Re: [PATCH 1/3] x86/mm: Use IPIs to synchronize LAM enablement

[Kirill A. Shutemov]

On Tue, Mar 12, 2024 at 03:59:49AM +0000, Yosry Ahmed wrote:
> LAM can only be enabled when a process is single-threaded.  But _kernel_
> threads can temporarily use a single-threaded process's mm.
> 
> If LAM is enabled by a userspace process while a kthread is using its
> mm, the kthread will not observe LAM enablement (i.e.  LAM will be
> disabled in CR3). This could be fine for the kthread itself, as LAM only
> affects userspace addresses. However, if the kthread context switches to
> a thread in the same userspace process, CR3 may or may not be updated
> because the mm_struct doesn't change (based on pending TLB flushes). If
> CR3 is not updated, the userspace thread will run incorrectly with LAM
> disabled, which may cause page faults when using tagged addresses.
> Example scenario:
> 
> CPU 1                                   CPU 2
> /* kthread */
> kthread_use_mm()
>                                         /* user thread */
>                                         prctl_enable_tagged_addr()
>                                         /* LAM enabled on CPU 2 */
> /* LAM disabled on CPU 1 */
>                                         context_switch() /* to CPU 1 */
> /* Switching to user thread */
> switch_mm_irqs_off()
> /* CR3 not updated */
> /* LAM is still disabled on CPU 1 */
> 
> Synchronize LAM enablement by sending an IPI from
> prctl_enable_tagged_addr() to all CPUs running with the mm_struct to
> enable LAM. This makes sure LAM is enabled on CPU 1 in the above
> scenario before prctl_enable_tagged_addr() returns and userspace starts
> using tagged addresses, and before it's possible to run the userspace
> process on CPU 1.
> 
> In switch_mm_irqs_off(), move reading the LAM mask until after
> mm_cpumask() is updated. This ensures that if an outdated LAM mask is
> written to CR3, an IPI is received to update it right after IRQs are
> re-enabled.
> 
> Fixes: 82721d8b25d7 ("x86/mm: Handle LAM on context switch")
> Suggested-by: Andy Lutomirski <luto@xxxxxxxxxx>
> Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>

Reviewed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov