Re: [PATCHv14 01/17] x86/mm: Rework address range check in get_user() and put_user()

[Peter Zijlstra]

On Wed, Jan 11, 2023 at 03:37:20PM +0300, Kirill A. Shutemov wrote:
> The functions get_user() and put_user() check that the target address
> range resides in the user space portion of the virtual address space.
> In order to perform this check, the functions compare the end of the
> range against TASK_SIZE_MAX.
> 
> For kernels compiled with CONFIG_X86_5LEVEL, this process requires some
> additional trickery using ALTERNATIVE, as TASK_SIZE_MAX depends on the
> paging mode in use.
> 
> Linus suggested that this check could be simplified for 64-bit kernels.
> It is sufficient to check bit 63 of the address to ensure that the range
> belongs to user space. Additionally, the use of branches can be avoided
> by setting the target address to all ones if bit 63 is set.
> 
> There's no need to check the end of the access range as there's huge
> gap between end of userspace range and start of the kernel range. The
> gap consists of canonical hole and unused ranges on both kernel and
> userspace sides.

So far I can follow, however

> If an address with bit 63 set is passed down, it will trigger a #GP
> exception. _ASM_EXTABLE_UA() complains about this. Replace it with
> plain _ASM_EXTABLE() as it is expected behaviour now.

here I don't. The new logic basically squishes every kernel address to
-1L -- a known unmapped address, but getting that address in
{get,put}_user() is still a fail, right?

We used to manually branch to bad_get_user when outside TASK_SIZE_MAX,
now we rely on #GP.

So why silence it?