Re: [RFC PATCH 3/3] mm/migrate: Create move_phys_pages syscall

[Gregory Price]

On Mon, Sep 11, 2023 at 07:26:45PM +0200, Arnd Bergmann wrote:
> On Sun, Sep 10, 2023, at 14:52, Gregory Price wrote:
> > I'll clean up the current implementation for what I have on a v2 of an
> > RFC, and then look at adding some pull-ahead patches to fix both
> > move_pages and move_phys_pages for compat processes.  Might take me a
> > bit, I've only done compat work once before and I remember it being
> > annoying to get right.
> 
> I think what you want is roughly this (untested):
> 
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -2159,6 +2159,7 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
>                          const int __user *nodes,
>                          int __user *status, int flags)
>  {
> +       struct compat_uptr_t __user *compat_pages = (void __user *)pages;
>         int current_node = NUMA_NO_NODE;
>         LIST_HEAD(pagelist);
>         int start, i;
> @@ -2171,8 +2172,17 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
>                 int node;
>  
>                 err = -EFAULT;
> -               if (get_user(p, pages + i))
> -                       goto out_flush;
> +               if (in_compat_syscall() {
> +                       compat_uptr_t cp;
> +
> +                       if (get_user(cp, compat_pages + i))
> +                               goto out_flush;
> +
> +                       p = compat_ptr(cp);
> +               } else {
> +                       if (get_user(p, pages + i))
> +                               goto out_flush;
> +               }
>                 if (get_user(node, nodes + i))
>                         goto out_flush;
>  
> alternatively you could use the get_compat_pages_array()
> helper that is already used in the do_pages_stat()
> function.
> 

Appreciated, i'll give it a hack before i submit V2.

Just to be clear, it sounds like you want move_pages to be converted
from (const __user * __user *pages) to (const __u64 __user *pages) as
well, correct?  That seems like a fairly trivial change.

> 
> >
> > This only requires plumbing new 2 flags through do_pages_move, and no
> > new user-exposed types or information.
> >
> > Is there an ick-factor with the idea of adding the following?
> >
> > MPOL_MF_PHYS_ADDR : Treat page migration addresses as physical
> > MPOL_MF_PFN : Treat page migration addresses as PFNs
> 
> I would strongly prefer supporting only one of the two, and
> a 64-bit physical address seems like the logical choice here.
> 
> I agree that this doesn't introduce any additional risk for rowhammer
> attacks, but it seems slightly more logical to me to use CAP_SYS_ADMIN
> if that is what the other interfaces use that handle physical addresses
> and may leak address information.
> 
>      Arnd

Fair enough, I'll swap to ADMIN and limit to phys_addr.

I suppose I could add /sys/kernel/mm/page_size accessible only by root
for the same purpose, so that PFNs from idle and such can be useful.

I don't know of another way for userland to determine the shift.

~Gregory