Re: [PATCH v5 1/6] mm/gup: remove unused vmas parameter from get_user_pages()

[Sean Christopherson]

On Sun, May 14, 2023, Lorenzo Stoakes wrote:
> No invocation of get_user_pages() use the vmas parameter, so remove it.
> 
> The GUP API is confusing and caveated. Recent changes have done much to
> improve that, however there is more we can do. Exporting vmas is a prime
> target as the caller has to be extremely careful to preclude their use
> after the mmap_lock has expired or otherwise be left with dangling
> pointers.
> 
> Removing the vmas parameter focuses the GUP functions upon their primary
> purpose - pinning (and outputting) pages as well as performing the actions
> implied by the input flags.
> 
> This is part of a patch series aiming to remove the vmas parameter
> altogether.
> 
> Suggested-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> Acked-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>
> Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Acked-by: Christian K�nig <christian.koenig@xxxxxxx> (for radeon parts)
> Acked-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
> Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> ---
>  arch/x86/kernel/cpu/sgx/ioctl.c     | 2 +-
>  drivers/gpu/drm/radeon/radeon_ttm.c | 2 +-
>  drivers/misc/sgi-gru/grufault.c     | 2 +-
>  include/linux/mm.h                  | 3 +--
>  mm/gup.c                            | 9 +++------
>  mm/gup_test.c                       | 5 ++---
>  virt/kvm/kvm_main.c                 | 2 +-
>  7 files changed, 10 insertions(+), 15 deletions(-)

Acked-by: Sean Christopherson <seanjc@xxxxxxxxxx> (KVM)

> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index cb5c13eee193..eaa5bb8dbadc 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2477,7 +2477,7 @@ static inline int check_user_page_hwpoison(unsigned long addr)
>  {
>  	int rc, flags = FOLL_HWPOISON | FOLL_WRITE;
>  
> -	rc = get_user_pages(addr, 1, flags, NULL, NULL);
> +	rc = get_user_pages(addr, 1, flags, NULL);
>  	return rc == -EHWPOISON;

Unrelated to this patch, I think there's a pre-existing bug here.  If gup() returns
a valid page, KVM will leak the refcount and unintentionally pin the page.  That's
highly unlikely as check_user_page_hwpoison() is called iff get_user_pages_unlocked()
fails (called by hva_to_pfn_slow()), but it's theoretically possible that userspace
could change the VMAs between hva_to_pfn_slow() and check_user_page_hwpoison() since
KVM doesn't hold any relevant locks at this point.

E.g. if there's no VMA during hva_to_pfn_{fast,slow}(), npages==-EFAULT and KVM
will invoke check_user_page_hwpoison().  If userspace installs a valid mapping
after hva_to_pfn_slow() but before KVM acquires mmap_lock, then gup() will find
a valid page.

I _think_ the fix is to simply delete this code. The bug was introduced by commit
fafc3dbaac64 ("KVM: Replace is_hwpoison_address with __get_user_pages").  At that
time, KVM didn't check for "npages == -EHWPOISON" from the first call to
get_user_pages_unlocked().  Later on, commit 0857b9e95c1a ("KVM: Enable async page
fault processing") reworked the caller to be:

	mmap_read_lock(current->mm);
	if (npages == -EHWPOISON ||
	      (!async && check_user_page_hwpoison(addr))) {
		pfn = KVM_PFN_ERR_HWPOISON;
		goto exit;
	}

where async really means NOWAIT, so that the hwpoison use of gup() didn't sleep.

    KVM: Enable async page fault processing
    
    If asynchronous hva_to_pfn() is requested call GUP with FOLL_NOWAIT to
    avoid sleeping on IO. Check for hwpoison is done at the same time,
    otherwise check_user_page_hwpoison() will call GUP again and will put
    vcpu to sleep.

There are other potential problems too, e.g. the hwpoison call doesn't honor
the recently introduced @interruptible flag.

I don't see any reason to keep check_user_page_hwpoison(), KVM can simply rely on
the "npages == -EHWPOISON" check.   get_user_pages_unlocked() is guaranteed to be
called with roughly equivalent flags, and the flags that aren't equivalent are
arguably bugs in check_user_page_hwpoison(), e.g. assuming FOLL_WRITE is wrong.

TL;DR: Go ahead with this change, I'll submit a separate patch to delete the
buggy KVM code.