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SubjectRe: [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@redhat.com>
> >>> Cc: <stable@vger.kernel.org>
> >>> Reported-by: Yu Zhao <yuzhao@google.com>
> >>> Signed-off-by: Peter Xu <peterx@redhat.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

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