linux-kernel - Re: [PATCH] mm: Fix NULL page->mapping dereference in page_is

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Date:   Fri, 8 Oct 2021 12:46:17 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, Mike Rapoport <rppt@...nel.org>,
        linux-mm@...ck.org, "Darrick J . Wong" <djwong@...nel.org>,
        Stephen <stephenackerman16@...il.com>
Subject: Re: [PATCH] mm: Fix NULL page->mapping dereference in
 page_is_secretmem()

On 08.10.21 01:15, Sean Christopherson wrote:
> Check for a NULL page->mapping before dereferencing the mapping in
> page_is_secretmem(), as the page's mapping can be nullified while gup()
> is running, e.g. by reclaim or truncation.
> 
>    BUG: kernel NULL pointer dereference, address: 0000000000000068
>    #PF: supervisor read access in kernel mode
>    #PF: error_code(0x0000) - not-present page
>    PGD 0 P4D 0
>    Oops: 0000 [#1] PREEMPT SMP NOPTI
>    CPU: 6 PID: 4173897 Comm: CPU 3/KVM Tainted: G        W
>    RIP: 0010:internal_get_user_pages_fast+0x621/0x9d0
>    Code: <48> 81 7a 68 80 08 04 bc 0f 85 21 ff ff 8 89 c7 be
>    RSP: 0018:ffffaa90087679b0 EFLAGS: 00010046
>    RAX: ffffe3f37905b900 RBX: 00007f2dd561e000 RCX: ffffe3f37905b934
>    RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffe3f37905b900
>    ...
>    CR2: 0000000000000068 CR3: 00000004c5898003 CR4: 00000000001726e0
>    Call Trace:
>     get_user_pages_fast_only+0x13/0x20
>     hva_to_pfn+0xa9/0x3e0
>     try_async_pf+0xa1/0x270
>     direct_page_fault+0x113/0xad0
>     kvm_mmu_page_fault+0x69/0x680
>     vmx_handle_exit+0xe1/0x5d0
>     kvm_arch_vcpu_ioctl_run+0xd81/0x1c70
>     kvm_vcpu_ioctl+0x267/0x670
>     __x64_sys_ioctl+0x83/0xa0
>     do_syscall_64+0x56/0x80
>     entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> Cc: Mike Rapoport <rppt@...nel.org>
> Cc: linux-mm@...ck.org
> Reported-by: Darrick J. Wong <djwong@...nel.org>
> Reported-by: Stephen <stephenackerman16@...il.com>
> Tested-by: Darrick J. Wong <djwong@...nel.org>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   include/linux/secretmem.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/secretmem.h b/include/linux/secretmem.h
> index 21c3771e6a56..988528b5da43 100644
> --- a/include/linux/secretmem.h
> +++ b/include/linux/secretmem.h
> @@ -23,7 +23,7 @@ static inline bool page_is_secretmem(struct page *page)
>   	mapping = (struct address_space *)
>   		((unsigned long)page->mapping & ~PAGE_MAPPING_FLAGS);
>   
> -	if (mapping != page->mapping)
> +	if (!mapping || mapping != page->mapping)
>   		return false;
>   
>   	return mapping->a_ops == &secretmem_aops;
> 

I assume we want to add

Fixes: 1507f51255c9 ("mm: introduce memfd_secret system call to create "secret" memory areas")
Cc: <stable@...r.kernel.org> # v5.14

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

-- 
Thanks,

David / dhildenb