0,

within page_is_buddy(), it still use the illegal buddy page to do 
the check. I think in most of cases, page_is_buddy() can return 
false,  but it still may return true with very low probablity.
Can you elaborate more on this? What kind of page can lead to 
page_is_buddy()
returning true? You said it is buddy_pfn is 0, but if the page is 
reserved,
if (!page_is_guard(buddy) && !PageBuddy(buddy)) should return false.
Maybe show us the dump_page() that offending page.

Thanks.

Let‘s take the issue we met on RISC-V arch for example,

pfn_base is 512 as we reserved 2M RAM for opensbi, mem_map's value is 
0xffffffe07e205000, which is the page address of PFN 512.

__find_buddy_pfn() returned 0 for PFN 0x2000 with order 0xd.
We know PFN 0 is not a valid pfn for buddy system, because 512 is the 
first PFN for buddy system.

Then it use below code to get buddy page with buddy_pfn 0:
buddy = page + (buddy_pfn - pfn);
So buddy page address is:
0xffffffe07e1fe000 = (struct page*)0xffffffe07e26e000 + (0 - 0x2000)

we can know this buddy page's address is less than 
mem_map(0xffffffe07e1fe000 < 0xffffffe07e205000),
actually 0xffffffe07e1fe000 is not a valid page's address. If we use 
0xffffffe07e1fe000
as the page's address to extract the value of a member in 'struct 
page', we may get an uncertain value.
That's why I say page_is_buddy() may return true with very low 
probablity.

So I think we need to add the code the verify buddy_pfn in the first 
place:
    pfn_valid(buddy_pfn)

The contents of the invalid page as below, all 0 in contents, but it may 
diff next time.

(gdb) p /x *(struct page *)0xffffffe07e1fe000

$15 = {flags = 0x0, {{lru = {next = 0x0, prev = 0x0}, mapping = 0x0, 
index = 0x0, private = 0x0}, {dma_addr = {0x0, 0x0}}, {{slab_list = 
{next = 0x0, prev = 0
          x0}, {next = 0x0, pages = 0x0, pobjects = 0x0}}, slab_cache = 
0x0, freelist = 0x0, {s_mem = 0x0, counters = 0x0, {inuse = 0x0, objects 
= 0x0
          , frozen = 0x0}}}, {compound_head = 0x0, compound_dtor = 0x0, 
compound_order = 0x0, compound_mapcount = {counter = 0x0}, compound_nr =
      0x0}, {_compound_pad_1 = 0x0, hpage_pinned_refcount = {counter = 
0x0}, deferred_list = {next = 0x0, prev = 0x0}}, {_pt_pad_1 = 0x0, pmd_hu
      ge_pte = 0x0, _pt_pad_2 = 0x0, {pt_mm = 0x0, pt_frag_refcount = 
{counter = 0x0}}, ptl = {{rlock = {raw_lock = {lock = 0x0}}}}}, {pgmap = 
0x0, zone
      _device_data = 0x0}, callback_head = {next = 0x0, func = 0x0}}, 
{_mapcount = {counter = 0x0}, page_type = 0x0, active = 0x0, units = 
0x0}, _refcount

