[RFC PATCH 3/6] mm, arm64: untag user addresses in memory syscalls

[Evgenii Stepanov]

On Fri, Mar 9, 2018 at 9:31 AM, Andrey Konovalov <andreyknvl at google.com> wrote:
> On Fri, Mar 9, 2018 at 4:53 PM, Catalin Marinas <catalin.marinas at arm.com> wrote:
>> On Fri, Mar 09, 2018 at 03:02:01PM +0100, Andrey Konovalov wrote:
>>> Memory subsystem syscalls accept user addresses as arguments, but don't use
>>> copy_from_user and other similar functions, so we need to handle this case
>>> separately.
>>>
>>> Untag user pointers passed to madvise, mbind, get_mempolicy, mincore,
>>> mlock, mlock2, brk, mmap_pgoff, old_mmap, munmap, remap_file_pages,
>>> mprotect, pkey_mprotect, mremap and msync.
>>>
>>> Signed-off-by: Andrey Konovalov <andreyknvl at google.com>
>>
>> Please keep the cc list small (maybe linux-arch, linux-arm-kernel) as
>> I'm sure some lists would consider this spam.
>
> OK.
>
>>
>>>  mm/madvise.c   | 2 ++
>>>  mm/mempolicy.c | 6 ++++++
>>>  mm/mincore.c   | 2 ++
>>>  mm/mlock.c     | 5 +++++
>>>  mm/mmap.c      | 9 +++++++++
>>>  mm/mprotect.c  | 2 ++
>>>  mm/mremap.c    | 2 ++
>>>  mm/msync.c     | 3 +++
>>
>> I'm not yet convinced these functions need to allow tagged pointers.
>> They are not doing memory accesses but rather dealing with the memory
>> range, hence an untagged pointer is better suited. There is probably a
>> reason why the "start" argument is "unsigned long" vs "void __user *"
>> (in the kernel, not the man page).
>
> So that would make the user to untag pointers before passing to these syscalls.
>
> Evgeniy, would that be possible to untag pointers in HWASan before
> using memory subsystem syscalls? Is there a reason for untagging them
> in the kernel?

Generally, no. It's possible to intercept a libc call using symbol
interposition, but I don't know how to rewrite arguments of a raw
system call other than through ptrace, and that creates more problems
than it solves.

AFAIU, it's valid for a program to pass an address obtained from
malloc or, better, posix_memalign to an mm syscall like mprotect().
These arguments are pointers from the userspace point of view.