Re: [RFC PATCH 2/6] arm64: untag user addresses in copy_from_user and others

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, Mar 09, 2018 at 03:03:09PM +0000, Mark Rutland wrote:
> On Fri, Mar 09, 2018 at 03:02:00PM +0100, Andrey Konovalov wrote:
> > copy_from_user (and a few other similar functions) are used to copy data
> > from user memory into the kernel memory or vice versa. Since a user can
> > provided a tagged pointer to one of the syscalls that use copy_from_user,
> > we need to correctly handle such pointers.
> 
> I don't think it makes sense to do this in the low-level uaccess
> primitives, given we're going to have to untag pointers before common
> code can use them, e.g. for comparisons against TASK_SIZE or
> user_addr_max().
> 
> I think we'll end up with subtle bugs unless we consistently untag
> pointers before we get to uaccess primitives. If core code does untag
> pointers, then it's redundant to do so here.

A quick "hack" below clears the tag on syscall entry (where the argument
is a __user pointer). However, we still have cases in core code where
the pointer is read from a structure or even passed as an unsigned long
as part of a command + argument (like in ptrace).

The "hack":

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