linux-kernel - Re: [PATCH] mm/mprotect: Only reference swap pfn page if type match

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Date:   Fri, 26 Aug 2022 12:04:27 -0400
From:   Peter Xu <peterx@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Huang Ying <ying.huang@...el.com>, stable@...r.kernel.org,
        Yu Zhao <yuzhao@...gle.com>
Subject: Re: [PATCH] mm/mprotect: Only reference swap pfn page if type match

On Fri, Aug 26, 2022 at 04:39:08PM +0200, David Hildenbrand wrote:
> On 26.08.22 16:25, Peter Xu wrote:
> > On Fri, Aug 26, 2022 at 12:49:37PM +0200, David Hildenbrand wrote:
> >> On 24.08.22 00:11, Peter Xu wrote:
> >>> Yu Zhao reported a bug after the commit "mm/swap: Add swp_offset_pfn() to
> >>> fetch PFN from swap entry" added a check in swp_offset_pfn() for swap type [1]:
> >>>
> >>>   kernel BUG at include/linux/swapops.h:117!
> >>>   CPU: 46 PID: 5245 Comm: EventManager_De Tainted: G S         O L 6.0.0-dbg-DEV #2
> >>>   RIP: 0010:pfn_swap_entry_to_page+0x72/0xf0
> >>>   Code: c6 48 8b 36 48 83 fe ff 74 53 48 01 d1 48 83 c1 08 48 8b 09 f6
> >>>   c1 01 75 7b 66 90 48 89 c1 48 8b 09 f6 c1 01 74 74 5d c3 eb 9e <0f> 0b
> >>>   48 ba ff ff ff ff 03 00 00 00 eb ae a9 ff 0f 00 00 75 13 48
> >>>   RSP: 0018:ffffa59e73fabb80 EFLAGS: 00010282
> >>>   RAX: 00000000ffffffe8 RBX: 0c00000000000000 RCX: ffffcd5440000000
> >>>   RDX: 1ffffffffff7a80a RSI: 0000000000000000 RDI: 0c0000000000042b
> >>>   RBP: ffffa59e73fabb80 R08: ffff9965ca6e8bb8 R09: 0000000000000000
> >>>   R10: ffffffffa5a2f62d R11: 0000030b372e9fff R12: ffff997b79db5738
> >>>   R13: 000000000000042b R14: 0c0000000000042b R15: 1ffffffffff7a80a
> >>>   FS:  00007f549d1bb700(0000) GS:ffff99d3cf680000(0000) knlGS:0000000000000000
> >>>   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> >>>   CR2: 0000440d035b3180 CR3: 0000002243176004 CR4: 00000000003706e0
> >>>   DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> >>>   DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> >>>   Call Trace:
> >>>    <TASK>
> >>>    change_pte_range+0x36e/0x880
> >>>    change_p4d_range+0x2e8/0x670
> >>>    change_protection_range+0x14e/0x2c0
> >>>    mprotect_fixup+0x1ee/0x330
> >>>    do_mprotect_pkey+0x34c/0x440
> >>>    __x64_sys_mprotect+0x1d/0x30
> >>>
> >>> It triggers because pfn_swap_entry_to_page() could be called upon e.g. a
> >>> genuine swap entry.
> >>>
> >>> Fix it by only calling it when it's a write migration entry where the page*
> >>> is used.
> >>>
> >>> [1] https://lore.kernel.org/lkml/CAOUHufaVC2Za-p8m0aiHw6YkheDcrO-C3wRGixwDS32VTS+k1w@mail.gmail.com/
> >>>
> >>> Fixes: 6c287605fd56 ("mm: remember exclusively mapped anonymous pages with PG_anon_exclusive")
> >>> Cc: David Hildenbrand <david@...hat.com>
> >>> Cc: <stable@...r.kernel.org>
> >>> Reported-by: Yu Zhao <yuzhao@...gle.com>
> >>> Signed-off-by: Peter Xu <peterx@...hat.com>
> >>> ---
> >>>  mm/mprotect.c | 3 ++-
> >>>  1 file changed, 2 insertions(+), 1 deletion(-)
> >>>
> >>> diff --git a/mm/mprotect.c b/mm/mprotect.c
> >>> index f2b9b1da9083..4549f5945ebe 100644
> >>> --- a/mm/mprotect.c
> >>> +++ b/mm/mprotect.c
> >>> @@ -203,10 +203,11 @@ static unsigned long change_pte_range(struct mmu_gather *tlb,
> >>>  			pages++;
> >>>  		} else if (is_swap_pte(oldpte)) {
> >>>  			swp_entry_t entry = pte_to_swp_entry(oldpte);
> >>> -			struct page *page = pfn_swap_entry_to_page(entry);
> >>>  			pte_t newpte;
> >>>  
> >>>  			if (is_writable_migration_entry(entry)) {
