From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id E540954723; Sun, 12 Jul 2026 11:16:45 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783855008; cv=none; b=eNWEvFBAuE7fpf5IzI/7X9Ts1TlP2/Beo93fQSqVVHL7WQuhFDKL935P99yICHDOv5HGuhuSsBj+8S3SGNyPIv1xf0Z8ihKzczAFS4TWvigt9F/JcDrVp4pa0dXFsyzCeVmgWPoj0THVDvrY7ktSt5uOAc2FXoS24+/ZUBlaVe0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783855008; c=relaxed/simple; bh=4w2FnFBScyw7YiWU7s5gk5e3O6Mlmlj6IlRNpHN4Kvs=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=e5wDhCt0w/2dqUZbNFzGv8EmuoZ62mRNm5yrHW7PHHlQzF/oCL3c15aAzEfZwSpoo/k8wlVn8vuc/DNEE9p0HqxeC5w2u8GUzlEmUCuhCOFkujrjdriilmVZa5aHZ+IMlgXyaXi4y5QYqjRUIBK7k/h0ZOd2T+9qz/WvwP6mc+c= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b=aizDZIet; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b="aizDZIet" Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 132A81682; Sun, 12 Jul 2026 04:16:41 -0700 (PDT) Received: from [10.163.128.224] (unknown [10.163.128.224]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 9374E3F85F; Sun, 12 Jul 2026 04:16:39 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1783855005; bh=4w2FnFBScyw7YiWU7s5gk5e3O6Mlmlj6IlRNpHN4Kvs=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=aizDZIetRuhqDZIAblfUpSHmN60c1g6V9Mt4KSqHWNxS7o8f22B5iWg89O/gWEy3U xlLQsPTatoZxElpkRlPCFNKVpRZ66ca6G7+pNhtMcqsfcCRZ9dRqxOG9MiqH109zRo TZbIaVaOnhkMQGxIsvq0cW4Q0p459JDKSW3WI/lI= Message-ID: <28b7e7d2-62d8-4141-a6e9-7ab1cbb2baa4@arm.com> Date: Sun, 12 Jul 2026 16:46:36 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] mm/vmalloc: acquire init_mm read lock on huge vmap promotion To: Lorenzo Stoakes Cc: Andrew Morton , Suren Baghdasaryan , "Liam R. Howlett" , Vlastimil Babka , Shakeel Butt , David Hildenbrand , Mike Rapoport , Michal Hocko , Uladzislau Rezki , Toshi Kani , Catalin Marinas , Will Deacon , David Carlier , Ryan Roberts , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, stable@vger.kernel.org, syzbot+fd95a72470f5a44e464c@syzkaller.appspotmail.com References: <20260710-series-vmap-race-fix-v1-0-5b3794c113fe@kernel.org> <20260710-series-vmap-race-fix-v1-1-5b3794c113fe@kernel.org> Content-Language: en-US From: Dev Jain In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 12/07/26 1:52 pm, Lorenzo Stoakes wrote: > On Sun, Jul 12, 2026 at 01:13:12PM +0530, Dev Jain wrote: >> >> >> [-----] >> >>> We also define a guard class for mmap_read_trylock() so we can use >>> cleanup.h to make the scope handling cleaner in the implementation. >>> >> >> Will this cause backport problems, I think this scoped guard thingy is >> not that old? > > I intentionally used it because it's the best way to solve this problem. > > If there's any issue I'll fix them up in the stable backports myself. > > I will likely resend this as a 4 patch series and just do the stable > backports manually anyway. > >> >> >>> One wrinkle here is commit fa93b45fd397 ("arm64: Enable vmalloc-huge with >>> ptdump"), which addresses the issue for arm64 only by explicitly acquiring >>> the mmap read lock on kernel page table freeing should a concurrent ptdump >>> be in progress. >>> >>> This is problematic as vmap may acquire the mmap read lock prior to ptdump >>> attempting to acquire an mmap write lock, leading to a deadlock when the >>> mmap read lock is slept upon on page table freeing due to rwsem >>> anti-starvation. >>> >>> We work around this by predicating the mmap lock being taken on >>> !CONFIG_ARM64 for the time being. >>> >>> With this patch applied, a follow up will partially revert commit >>> fa93b45fd397 ("arm64: Enable vmalloc-huge with ptdump") and at that stage >>> remove the arm64 ifdeffery. >>> >>> We also update walk_page_range_debug() to