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xen.28172
xsa402-2.patch
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File xsa402-2.patch of Package xen.28172
From: Andrew Cooper <andrew.cooper3@citrix.com> Subject: x86: Don't change the cacheability of the directmap Changeset 55f97f49b7ce ("x86: Change cache attributes of Xen 1:1 page mappings in response to guest mapping requests") attempted to keep the cacheability consistent between different mappings of the same page. The reason wasn't described in the changelog, but it is understood to be in regards to a concern over machine check exceptions, owing to errata when using mixed cacheabilities. It did this primarily by updating Xen's mapping of the page in the direct map when the guest mapped a page with reduced cacheability. Unfortunately, the logic didn't actually prevent mixed cacheability from occurring: * A guest could map a page normally, and then map the same page with different cacheability; nothing prevented this. * The cacheability of the directmap was always latest-takes-precedence in terms of guest requests. * Grant-mapped frames with lesser cacheability didn't adjust the page's cacheattr settings. * The map_domain_page() function still unconditionally created WB mappings, irrespective of the page's cacheattr settings. Additionally, update_xen_mappings() had a bug where the alias calculation was wrong for mfn's which were .init content, which should have been treated as fully guest pages, not Xen pages. Worse yet, the logic introduced a vulnerability whereby necessary pagetable/segdesc adjustments made by Xen in the validation logic could become non-coherent between the cache and main memory. The CPU could subsequently operate on the stale value in the cache, rather than the safe value in main memory. The directmap contains primarily mappings of RAM. PAT/MTRR conflict resolution is asymmetric, and generally for MTRR=WB ranges, PAT of lesser cacheability resolves to being coherent. The special case is WC mappings, which are non-coherent against MTRR=WB regions (except for fully-coherent CPUs). Xen must not have any WC cacheability in the directmap, to prevent Xen's actions from creating non-coherency. (Guest actions creating non-coherency is dealt with in subsequent patches.) As all memory types for MTRR=WB ranges inter-operate coherently, so leave Xen's directmap mappings as WB. Only PV guests with access to devices can use reduced-cacheability mappings to begin with, and they're trusted not to mount DoSs against the system anyway. Drop PGC_cacheattr_{base,mask} entirely, and the logic to manipulate them. Shift the later PGC_* constants up, to gain 3 extra bits in the main reference count. Retain the check in get_page_from_l1e() for special_pages() because a guest has no business using reduced cacheability on these. This reverts changeset 55f97f49b7ce6c3520c555d19caac6cf3f9a5df0 This is CVE-2022-26363, part of XSA-402. Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com> Reviewed-by: George Dunlap <george.dunlap@citrix.com> --- a/xen/arch/x86/mm.c +++ b/xen/arch/x86/mm.c @@ -786,24 +786,6 @@ bool is_iomem_page(mfn_t mfn) return (page_get_owner(page) == dom_io); } -static int update_xen_mappings(unsigned long mfn, unsigned int cacheattr) -{ - int err = 0; - bool alias = mfn >= PFN_DOWN(xen_phys_start) && - mfn < PFN_UP(xen_phys_start + xen_virt_end - XEN_VIRT_START); - unsigned long xen_va = - XEN_VIRT_START + ((mfn - PFN_DOWN(xen_phys_start)) << PAGE_SHIFT); - - if ( unlikely(alias) && cacheattr ) - err = map_pages_to_xen(xen_va, _mfn(mfn), 1, 0); - if ( !err ) - err = map_pages_to_xen((unsigned