On Fri, Aug 02, 2019 at 06:06:14PM +0200, Vlastimil Babka wrote:
> The x86 version of get_user_pages_fast() relies on disabled interrupts to
> synchronize gup_pte_range() between gup_get_pte(ptep); and get_page() against
> a parallel munmap. The munmap side nulls the pte, then flushes TLBs, then
> releases the page. As TLB flush is done synchronously via IPI disabling
> interrupts blocks the page release, and get_page(), which assumes existing
> reference on page, is thus safe.
> However when TLB flush is done by a hypercall, e.g. in a Xen PV guest, there
> is
> no blocking thanks to disabled interrupts, and get_page() can succeed on a
> page
> that was already freed or even reused.
>
> We have recently seen this happen with our 4.4 and 4.12 based kernels, with
> userspace (java) that exits a thread, where mm_release() performs a
> futex_wake()
> on tsk->clear_child_tid, and another thread in parallel unmaps the page where
> tsk->clear_child_tid points to. The spurious get_page() succeeds, but futex
> code
> immediately releases the page again, while it's already on a freelist.
> Symptoms
> include a bad page state warning, general protection faults acessing a
> poisoned
> list prev/next pointer in the freelist, or free page pcplists of two cpus
> joined
> together in a single list. Oscar has also reproduced this scenario, with a
> patch inserting delays before the get_page() to make the race window larger.
>
> Fix this by removing the dependency on TLB flush interrupts the same way as
> the
> generic get_user_pages_fast() code by using page_cache_add_speculative() and
> revalidating the PTE contents after pinning the page. Mainline is safe since
> 4.13 where the x86 gup code was removed in favor of the common code. Accessing
> the page table itself safely also relies on disabled interrupts and TLB flush
> IPIs that don't happen with hypercalls, which was acknowledged in commit
> 9e52fc2b50de ("x86/mm: Enable RCU based page table freeing
> (CONFIG_HAVE_RCU_TABLE_FREE=y)"). That commit with follups should also be
> backported for full safety, although our reproducer didn't hit a problem
> without that backport.
>
> Reproduced-by: Oscar Salvador <[email protected]>
> Signed-off-by: Vlastimil Babka <[email protected]>
> Cc: Thomas Gleixner <[email protected]>
> Cc: Ingo Molnar <[email protected]>
> Cc: Peter Zijlstra <[email protected]>
> Cc: Juergen Gross <[email protected]>
> Cc: Kirill A. Shutemov <[email protected]>
> Cc: Vitaly Kuznetsov <[email protected]>
> Cc: Linus Torvalds <[email protected]>
> Cc: Borislav Petkov <[email protected]>
> Cc: Dave Hansen <[email protected]>
> Cc: Andy Lutomirski <[email protected]>
> ---
>
> Hi, I'm sending this stable-only patch for consideration because it's probably
> unrealistic to backport the 4.13 switch to generic GUP. I can look at 4.4 and
> 3.16 if accepted. The RCU page table freeing could be also considered.
> Note the patch also includes page refcount protection. I found out that
> 8fde12ca79af ("mm: prevent get_user_pages() from overflowing page refcount")
> backport to 4.9 missed the arch-specific gup implementations:
> https://lore.kernel.org/lkml/[email protected]/
This looks sane to me, thank you for the backport. I've queued it up
now, and if anyone has any objections, please let me know.
thanks,
greg k-h
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