Re: [PATCH v2 2/3] mm/page_table_check: check entries at pud and pmd levels

From: Pasha Tatashin
Date: Fri Jan 21 2022 - 14:33:38 EST

On Thu, Jan 20, 2022 at 2:19 PM Wei Xu <weixugc@xxxxxxxxxx> wrote:
>
> On Thu, Jan 20, 2022 at 11:12 AM Pasha Tatashin
> <pasha.tatashin@xxxxxxxxxx> wrote:
> >
> > syzbot detected a case where the page table counters were not properly
> > updated.
> >
> > syzkaller login: ------------[ cut here ]------------
> > kernel BUG at mm/page_table_check.c:162!
> > invalid opcode: 0000 [#1] PREEMPT SMP KASAN
> > CPU: 0 PID: 3099 Comm: pasha Not tainted 5.16.0+ #48
> > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIO4
> > RIP: 0010:__page_table_check_zero+0x159/0x1a0
> > Code: 7d 3a b2 ff 45 39 f5 74 2a e8 43 38 b2 ff 4d 85 e4 01
> > RSP: 0018:ffff888010667418 EFLAGS: 00010293
> > RAX: 0000000000000000 RBX: 0000000000000001 RCX: 0000000000
> > RDX: ffff88800cea8680 RSI: ffffffff81becaf9 RDI: 0000000003
> > RBP: ffff888010667450 R08: 0000000000000001 R09: 0000000000
> > R10: ffffffff81becaab R11: 0000000000000001 R12: ffff888008
> > R13: 0000000000000001 R14: 0000000000000200 R15: dffffc0000
> > FS: 0000000000000000(0000) GS:ffff888035e00000(0000) knlG0
> > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 00007ffd875cad00 CR3: 00000000094ce000 CR4: 0000000000
> > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000
> > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000
> > Call Trace:
> > <TASK>
> > free_pcp_prepare+0x3be/0xaa0
> > free_unref_page+0x1c/0x650
> > ? trace_hardirqs_on+0x6a/0x1d0
> > free_compound_page+0xec/0x130
> > free_transhuge_page+0x1be/0x260
> > __put_compound_page+0x90/0xd0
> > release_pages+0x54c/0x1060
> > ? filemap_remove_folio+0x161/0x210
> > ? lock_downgrade+0x720/0x720
> > ? __put_page+0x150/0x150
> > ? filemap_free_folio+0x164/0x350
> > __pagevec_release+0x7c/0x110
> > shmem_undo_range+0x85e/0x1250
> > ...
> >
> > The repro involved having a huge page that is split due to uprobe event
> > temporarily replacing one of the pages in the huge page. Later the huge
> > page was combined again, but the counters were off, as the PTE level
> > was not properly updated.
> >
> > Make sure that not only huge page but also small pages are updated when
> > a new entry is set or cleared.
> >
> > Fixes: df4e817b7108 ("mm: page table check")
> >
> > Signed-off-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
> > ---
> > mm/page_table_check.c | 60 ++++++++++++++++++++++++++-----------------
> > 1 file changed, 36 insertions(+), 24 deletions(-)
> >
> > diff --git a/mm/page_table_check.c b/mm/page_table_check.c
> > index 7504e7caa2a1..877d967742bc 100644
> > --- a/mm/page_table_check.c
> > +++ b/mm/page_table_check.c
> > @@ -145,6 +145,30 @@ static void page_table_check_set(struct mm_struct *mm, unsigned long addr,
> > }
> > }
> >
> > +static void pte_clear_level(struct mm_struct *mm, unsigned long addr,
> > + pte_t *ptep)
> > +{
> > + unsigned long i;
> > +
> > + for (i = 0; i < PTRS_PER_PTE; i++) {
> > + __page_table_check_pte_clear(mm, addr, *ptep);
> > + addr += PAGE_SIZE;
> > + ptep++;
> > + }
> > +}
> > +
> > +static void pmd_clear_level(struct mm_struct *mm, unsigned long addr,
> > + pmd_t *pmdp)
> > +{
> > + unsigned long i;
> > +
> > + for (i = 0; i < PTRS_PER_PMD; i++) {