long)mfn_to_virt(mfn), _mfn(mfn), 1, - PAGE_HYPERVISOR | cacheattr_to_pte_flags(cacheattr)); - if ( unlikely(alias) && !cacheattr && !err ) - err = map_pages_to_xen(xen_va, _mfn(mfn), 1, PAGE_HYPERVISOR); - return err; -} - #ifndef NDEBUG struct mmio_emul_range_ctxt { const struct domain *d; @@ -1008,47 +990,14 @@ get_page_from_l1e( goto could_not_pin; } - if ( pte_flags_to_cacheattr(l1f) != - ((page->count_info & PGC_cacheattr_mask) >> PGC_cacheattr_base) ) + if ( (l1f & PAGE_CACHE_ATTRS) != _PAGE_WB && is_xen_heap_page(page) ) { - unsigned long x, nx, y = page->count_info; - unsigned long cacheattr = pte_flags_to_cacheattr(l1f); - int err; - - if ( is_xen_heap_page(page) ) - { - if ( write ) - put_page_type(page); - put_page(page); - gdprintk(XENLOG_WARNING, - "Attempt to change cache attributes of Xen heap page\n"); - return -EACCES; - } - - do { - x = y; - nx = (x & ~PGC_cacheattr_mask) | (cacheattr << PGC_cacheattr_base); - } while ( (y = cmpxchg(&page->count_info, x, nx)) != x ); - - err = update_xen_mappings(mfn, cacheattr); - if ( unlikely(err) ) - { - cacheattr = y & PGC_cacheattr_mask; - do { - x = y; - nx = (x & ~PGC_cacheattr_mask) | cacheattr; - } while ( (y = cmpxchg(&page->count_info, x, nx)) != x ); - - if ( write ) - put_page_type(page); - put_page(page); - - gdprintk(XENLOG_WARNING, "Error updating mappings for mfn %" PRI_mfn - " (pfn %" PRI_pfn ", from L1 entry %" PRIpte ") for d%d\n", - mfn, get_gpfn_from_mfn(mfn), - l1e_get_intpte(l1e), l1e_owner->domain_id); - return err; - } + if ( write ) + put_page_type(page); + put_page(page); + gdprintk(XENLOG_WARNING, + "Attempt to change cache attributes of Xen heap page\n"); + return -EACCES; } return 0; @@ -2541,25 +2490,10 @@ static int mod_l4_entry(l4_pgentry_t *pl */ static int cleanup_page_mappings(struct page_info *page) { - unsigned int cacheattr = - (page->count_info & PGC_cacheattr_mask) >> PGC_cacheattr_base; int rc = 0; unsigned long mfn = mfn_x(page_to_mfn(page)); /* - * If we've modified xen mappings as a result of guest cache - * attributes, restore them to the "normal" state. - */ - if ( unlikely(cacheattr) ) - { - page->count_info &= ~PGC_cacheattr_mask; - - BUG_ON(is_xen_heap_page(page)); - - rc = update_xen_mappings(mfn, 0); - } - - /* * If this may be in a PV domain's IOMMU, remove it. * * NB that writable xenheap pages have their type set and cleared by --- a/xen/include/asm-x86/mm.h +++ b/xen/include/asm-x86/mm.h @@ -64,22 +64,19 @@ /* Set when is using a page as a page table */ #define _PGC_page_table PG_shift(3) #define PGC_page_table PG_mask(1, 3) - /* 3-bit PAT/PCD/PWT cache-attribute hint. */ -#define PGC_cacheattr_base PG_shift(6) -#define PGC_cacheattr_mask PG_mask(7, 6) /* Page is broken? */ -#define _PGC_broken PG_shift(7) -#define PGC_broken PG_mask(1, 7) +#define _PGC_broken PG_shift(4) +#define PGC_broken PG_mask(1, 4) /* Mutually-exclusive page states: { inuse, offlining, offlined, free }. */ -#define PGC_state PG_mask(3, 9) -#define PGC_state_inuse PG_mask(0, 9) -#define PGC_state_offlining PG_mask(1, 9) -#define PGC_state_offlined PG_mask(2, 9) -#define PGC_state_free PG_mask(3, 9) +#define PGC_state PG_mask(3, 6) +#define PGC_state_inuse PG_mask(0, 6) +#define PGC_state_offlining PG_mask(1, 6) +#define PGC_state_offlined PG_mask(2, 6) +#define PGC_state_free PG_mask(3, 6) #define page_state_is(pg, st) (((pg)->count_info&PGC_state) == PGC_state_##st) /* Count of references to this frame. */ -#define PGC_count_width PG_shift(9) +#define PGC_count_width PG_shift(6) #define PGC_count_mask ((1UL<<PGC_count_width)-1) /*
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