linux-kernel - Re: [BUG?] mm/secretmem: memory address mapped to memfd

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Date:   Mon, 13 Nov 2023 15:42:44 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Theodore Ts'o <tytso@....edu>
Cc:     David Wang <00107082@....com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Mike Rapoport <rppt@...ux.ibm.com>
Subject: Re: [BUG?] mm/secretmem: memory address mapped to memfd_secret can be
 used in write syscall.

On 13.11.23 14:26, Theodore Ts'o wrote:
> On Mon, Nov 13, 2023 at 10:15:05AM +0100, David Hildenbrand wrote:
>>
>> According to the man page:
>>
>> "The  memory areas backing the file created with memfd_secret(2) are visible
>> only to the processes that have access to the file descriptor. The memory
>> region is removed from the kernel page tables and only the page tables  of
>> the  processes  holding  the file descriptor map the corresponding physical
>> memory.  (Thus, the pages in the region can't be accessed by the kernel
>> itself, so that, for example, pointers  to  the region can't be passed to
>> system calls.)
>>
>> I'm not sure if the last part is actually true, if the syscalls end up
>> walking user page tables to copy data in/out.
> 
> The idea behind removing it from the kernel page tables is so that
> kernel code running in some other process context won't be able to
> reference the memory via the kernel address space.  (So if there is
> some kind of kernel zero-day which allows arbitrary code execution,
> the injected attack code would have to play games with page tables
> before being able to reference the memory --- this is not
> *impossible*, just more annoying.)
> 
> But if you are doing a buffered write, the copy from the user-supplied
> buffer to the page cache is happening in the process's context.  So
> "foreground kernel code" can dereference the user-supplied pointer
> just fine.

Right, so the statement in the man page is imprecise.

-- 
Cheers,

David / dhildenb