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

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Date:   Fri, 26 Aug 2022 12:49:37 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Cc:     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 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?

-- 
Thanks,

David / dhildenb