1 files changed, 36 insertions, 0 deletions

diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
index ffd08c414e9..64effdc6da9 100644
--- a/arch/x86/xen/p2m.c
+++ b/arch/x86/xen/p2m.c
@@ -706,6 +706,7 @@ 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)) {
@@ -741,6 +742,24 @@ int m2p_add_override(unsigned long mfn, struct page *page,
         list_add(&page->lru,  &m2p_overrides[mfn_hash(mfn)]);
         spin_unlock_irqrestore(&m2p_override_lock, flags);
 
+        /* p2m(m2p(mfn)) == mfn: the mfn is already present somewhere in
+         * this domain. Set the FOREIGN_FRAME_BIT in the p2m for the other
+         * pfn so that the following mfn_to_pfn(mfn) calls will return the
+         * pfn from the m2p_override (the backend pfn) instead.
+         * We need to do this because the pages shared by the frontend
+         * (xen-blkfront) can be already locked (lock_page, called by
+         * do_read_cache_page); when the userspace backend tries to use them
+         * with direct_IO, mfn_to_pfn returns the pfn of the frontend, so
+         * do_blockdev_direct_IO is going to try to lock the same pages
+         * again resulting in a deadlock.
+         * As a side effect get_user_pages_fast might not be safe on the
+         * 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)
+                set_phys_to_machine(pfn, FOREIGN_FRAME(mfn));
+
         return 0;
 }
 EXPORT_SYMBOL_GPL(m2p_add_override);
@@ -752,6 +771,7 @@ int m2p_remove_override(struct page *page, bool clear_pte)
         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);
@@ -821,6 +841,22 @@ int m2p_remove_override(struct page *page, bool clear_pte)
         } else
                 set_phys_to_machine(pfn, page->index);
 
+        /* p2m(m2p(mfn)) == FOREIGN_FRAME(mfn): the mfn is already present
+         * somewhere in this domain, even before being added to the
+         * m2p_override (see comment above in m2p_add_override).
+         * If there are no other entries in the m2p_override corresponding
+         * to this mfn, then remove the FOREIGN_FRAME_BIT from the p2m for
+         * the original pfn (the one shared by the frontend): the backend
+         * cannot do any IO on this page anymore because it has been
+         * unshared. Removing the FOREIGN_FRAME_BIT from the p2m entry of
+         * 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) &&
+                        m2p_find_override(mfn) == NULL)
+                set_phys_to_machine(pfn, mfn);
+
         return 0;
 }
 EXPORT_SYMBOL_GPL(m2p_remove_override);