Re: [PATCHv3] xen/p2m: check MFN is in range before using the m2p table

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Fri, Sep 13, 2013 at 03:13:30PM +0100, David Vrabel wrote:
> From: David Vrabel <david.vrabel@citrix.com>
> 
> On hosts with more than 168 GB of memory, a 32-bit guest may attempt
> to grant map an MFN that is not cannot lookup in its mapping of the
> m2p table.  There is an m2p lookup as part of m2p_add_override() and
> m2p_remove_override().  The lookup falls off the end of the mapped
> portion of the m2p and (because the mapping is at the highest virtual
> address) wraps around and the lookup causes a fault on what appears to
> be a user space address.
> 
> do_page_fault() (thinking it's a fault to a userspace address), tries
> to lock mm->mmap_sem.  If the gntdev device is used for the grant map,
> m2p_add_override() is called from from gnttab_mmap() with mm->mmap_sem
> already locked.  do_page_fault() then deadlocks.
> 
> The deadlock would most commonly occur when a 64-bit guest is started
> and xenconsoled attempts to grant map its console ring.
> 
> Introduce mfn_to_pfn_no_overrides() which checks the MFN is within the
> mapped portion of the m2p table before accessing the table and use
> this in m2p_add_override(), m2p_remove_override(), and mfn_to_pfn()
> (which already had the correct range check).
> 
> All faults caused by accessing the non-existant parts of the m2p are
> thus within the kernel address space and exception_fixup() is called
> without trying to lock mm->mmap_sem.
> 
> This means that for MFNs that are outside the mapped range of the m2p
> then mfn_to_pfn() will always look in the m2p overrides.  This is
> correct because it must be a foreign MFN (and the PFN in the m2p in
> this case is only relevant for the other domain).
> 
> Signed-off-by: David Vrabel <david.vrabel@citrix.com>
> Cc: Stefano Stabellini <stefano.stabellini@citrix.com>
> Cc: Jan Beulich <JBeulich@suse.com>

This looks like it should also go in stable?

> --
> v3: check for auto_translated_physmap in mfn_to_pfn_no_overrides()
> v2: in mfn_to_pfn() look in m2p_overrides if the MFN is out of
>     range as it's probably foreign.
> ---
>  arch/x86/include/asm/xen/page.h |   31 ++++++++++++++++++++-----------
>  arch/x86/xen/p2m.c              |   10 ++++------
>  2 files changed, 24 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/x86/include/asm/xen/page.h b/arch/x86/include/asm/xen/page.h
> index 6aef9fb..b913915 100644
> --- a/arch/x86/include/asm/xen/page.h
> +++ b/arch/x86/include/asm/xen/page.h
> @@ -79,30 +79,38 @@ static inline int phys_to_machine_mapping_valid(unsigned long pfn)
>  	return get_phys_to_machine(pfn) != INVALID_P2M_ENTRY;
>  }
>  
> -static inline unsigned long mfn_to_pfn(unsigned long mfn)
> +static inline unsigned long mfn_to_pfn_no_overrides(unsigned long mfn)
>  {
>  	unsigned long pfn;
> -	int ret = 0;
> +	int ret;
>  
>  	if (xen_feature(XENFEAT_auto_translated_physmap))
>  		return mfn;
>  
> -	if (unlikely(mfn >= machine_to_phys_nr)) {
> -		pfn = ~0;
> -		goto try_override;
> -	}
> -	pfn = 0;
> +	if (unlikely(mfn >= machine_to_phys_nr))
> +		return ~0;
> +
>  	/*
>  	 * The array access can fail (e.g., device space beyond end of RAM).
>  	 * In such cases it doesn't matter what we return (we return garbage),
>  	 * but we must handle the fault without crashing!
>  	 */
>  	ret = __get_user(pfn, &machine_to_phys_mapping[mfn]);
> -try_override:
> -	/* ret might be < 0 if there are no entries in the m2p for mfn */
>  	if (ret < 0)
> -		pfn = ~0;
> -	else if (get_phys_to_machine(pfn) != mfn)
> +		return ~0;
> +
> +	return pfn;
> +}
> +
> +static inline unsigned long mfn_to_pfn(unsigned long mfn)
> +{
> +	unsigned long pfn;
> +
> +	if (xen_feature(XENFEAT_auto_translated_physmap))
> +		return mfn;
> +
> +	pfn = mfn_to_pfn_no_overrides(mfn);
> +	if (get_phys_to_machine(pfn) != mfn) {
>  		/*
>  		 * If this appears to be a foreign mfn (because the pfn
>  		 * doesn't map back to the mfn), then check the local override
> @@ -111,6 +119,7 @@ try_override:
>  		 * m2p_find_override_pfn returns ~0 if it doesn't find anything.
>  		 */
>  		pfn = m2p_find_override_pfn(mfn, ~0);
> +	}
>  
>  	/* 
>  	 * pfn is ~0 if there are no entries in the m2p for mfn or if the
> diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
> index 8b901e8..a61c7d5 100644
> --- a/arch/x86/xen/p2m.c
> +++ b/arch/x86/xen/p2m.c
> @@ -879,7 +879,6 @@ int m2p_add_override(unsigned long mfn, struct page *page,
>  	unsigned long uninitialized_var(address);
>  	unsigned level;
>  	pte_t *ptep = NULL;
> -	int ret = 0;
>  
>  	pfn = page_to_pfn(page);
>  	if (!PageHighMem(page)) {
> @@ -926,8 +925,8 @@ int m2p_add_override(unsigned long mfn, struct page *page,
>  	 * frontend pages while they are being shared with the backend,
>  	 * because mfn_to_pfn (that ends up being called by GUPF) will
>  	 * return the backend pfn rather than the frontend pfn. */
> -	ret = __get_user(pfn, &machine_to_phys_mapping[mfn]);
> -	if (ret == 0 && get_phys_to_machine(pfn) == mfn)
> +	pfn = mfn_to_pfn_no_overrides(mfn);
> +	if (get_phys_to_machine(pfn) == mfn)
>  		set_phys_to_machine(pfn, FOREIGN_FRAME(mfn));
>  
>  	return 0;
> @@ -942,7 +941,6 @@ int m2p_remove_override(struct page *page,
>  	unsigned long uninitialized_var(address);
>  	unsigned level;
>  	pte_t *ptep = NULL;
> -	int ret = 0;
>  
>  	pfn = page_to_pfn(page);
>  	mfn = get_phys_to_machine(pfn);
> @@ -1029,8 +1027,8 @@ int m2p_remove_override(struct page *page,
>  	 * the original pfn causes mfn_to_pfn(mfn) to return the frontend
>  	 * pfn again. */
>  	mfn &= ~FOREIGN_FRAME_BIT;
> -	ret = __get_user(pfn, &machine_to_phys_mapping[mfn]);
> -	if (ret == 0 && get_phys_to_machine(pfn) == FOREIGN_FRAME(mfn) &&
> +	pfn = mfn_to_pfn_no_overrides(mfn);
> +	if (get_phys_to_machine(pfn) == FOREIGN_FRAME(mfn) &&
>  			m2p_find_override(mfn) == NULL)
>  		set_phys_to_machine(pfn, mfn);
>  
> -- 
> 1.7.2.5
>