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

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Date:   Mon, 13 Nov 2023 23:42:53 +0800 (CST)
From:   "David Wang" <00107082@....com>
To:     "Theodore Ts'o" <tytso@....edu>
Cc:     "David Hildenbrand" <david@...hat.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.



At 2023-11-13 21:26:21, "Theodore Ts'o" <tytso@....edu> 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.
>

But the  inconsistent treatment in kernel,   memfd denied while  mmaped-address allowed,   is kind of confusing...
I thought those two should be treated the same way....

Thanks
David Wang