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=-15.7 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,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 E0450C2BB40 for ; Tue, 15 Dec 2020 03:06:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A7411224DE for ; Tue, 15 Dec 2020 03:06:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725931AbgLODGm (ORCPT ); Mon, 14 Dec 2020 22:06:42 -0500 Received: from mail.kernel.org ([198.145.29.99]:34386 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726518AbgLODGd (ORCPT ); Mon, 14 Dec 2020 22:06:33 -0500 Date: Mon, 14 Dec 2020 19:05:44 -0800 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1608001546; bh=EhPK13ipLfockHPv4s4hehPGqjICAvzk5fTJ3Bo/ARM=; h=From:To:Subject:In-Reply-To:From; b=l/ObnCYHu3ilYyzGEzxRxN+3anFVpB0/EcrIEAOvjNws3Y0nv9dDeAvqbnsAr0RXv yYuynrix+wl+3trjr9LhzyyoC8qTr5Xa+hkv+wHKmQfe9yC6wRd05M7AUTcjUp3n9/ +hngHUvIuh7ueCzn1oaJ3AugqAZuAIakvlsPrrz0= From: Andrew Morton To: a.darwish@linutronix.de, aarcange@redhat.com, akpm@linux-foundation.org, aneesh.kumar@linux.ibm.com, hch@lst.de, hughd@google.com, jack@suse.cz, jannh@google.com, jgg@nvidia.com, jhubbard@nvidia.com, kirill@shutemov.name, ktkhai@virtuozzo.com, leonro@nvidia.com, linux-mm@kvack.org, mhocko@suse.com, mm-commits@vger.kernel.org, oleg@redhat.com, peterx@redhat.com, torvalds@linux-foundation.org Subject: [patch 044/200] mm/gup: prevent gup_fast from racing with COW during fork Message-ID: <20201215030544.JXnn3g5Dw%akpm@linux-foundation.org> In-Reply-To: <20201214190237.a17b70ae14f129e2dca3d204@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: Jason Gunthorpe Subject: mm/gup: prevent gup_fast from racing with COW during fork Since commit 70e806e4e645 ("mm: Do early cow for pinned pages during fork() for ptes") pages under a FOLL_PIN will not be write protected during COW for fork. This means that pages returned from pin_user_pages(FOLL_WRITE) should not become write protected while the pin is active. However, there is a small race where get_user_pages_fast(FOLL_PIN) can establish a FOLL_PIN at the same time copy_present_page() is write protecting it: CPU 0 CPU 1 get_user_pages_fast() internal_get_user_pages_fast() copy_page_range() pte_alloc_map_lock() copy_present_page() atomic_read(has_pinned) == 0 page_maybe_dma_pinned() == false atomic_set(has_pinned, 1); gup_pgd_range() gup_pte_range() pte_t pte = gup_get_pte(ptep) pte_access_permitted(pte) try_grab_compound_head() pte = pte_wrprotect(pte) set_pte_at(); pte_unmap_unlock() // GUP now returns with a write protected page The first attempt to resolve this by using the write protect caused problems (and was missing a barrrier), see commit f3c64eda3e50 ("mm: avoid early COW write protect games during fork()") Instead wrap copy_p4d_range() with the write side of a seqcount and check the read side around gup_pgd_range(). If there is a collision then get_user_pages_fast() fails and falls back to slow GUP. Slow GUP is safe against this race because copy_page_range() is only called while holding the exclusive side of the mmap_lock on the src mm_struct. [akpm@linux-foundation.org: coding style fixes] Link: https://lkml.kernel.org/r/2-v4-908497cf359a+4782-gup_fork_jgg@nvidia.com Fixes: f3c64eda3e50 ("mm: avoid early COW write protect games during fork()") Signed-off-by: Jason Gunthorpe Suggested-by: Linus Torvalds Link: https://lore.kernel.org/r/CAHk-=wi=iCnYCARbPGjkVJu9eyYeZ13N64tZYLdOB8CP5Q_PLw@mail.gmail.com Reviewed-by: John Hubbard Reviewed-by: Jan Kara Reviewed-by: Peter Xu Acked-by: "Ahmed S. Darwish" [seqcount_t parts] Cc: Andrea Arcangeli Cc: "Aneesh Kumar K.V" Cc: Christoph Hellwig Cc: Hugh Dickins Cc: Jann Horn Cc: Kirill Shutemov Cc: Kirill Tkhai Cc: Leon Romanovsky Cc: Michal Hocko Cc: Oleg Nesterov Signed-off-by: Andrew Morton --- arch/x86/kernel/tboot.c | 1 + drivers/firmware/efi/efi.c | 1 + include/linux/mm_types.h | 8 ++++++++ kernel/fork.c | 1 + mm/gup.c | 18 ++++++++++++++++++ mm/init-mm.c | 1 + mm/memory.c | 13 ++++++++++++- 7 files changed, 42 insertions(+), 1 deletion(-) --- a/arch/x86/kernel/tboot.c~mm-prevent-gup_fast-from-racing-with-cow-during-fork +++ a/arch/x86/kernel/tboot.c @@ -93,6 +93,7 @@ static struct mm_struct tboot_mm = { .pgd = swapper_pg_dir, .mm_users = ATOMIC_INIT(2), .mm_count = ATOMIC_INIT(1), + .write_protect_seq = SEQCNT_ZERO(tboot_mm.write_protect_seq), MMAP_LOCK_INITIALIZER(init_mm) .page_table_lock = __SPIN_LOCK_UNLOCKED(init_mm.page_table_lock), .mmlist = LIST_HEAD_INIT(init_mm.mmlist), --- a/drivers/firmware/efi/efi.c~mm-prevent-gup_fast-from-racing-with-cow-during-fork +++ a/drivers/firmware/efi/efi.c @@ -57,6 +57,7 @@ struct mm_struct efi_mm = { .mm_rb = RB_ROOT, .mm_users = ATOMIC_INIT(2), .mm_count = ATOMIC_INIT(1), + .write_protect_seq = SEQCNT_ZERO(efi_mm.write_protect_seq), MMAP_LOCK_INITIALIZER(efi_mm) .page_table_lock = __SPIN_LOCK_UNLOCKED(efi_mm.page_table_lock), .mmlist = LIST_HEAD_INIT(efi_mm.mmlist), --- a/include/linux/mm_types.h~mm-prevent-gup_fast-from-racing-with-cow-during-fork +++ a/include/linux/mm_types.h @@ -14,6 +14,7 @@ #include #include #include +#include #include @@ -446,6 +447,13 @@ struct mm_struct { */ atomic_t has_pinned; + /** + * @write_protect_seq: Locked when any thread is write + * protecting pages mapped by this mm to enforce a later COW, + * for instance during page table copying for fork(). + */ + seqcount_t write_protect_seq; + #ifdef CONFIG_MMU atomic_long_t pgtables_bytes; /* PTE page table pages */ #endif --- a/kernel/fork.c~mm-prevent-gup_fast-from-racing-with-cow-during-fork +++ a/kernel/fork.c @@ -1007,6 +1007,7 @@ static struct mm_struct *mm_init(struct mm->vmacache_seqnum = 0; atomic_set(&mm->mm_users, 1); atomic_set(&mm->mm_count, 1); + seqcount_init(&mm->write_protect_seq); mmap_init_lock(mm); INIT_LIST_HEAD(&mm->mmlist); mm->core_state = NULL; --- a/mm/gup.c~mm-prevent-gup_fast-from-racing-with-cow-during-fork +++ a/mm/gup.c @@ -2684,11 +2684,18 @@ static unsigned long lockless_pages_from { unsigned long flags; int nr_pinned = 0; + unsigned seq; if (!IS_ENABLED(CONFIG_HAVE_FAST_GUP) || !gup_fast_permitted(start, end)) return 0; + if (gup_flags & FOLL_PIN) { + seq = raw_read_seqcount(¤t->mm->write_protect_seq); + if (seq & 1) + return 0; + } + /* * Disable interrupts. The nested form is used, in order to allow full, * general purpose use of this routine. @@ -2703,6 +2710,17 @@ static unsigned long lockless_pages_from local_irq_save(flags); gup_pgd_range(start, end, gup_flags, pages, &nr_pinned); local_irq_restore(flags); + + /* + * When pinning pages for DMA there could be a concurrent write protect + * from fork() via copy_page_range(), in this case always fail fast GUP. + */ + if (gup_flags & FOLL_PIN) { + if (read_seqcount_retry(¤t->mm->write_protect_seq, seq)) { + unpin_user_pages(pages, nr_pinned); + return 0; + } + } return nr_pinned; } --- a/mm/init-mm.c~mm-prevent-gup_fast-from-racing-with-cow-during-fork +++ a/mm/init-mm.c @@ -31,6 +31,7 @@ struct mm_struct init_mm = { .pgd = swapper_pg_dir, .mm_users = ATOMIC_INIT(2), .mm_count = ATOMIC_INIT(1), + .write_protect_seq = SEQCNT_ZERO(init_mm.write_protect_seq), MMAP_LOCK_INITIALIZER(init_mm) .page_table_lock = __SPIN_LOCK_UNLOCKED(init_mm.page_table_lock), .arg_lock = __SPIN_LOCK_UNLOCKED(init_mm.arg_lock), --- a/mm/memory.c~mm-prevent-gup_fast-from-racing-with-cow-during-fork +++ a/mm/memory.c @@ -1171,6 +1171,15 @@ copy_page_range(struct vm_area_struct *d mmu_notifier_range_init(&range, MMU_NOTIFY_PROTECTION_PAGE, 0, src_vma, src_mm, addr, end); mmu_notifier_invalidate_range_start(&range); + /* + * Disabling preemption is not needed for the write side, as + * the read side doesn't spin, but goes to the mmap_lock. + * + * Use the raw variant of the seqcount_t write API to avoid + * lockdep complaining about preemptibility. + */ + mmap_assert_write_locked(src_mm); + raw_write_seqcount_begin(&src_mm->write_protect_seq); } ret = 0; @@ -1187,8 +1196,10 @@ copy_page_range(struct vm_area_struct *d } } while (dst_pgd++, src_pgd++, addr = next, addr != end); - if (is_cow) + if (is_cow) { + raw_write_seqcount_end(&src_mm->write_protect_seq); mmu_notifier_invalidate_range_end(&range); + } return ret; } _