   = {counter = 0x0}}

This is the first normal page in mem_map,

(gdb) p /x *(struct page *)0xffffffe07e26e000
$16 = {flags = 0x0, {{lru = {next = 0x100, prev = 0x122}, mapping = 0x0, 
index = 0x0, private = 0x0}, {dma_addr = {0x100, 0x122}}, {{slab_list = 
{next = 0x100
          , prev = 0x122}, {next = 0x100, pages = 0x122, pobjects = 
0x0}}, slab_cache = 0x0, freelist = 0x0, {s_mem = 0x0, counters = 0x0, 
{inuse
          = 0x0, objects = 0x0, frozen = 0x0}}}, {compound_head = 
0x100, compound_dtor = 0x22, compound_order = 0x1, compound_mapcount = 
{counter = 0x0}
        , compound_nr = 0x0}, {_compound_pad_1 = 0x100, 
hpage_pinned_refcount = {counter = 0x122}, deferred_list = {next = 0x0, 
prev = 0x0}}, {_
      pt_pad_1 = 0x100, pmd_huge_pte = 0x122, _pt_pad_2 = 0x0, {pt_mm = 
0x0, pt_frag_refcount = {counter = 0x0}}, ptl = {{rlock = {raw_lock = 
{lock = 0x0}}}}
              }, {pgmap = 0x100, zone_device_data = 0x122}, 
callback_head = {next = 0x100, func = 0x122}}, {_mapcount = {counter = 
0xffffffff},
      page_type = 0xffffffff, active = 0xffffffff, units = 0xffffffff}, 
_refcount = {counter = 0x0}}

I think we need to add some code to verify buddy_pfn in the first 
place.

Could you give some suggestions for this idea?

Actually, this issue is involved by commit:
       commit d9dddbf55667 ("mm/page_alloc: prevent merging 
between isolated and other pageblocks")

For RISC-V arch, the first 2M is reserved for sbi, so the 
start PFN is 512,
but it got buddy PFN 0 for PFN 0x2000:
       0 = 0x2000 ^ (1 << 12)
With the illegal buddy PFN 0, it got an illegal buddy page, 
which caused
crash in __get_pfnblock_flags_mask().
It seems that the RISC-V arch reveals a similar bug from 
d9dddbf55667.
Basically, this bug will only happen when PFN=0x2000 is merging 
up and
there are some isolated pageblocks.
Not PFN=0x2000, it's PFN=0x1000, I guess.

RISC-V's first 2MB RAM could reserve for opensbi, so it would have
riscv_pfn_base=512 and mem_map began with 512th PFN when
CONFIG_FLATMEM=y.
(Also, csky has the same issue: a non-zero pfn_base in some 
scenarios.)

But __find_buddy_pfn algorithm thinks the start address is 0, it 
could
get 0 pfn or less than the pfn_base value. We need another check to
prevent that.

BTW, what does first reserved 2MB imply? All 4KB pages from 
first 2MB are
set to PageReserved?

With the patch, it can avoid the calling of 
get_pageblock_migratetype() if
it isn't buddy page.
You might miss the __find_buddy_pfn() caller in 
unset_migratetype_isolate()
from mm/page_isolation.c, if you are talking about linux-5.17.y 
and former
version. There, page_is_buddy() is also not called and 
is_migrate_isolate_page()
is called, which calls get_pageblock_migratetype() too.

Fixes: d9dddbf55667 ("mm/page_alloc: prevent merging between 
isolated and other pageblocks")
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: zjb194813@xxxxxxxxxxxxxxx
Reported-by: tianhu.hh@xxxxxxxxxxxxxxx
Signed-off-by: Xianting Tian <xianting.tian@xxxxxxxxxxxxxxxxx>
---
   mm/page_alloc.c | 3 +++
   1 file changed, 3 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b1caa1c6c887..5b423caa68fd 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1129,6 +1129,9 @@ static inline void 
__free_one_page(struct page *page,

                        buddy_pfn = __find_buddy_pfn(pfn, order);
                        buddy = page + (buddy_pfn - pfn);
+
+                     if (!page_is_buddy(page, buddy, order))
+                             goto done_merging;
                        buddy_mt = 
get_pageblock_migratetype(buddy);

                        if (migratetype != buddy_mt
--
2.17.1
--
Best Regards,
Yan, Zi

--
Best Regards
   Guo Ren

ML: https://lore.kernel.org/linux-csky/
--
Best Regards,
Yan, Zi

--
Best Regards
   Guo Ren

ML: https://lore.kernel.org/linux-csky/
--
Best Regards,
Yan, Zi
--
Best Regards,
Yan, Zi







