From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-20.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2B286C11F64 for ; Tue, 29 Jun 2021 02:40:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0FB6161D11 for ; Tue, 29 Jun 2021 02:40:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231799AbhF2Cm1 (ORCPT ); Mon, 28 Jun 2021 22:42:27 -0400 Received: from mail.kernel.org ([198.145.29.99]:34806 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231750AbhF2Cm1 (ORCPT ); Mon, 28 Jun 2021 22:42:27 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id BA3E561D0B; Tue, 29 Jun 2021 02:39:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1624934400; bh=JcK5J0ohugo1hHKbFqfwL/WIOI+F2ABAr36f1h6aw8U=; h=Date:From:To:Subject:In-Reply-To:From; b=RkmtFlfRIktyi0TjQd/Ftz4+4RZQWHIELlROefuBPQp8yWnMtF5zl0cM9+jAvI6Um ifyc3Kqh9FDdGdDukAlU0y3MpB456zljhz2SBGM9/PAuhrb54fwzZD0MLSun8bMp51 ceC24upNIvMjQk3erZGciCjLr4gdm7me5g+2L3hw= Date: Mon, 28 Jun 2021 19:39:59 -0700 From: Andrew Morton To: aarcange@redhat.com, akpm@linux-foundation.org, eugenis@google.com, kostyak@google.com, linux-mm@kvack.org, mm-commits@vger.kernel.org, pcc@google.com, peterx@redhat.com, torvalds@linux-foundation.org Subject: [patch 128/192] mm: improve mprotect(R|W) efficiency on pages referenced once Message-ID: <20210629023959.4ZAFiI8oZ%akpm@linux-foundation.org> In-Reply-To: <20210628193256.008961950a714730751c1423@linux-foundation.org> User-Agent: s-nail v14.8.16 Precedence: bulk Reply-To: linux-kernel@vger.kernel.org List-ID: X-Mailing-List: mm-commits@vger.kernel.org From: Peter Collingbourne Subject: mm: improve mprotect(R|W) efficiency on pages referenced once In the Scudo memory allocator [1] we would like to be able to detect use-after-free vulnerabilities involving large allocations by issuing mprotect(PROT_NONE) on the memory region used for the allocation when it is deallocated. Later on, after the memory region has been "quarantined" for a sufficient period of time we would like to be able to use it for another allocation by issuing mprotect(PROT_READ|PROT_WRITE). Before this patch, after removing the write protection, any writes to the memory region would result in page faults and entering the copy-on-write code path, even in the usual case where the pages are only referenced by a single PTE, harming performance unnecessarily. Make it so that any pages in anonymous mappings that are only referenced by a single PTE are immediately made writable during the mprotect so that we can avoid the page faults. This program shows the critical syscall sequence that we intend to use in the allocator: #include #include enum { kSize = 131072 }; int main(int argc, char **argv) { char *addr = (char *)mmap(0, kSize, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0); for (int i = 0; i != 100000; ++i) { memset(addr, i, kSize); mprotect((void *)addr, kSize, PROT_NONE); mprotect((void *)addr, kSize, PROT_READ | PROT_WRITE); } } The effect of this patch on the above program was measured on a DragonBoard 845c by taking the median real time execution time of 10 runs. Before: 2.94s After: 0.66s The effect was also measured using one of the microbenchmarks that we normally use to benchmark the allocator [2], after modifying it to make the appropriate mprotect calls [3]. With an allocation size of 131072 bytes to trigger the allocator's "large allocation" code path the per-iteration time was measured as follows: Before: 27450ns After: 6010ns This patch means that we do more work during the mprotect call itself in exchange for less work when the pages are accessed. In the worst case, the pages are not accessed at all. The effect of this patch in such cases was measured using the following program: #include #include enum { kSize = 131072 }; int main(int argc, char **argv) { char *addr = (char *)mmap(0, kSize, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0); memset(addr, 1, kSize); for (int i = 0; i != 100000; ++i) { #ifdef PAGE_FAULT memset(addr + (i * 4096) % kSize, i, 4096); #endif mprotect((void *)addr, kSize, PROT_NONE); mprotect((void *)addr, kSize, PROT_READ | PROT_WRITE); } } With PAGE_FAULT undefined (0 pages touched after removing write protection) the median real time execution time of 100 runs was measured as follows: Before: 0.330260s After: 0.338836s With