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

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Message-ID: <b90c64fd-1690-aa70-691f-c249586f20d4@redhat.com>
Date:   Wed, 24 Aug 2022 08:58:41 +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!

Hi Peter,

Note that Linus recently complained to me that we should not add any new
BUG_ONs (not even VM_BUG_ONs), and even convert all of them to
WARN_ON_ONCE. So  we might want to change that to a WARN_ON_ONCE before
sending it upstream.

>   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.

Right, but the page is not touched in that case and it would simply be
an unused garbage pointer. So the real issue is that we might call
pfn_to_page() on a wrong PFN.

For !FLATMEM I think we just don't care. For SPARSEMEM, however, we
could end up getting a NULL pointer from __pfn_to_section() and end up
de-referencing it when looking up the memmap address inside the section
via __section_mem_map_addr().

I guess that could happen before your changes, for example, when we'd
have a swap offset that's larger than the biggest PFN in the system.

Reviewed-by: David Hildenbrand <david@...hat.com>

> 
> 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


-- 
Thanks,

David / dhildenb