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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCHv2] xen/p2m: check MFN is in range before using the m2p table
On Thu, 29 Aug 2013, David Vrabel wrote:
> From: David Vrabel <david.vrabel@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> Cc: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
> Cc: Jan Beulich <JBeulich@xxxxxxxx>
> --
> v2: in mfn_to_pfn() look in m2p_overrides if the MFN is is out of
> range as it's probably foreign.
> ---
> arch/x86/include/asm/xen/page.h | 32 +++++++++++++++++++-------------
> arch/x86/xen/p2m.c | 10 ++++------
> 2 files changed, 23 insertions(+), 19 deletions(-)
>
> diff --git a/arch/x86/include/asm/xen/page.h b/arch/x86/include/asm/xen/page.h
> index 6aef9fb..979d7a2 100644
> --- a/arch/x86/include/asm/xen/page.h
> +++ b/arch/x86/include/asm/xen/page.h
> @@ -79,30 +79,35 @@ 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))
> + return ~0;
>
> - if (unlikely(mfn >= machine_to_phys_nr)) {
> - pfn = ~0;
> - goto try_override;
> - }
I think we need to keep an XENFEAT_auto_translated_physmap test at the
beginning of mfn_to_pfn_no_overrides: this function is exported in
page.h, anybody could start using it in common code paths.
Other than that, I think it's OK.
> - pfn = 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 +116,7 @@ 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 95fb2aa..1563444 100644
> --- a/arch/x86/xen/p2m.c
> +++ b/arch/x86/xen/p2m.c
> @@ -878,7 +878,6 @@ 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)) {
> @@ -925,8 +924,8 @@ 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;
> @@ -941,7 +940,6 @@ 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);
> @@ -1019,8 +1017,8 @@ 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
>
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