Messages in this thread | | | Date | Mon, 13 Nov 2023 15:42:44 +0100 | Subject | Re: [BUG?] mm/secretmem: memory address mapped to memfd_secret can be used in write syscall. | From | David Hildenbrand <> |
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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
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