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

[Peter Xu <peterx@redhat.com>]

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.

Thanks,

-- 
Peter Xu