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

[Andrey Konovalov]

On Fri, Mar 9, 2018 at 6:42 PM, Evgenii Stepanov <eugenis at google.com> wrote:
> 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:
>>> 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.

Catalin, do you think this is a good reason to have the untagging done
in the kernel?