RE: [PATCH v3 for-4.14] x86/vmx: use P2M_ALLOC in vmx_load_pdptrs instead of P2M_UNSHARE

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Lengyel, Tamas <tamas.lengyel@xxxxxxxxx>
> Sent: Thursday, June 18, 2020 10:39 PM
> 
> While forking VMs running a small RTOS system (Zephyr) a Xen crash has
> been
> observed due to a mm-lock order violation while copying the HVM CPU
> context
> from the parent. This issue has been identified to be due to
> hap_update_paging_modes first getting a lock on the gfn using get_gfn. This
> call also creates a shared entry in the fork's memory map for the cr3 gfn. The
> function later calls hap_update_cr3 while holding the paging_lock, which
> results in the lock-order violation in vmx_load_pdptrs when it tries to
> unshare
> the above entry when it grabs the page with the P2M_UNSHARE flag set.
> 
> Since vmx_load_pdptrs only reads from the page its usage of P2M_UNSHARE
> was
> unnecessary to start with. Using P2M_ALLOC is the appropriate flag to ensure
> the p2m is properly populated.
> 
> Note that the lock order violation is avoided because before the paging_lock
> is
> taken a lookup is performed with P2M_ALLOC that forks the page, thus the
> second
> lookup in vmx_load_pdptrs succeeds without having to perform the fork. We
> keep
> P2M_ALLOC in vmx_load_pdptrs because there are code-paths leading up to
> it
> which don't take the paging_lock and that have no previous lookup.
> Currently no
> other code-path exists leading there with the paging_lock taken, thus no
> further adjustments are necessary.
> 
> Signed-off-by: Tamas K Lengyel <tamas.lengyel@xxxxxxxxx>

Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>

> ---
> v3: expand commit message to explain why there is no lock-order violation
> ---
>  xen/arch/x86/hvm/vmx/vmx.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
> index ab19d9424e..cc6d4ece22 100644
> --- a/xen/arch/x86/hvm/vmx/vmx.c
> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> @@ -1325,7 +1325,7 @@ static void vmx_load_pdptrs(struct vcpu *v)
>      if ( (cr3 & 0x1fUL) && !hvm_pcid_enabled(v) )
>          goto crash;
> 
> -    page = get_page_from_gfn(v->domain, cr3 >> PAGE_SHIFT, &p2mt,
> P2M_UNSHARE);
> +    page = get_page_from_gfn(v->domain, cr3 >> PAGE_SHIFT, &p2mt,
> P2M_ALLOC);
>      if ( !page )
>      {
>          /* Ideally you don't want to crash but rather go into a wait
> --
> 2.25.1