From mboxrd@z Thu Jan 1 00:00:00 1970 From: Khalid Aziz Subject: [RFC PATCH v7 13/16] xpfo, mm: optimize spinlock usage in xpfo_kunmap Date: Thu, 10 Jan 2019 14:09:45 -0700 Message-Id: <95b6fa40ce6c7afb4a9e58f8d747d86aa7a94177.1547153058.git.khalid.aziz@oracle.com> In-Reply-To: References: In-Reply-To: References: To: juergh@gmail.com, tycho@tycho.ws, jsteckli@amazon.de, ak@linux.intel.com, torvalds@linux-foundation.org, liran.alon@oracle.com, keescook@google.com, konrad.wilk@oracle.com Cc: deepa.srinivasan@oracle.com, chris.hyser@oracle.com, tyhicks@canonical.com, dwmw@amazon.co.uk, andrew.cooper3@citrix.com, jcm@redhat.com, boris.ostrovsky@oracle.com, kanth.ghatraju@oracle.com, joao.m.martins@oracle.com, jmattson@google.com, pradeep.vincent@oracle.com, john.haxby@oracle.com, tglx@linutronix.de, kirill.shutemov@linux.intel.com, hch@lst.de, steven.sistare@oracle.com, kernel-hardening@lists.openwall.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, x86@kernel.org, "Vasileios P . Kemerlis" , Juerg Haefliger , Tycho Andersen , Marco Benatto , David Woodhouse , Khalid Aziz List-ID: From: Julian Stecklina Only the xpfo_kunmap call that needs to actually unmap the page needs to be serialized. We need to be careful to handle the case, where after the atomic decrement of the mapcount, a xpfo_kmap increased the mapcount again. In this case, we can safely skip modifying the page table. Model-checked with up to 4 concurrent callers with Spin. Signed-off-by: Julian Stecklina Cc: x86@kernel.org Cc: kernel-hardening@lists.openwall.com Cc: Vasileios P. Kemerlis Cc: Juerg Haefliger Cc: Tycho Andersen Cc: Marco Benatto Cc: David Woodhouse Signed-off-by: Khalid Aziz --- mm/xpfo.c | 22 ++++++++++++---------- 1 file changed, 12 insertions(+), 10 deletions(-) diff --git a/mm/xpfo.c b/mm/xpfo.c index cbfeafc2f10f..dbf20efb0499 100644 --- a/mm/xpfo.c +++ b/mm/xpfo.c @@ -149,22 +149,24 @@ void xpfo_kunmap(void *kaddr, struct page *page) if (!PageXpfoUser(page)) return; - spin_lock(&page->xpfo_lock); - /* * The page is to be allocated back to user space, so unmap it from the * kernel, flush the TLB and tag it as a user page. */ if (atomic_dec_return(&page->xpfo_mapcount) == 0) { -#ifdef CONFIG_XPFO_DEBUG - BUG_ON(PageXpfoUnmapped(page)); -#endif - SetPageXpfoUnmapped(page); - set_kpte(kaddr, page, __pgprot(0)); - xpfo_cond_flush_kernel_tlb(page, 0); - } + spin_lock(&page->xpfo_lock); - spin_unlock(&page->xpfo_lock); + /* + * In the case, where we raced with kmap after the + * atomic_dec_return, we must not nuke the mapping. + */ + if (atomic_read(&page->xpfo_mapcount) == 0) { + SetPageXpfoUnmapped(page); + set_kpte(kaddr, page, __pgprot(0)); + xpfo_cond_flush_kernel_tlb(page, 0); + } + spin_unlock(&page->xpfo_lock); + } } EXPORT_SYMBOL(xpfo_kunmap); -- 2.17.1