> >>> +				struct page *page = pfn_swap_entry_to_page(entry);
> >>> +
> >>>  				/*
> >>>  				 * A protection check is difficult so
> >>>  				 * just be safe and disable write
> >>
> >>
> >> Stumbling over the THP code, I was wondering if we also want to adjust change_huge_pmd()
> >> and hugetlb_change_protection. There are no actual swap entries, so I assume we're fine.
> >>
> >>
> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> >> index 482c1826e723..466364e7fc5f 100644
> >> --- a/mm/huge_memory.c
> >> +++ b/mm/huge_memory.c
> >> @@ -1798,10 +1798,10 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
> >>  #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
> >>         if (is_swap_pmd(*pmd)) {
> >>                 swp_entry_t entry = pmd_to_swp_entry(*pmd);
> >> -               struct page *page = pfn_swap_entry_to_page(entry);
> >>  
> >>                 VM_BUG_ON(!is_pmd_migration_entry(*pmd));
> >>                 if (is_writable_migration_entry(entry)) {
> >> +                       struct page *page = pfn_swap_entry_to_page(entry);
> >>                         pmd_t newpmd;
> >>                         /*
> >>                          * A protection check is difficult so
> >> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> >> index 2480ba627aa5..559465fae5cd 100644
> >> --- a/mm/hugetlb.c
> >> +++ b/mm/hugetlb.c
> >> @@ -6370,9 +6370,9 @@ unsigned long hugetlb_change_protection(struct vm_area_struct *vma,
> >>                 }
> >>                 if (unlikely(is_hugetlb_entry_migration(pte))) {
> >>                         swp_entry_t entry = pte_to_swp_entry(pte);
> >> -                       struct page *page = pfn_swap_entry_to_page(entry);
> >>  
> >>                         if (!is_readable_migration_entry(entry)) {
> >> +                               struct page *page = pfn_swap_entry_to_page(entry);
> >>                                 pte_t newpte;
> >>  
> >>                                 if (PageAnon(page))
> >>
> >>
> >> @Peter, what's your thought?
> > 
> > IMHO they're not needed?
> > 
> > The rule is simple in my mind: we should only pass in a pfn-typed swap
> > entry into pfn_swap_entry_to_page() (or the new swp_offset_pfn()), or it's
> > a violation of the API.  In these two cases they do not violate the API and
> > they're always safe because they're guaranteed to be pfn swap entries when
> > calling.
> 
> I was wondering about extreme corner cases regarding the struct page.
> 
> Assume we have a hwpoison_entry that pointed at a valid struct page. We
> can succeed in offlining+removing the section it's located on (I was
> recently challenging if we want to keep that behavior as it's really
> shaky already), freeing the relevant memmap entry and the memory section.
> 
> pfn_swap_entry_to_page() -> pfn_to_page() would be problematic if there
> is no memmap anymore.
> 
> 
> I assume it's ok to always call it for is_pfn_swap_entry(), but in the
> PMD case we only check for is_swap_pmd()? Isn't that problematic?

I don't know extensively enough on hwpoison on validity of fetching page
from pfn inside on online/offline ops, but.. if the only concern is about
hwpoison entry existance here I think its fine?  Because iirc we'l split
thp when any of the subpage got poisoned, so we should never hit a hwpoison
entry in thp path.

> 
> 
> I was confused by the hugetlb case, it's indeed fine as we check for
> is_hugetlb_entry_migration().

Right, it's more straightforward in the hugetlb case.

-- 
Peter Xu