PAGE_FAULT defined (1 page touched) the measurements were as follows: Before: 0.438048s After: 0.355661s So it seems that even with a single page fault the new approach is faster. I saw similar results if I adjusted the programs to use a larger mapping size. With kSize = 1048576 I get these numbers with PAGE_FAULT undefined: Before: 1.428988s After: 1.512016s i.e. around 5.5%. And these with PAGE_FAULT defined: Before: 1.518559s After: 1.524417s i.e. about the same. What I think we may conclude from these results is that for smaller mappings the advantage of the previous approach, although measurable, is wiped out by a single page fault. I think we may expect that there should be at least one access resulting in a page fault (under the previous approach) after making the pages writable, since the program presumably made the pages writable for a reason. For larger mappings we may guesstimate that the new approach wins if the density of future page faults is > 0.4%. But for the mappings that are large enough for density to matter (not just the absolute number of page faults) it doesn't seem like the increase in mprotect latency would be very large relative to the total mprotect execution time. [pcc@google.com: add comments, prohibit optimization for NUMA pages] Link: https://lkml.kernel.org/r/20210601185926.2623183-1-pcc@google.com Link: https://lkml.kernel.org/r/20210527190453.1259020-1-pcc@google.com Link: https://linux-review.googlesource.com/id/I98d75ef90e20330c578871c87494d64b1df3f1b8 Link: [1] https://source.android.com/devices/tech/debug/scudo Link: [2] https://cs.android.com/android/platform/superproject/+/master:bionic/benchmarks/stdlib_benchmark.cpp;l=53;drc=e8693e78711e8f45ccd2b610e4dbe0b94d551cc9 Link: [3] https://github.com/pcc/llvm-project/commit/scudo-mprotect-secondary2 Signed-off-by: Peter Collingbourne Reviewed-by: Peter Xu Cc: Kostya Kortchinsky Cc: Evgenii Stepanov Cc: Andrea Arcangeli Signed-off-by: Andrew Morton --- mm/mprotect.c | 52 ++++++++++++++++++++++++++++++++++++++++++------ 1 file changed, 46 insertions(+), 6 deletions(-) --- a/mm/mprotect.c~mm-improve-mprotectrw-efficiency-on-pages-referenced-once +++ a/mm/mprotect.c @@ -35,6 +35,51 @@ #include "internal.h" +/* Determine whether we can avoid taking write faults for known dirty pages. */ +static bool may_avoid_write_fault(pte_t pte, struct vm_area_struct *vma, + unsigned long cp_flags) +{ + /* + * The dirty accountable bit indicates that we can always make the page + * writable regardless of the number of references. + */ + if (!(cp_flags & MM_CP_DIRTY_ACCT)) { + /* Otherwise, we must have exclusive access to the page. */ + if (!(vma_is_anonymous(vma) && (vma->vm_flags & VM_WRITE))) + return false; + + if (page_count(pte_page(pte)) != 1) + return false; + } + + /* + * Don't do this optimization for clean pages as we need to be notified + * of the transition from clean to dirty. + */ + if (!pte_dirty(pte)) + return false; + + /* Same for softdirty. */ + if (!pte_soft_dirty(pte) && (vma->vm_flags & VM_SOFTDIRTY)) + return false; + + /* + * For userfaultfd the user program needs to monitor write faults so we + * can't do this optimization. + */ + if (pte_uffd_wp(pte)) + return false; + + /* + * It is unclear whether this optimization can be done safely for NUMA + * pages. + */ + if (cp_flags & MM_CP_PROT_NUMA) + return false; + + return true; +} + static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd, unsigned long addr, unsigned long end, pgprot_t newprot, unsigned long cp_flags) @@ -43,7 +88,6 @@ static unsigned long change_pte_range(st spinlock_t *ptl; unsigned long pages = 0; int target_node = NUMA_NO_NODE; - bool dirty_accountable = cp_flags & MM_CP_DIRTY_ACCT; bool prot_numa = cp_flags & MM_CP_PROT_NUMA; bool uffd_wp = cp_flags & MM_CP_UFFD_WP; bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE; @@ -131,12 +175,8 @@ static unsigned long change_pte_range(st ptent = pte_clear_uffd_wp(ptent); } - /* Avoid taking write faults for known dirty pages */ - if (dirty_accountable && pte_dirty(ptent) && - (pte_soft_dirty(ptent) || - !(vma->vm_flags & VM_SOFTDIRTY))) { + if (may_avoid_write_fault(ptent, vma, cp_flags)) ptent = pte_mkwrite(ptent); - } ptep_modify_prot_commit(vma, addr, pte, oldpte, ptent); pages++; } else if (is_swap_pte(oldpte)) { _