> > + __page_table_check_pmd_clear(mm, addr, *pmdp);
> > + addr += PMD_PAGE_SIZE;
> > + pmdp++;
> > + }
> > +}
> > +
> > /*
> > * page is on free list, or is being allocated, verify that counters are zeroes
> > * crash if they are not.
> > @@ -186,6 +210,11 @@ void __page_table_check_pmd_clear(struct mm_struct *mm, unsigned long addr,
> > if (pmd_user_accessible_page(pmd)) {
> > page_table_check_clear(mm, addr, pmd_pfn(pmd),
> > PMD_PAGE_SIZE >> PAGE_SHIFT);
> > + } else if (!pmd_bad(pmd) && !pmd_leaf(pmd)) {
> > + pte_t *ptep = pte_offset_map(&pmd, addr);
> > +
> > + pte_clear_level(mm, addr, ptep);
> > + pte_unmap(ptep);
> > }
> > }
> > EXPORT_SYMBOL(__page_table_check_pmd_clear);
> > @@ -199,6 +228,10 @@ void __page_table_check_pud_clear(struct mm_struct *mm, unsigned long addr,
> > if (pud_user_accessible_page(pud)) {
> > page_table_check_clear(mm, addr, pud_pfn(pud),
> > PUD_PAGE_SIZE >> PAGE_SHIFT);
> > + } else if (!pud_bad(pud) && !pud_leaf(pud)) {
> > + pmd_t *pmdp = pmd_offset(&pud, addr);
> > +
> > + pmd_clear_level(mm, addr, pmdp);
> > }
> > }
> > EXPORT_SYMBOL(__page_table_check_pud_clear);
> > @@ -206,17 +239,10 @@ EXPORT_SYMBOL(__page_table_check_pud_clear);
> > void __page_table_check_pte_set(struct mm_struct *mm, unsigned long addr,
> > pte_t *ptep, pte_t pte)
> > {
> > - pte_t old_pte;
> > -
> > if (&init_mm == mm)
> > return;
> >
> > - old_pte = *ptep;
> > - if (pte_user_accessible_page(old_pte)) {
> > - page_table_check_clear(mm, addr, pte_pfn(old_pte),
> > - PAGE_SIZE >> PAGE_SHIFT);
> > - }
> > -
> > + __page_table_check_pte_clear(mm, addr, *ptep);
> > if (pte_user_accessible_page(pte)) {
> > page_table_check_set(mm, addr, pte_pfn(pte),
> > PAGE_SIZE >> PAGE_SHIFT,
> > @@ -228,17 +254,10 @@ EXPORT_SYMBOL(__page_table_check_pte_set);
> > void __page_table_check_pmd_set(struct mm_struct *mm, unsigned long addr,
> > pmd_t *pmdp, pmd_t pmd)
> > {
> > - pmd_t old_pmd;
> > -
> > if (&init_mm == mm)
> > return;
> >
> > - old_pmd = *pmdp;
> > - if (pmd_user_accessible_page(old_pmd)) {
> > - page_table_check_clear(mm, addr, pmd_pfn(old_pmd),
> > - PMD_PAGE_SIZE >> PAGE_SHIFT);
> > - }
> > -
> > + __page_table_check_pmd_clear(mm, addr, *pmdp);
> > if (pmd_user_accessible_page(pmd)) {
> > page_table_check_set(mm, addr, pmd_pfn(pmd),
> > PMD_PAGE_SIZE >> PAGE_SHIFT,
> > @@ -250,17 +269,10 @@ EXPORT_SYMBOL(__page_table_check_pmd_set);
> > void __page_table_check_pud_set(struct mm_struct *mm, unsigned long addr,
> > pud_t *pudp, pud_t pud)
> > {
> > - pud_t old_pud;
> > -
> > if (&init_mm == mm)
> > return;
> >
> > - old_pud = *pudp;
> > - if (pud_user_accessible_page(old_pud)) {
> > - page_table_check_clear(mm, addr, pud_pfn(old_pud),
> > - PUD_PAGE_SIZE >> PAGE_SHIFT);
> > - }
> > -
> > + __page_table_check_pud_clear(mm, addr, *pudp);
> > if (pud_user_accessible_page(pud)) {
> > page_table_check_set(mm, addr, pud_pfn(pud),
> > PUD_PAGE_SIZE >> PAGE_SHIFT,
> > --
> > 2.34.1.703.g22d0c6ccf7-goog
> >
>
> Reviewed-by: Wei Xu <weixugc@xxxxxxxxxx>

Andrew, please hold off on taking this patch. During testing, I have
found that the proposed fix does not work with khugepaged: we have
double clearing of PTE: first remove PMD entry and clear PTEs, but
later clear PTEs again in __collapse_huge_page_copy(). I am looking
for a more complete fix.

Thanks,
Pasha