assert the mmap write lock >>> unconditionally and update the comment here to reflect this change. >>> >>> The issue has existed as long as ptdump was available and vmap freed page >>> tables when promoting to a huge leaf entry, that is, since commit >>> b6bdb7517c3d ("mm/vmalloc: add interfaces to free unmapped page table") for >>> huge ioremap, and commit 121e6f3258fe ("mm/vmalloc: hugepage vmalloc >>> mappings") for huge vmalloc. >>> >>> Since the former is the earlier of the two we choose that for our Fixes >>> tag. >>> >>> This patch is based on work by David Carlier (linked), with gratitude! >>> >>> Fixes: b6bdb7517c3d ("mm/vmalloc: add interfaces to free unmapped page table") >>> Cc: >>> Reported-by: syzbot+fd95a72470f5a44e464c@syzkaller.appspotmail.com >>> Closes: https://lore.kernel.org/all/6a287988.39669fcc.33b062.00a0.GAE@google.com/T/ >>> Link: https://lore.kernel.org/linux-mm/20260706203128.162335-1-devnexen@gmail.com/ >>> Signed-off-by: Lorenzo Stoakes >>> --- >>> include/linux/mmap_lock.h | 1 + >>> mm/pagewalk.c | 22 +++++++++++---------- >>> mm/vmalloc.c | 50 ++++++++++++++++++++++++++++++++++++++--------- >>> 3 files changed, 54 insertions(+), 19 deletions(-) >>> >>> diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h >>> index 04b8f61ece5d..6b5c2390cc30 100644 >>> --- a/include/linux/mmap_lock.h >>> +++ b/include/linux/mmap_lock.h >>> @@ -621,6 +621,7 @@ static inline void mmap_read_unlock(struct mm_struct *mm) >>> >>> DEFINE_GUARD(mmap_read_lock, struct mm_struct *, >>> mmap_read_lock(_T), mmap_read_unlock(_T)) >>> +DEFINE_GUARD_COND(mmap_read_lock, _try, mmap_read_trylock(_T)) >>> >>> static inline void mmap_read_unlock_non_owner(struct mm_struct *mm) >>> { >>> diff --git a/mm/pagewalk.c b/mm/pagewalk.c >>> index 3ae2586ff45b..bbcfd68d0907 100644 >>> --- a/mm/pagewalk.c >>> +++ b/mm/pagewalk.c >>> @@ -678,6 +678,8 @@ int walk_kernel_page_table_range_lockless(unsigned long start, unsigned long end >>> * will also not lock the PTEs for the pte_entry() callback. >>> * >>> * This is for debugging purposes ONLY. >>> + * >>> + * The mmap write lock must be held. >>> */ >>> int walk_page_range_debug(struct mm_struct *mm, unsigned long start, >>> unsigned long end, const struct mm_walk_ops *ops, >>> @@ -691,6 +693,16 @@ int walk_page_range_debug(struct mm_struct *mm, unsigned long start, >>> .no_vma = true >>> }; >>> >>> + /* >>> + * When walking userland page tables, an mmap write lock must be held to >>> + * account for munmap() downgrading to an mmap read lock when tearing >>> + * down page tables. >>> + * >>> + * When walking kernel page tables, an mmap write lock must also be held >>> + * to account for page table freeing on vmap huge page mapping. >>> + */ >>> + mmap_assert_write_locked(mm); >>> + >>> /* For convenience, we allow traversal of kernel mappings. */ >>> if (mm == &init_mm) >>> return walk_kernel_page_table_range(start, end, ops, >>> @@ -700,16 +712,6 @@ int walk_page_range_debug(struct mm_struct *mm, unsigned long start, >>> if (!check_ops_safe(ops)) >>> return -EINVAL; >>> >>> - /* >>> - * The mmap lock protects the page walker from changes to the page >>> - * tables during the walk. However a read lock is insufficient to >>> - * protect those areas which don't have a VMA as munmap() detaches >>> - * the VMAs before downgrading to a read lock and actually tearing >>> - * down PTEs/page tables. In which case, the mmap write lock should >>> - * be held. >>> - */ >>> - mmap_assert_write_locked(mm); >>> - >>> return walk_pgd_range(start, end, &walk); >>> } >>> >>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c >>> index 1afca3568b9b..9d0f1fdd6af3 100644 >>> --- a/mm/vmalloc.c >>> +++ b/mm/vmalloc.c >>> @@ -43,6 +43,7 @@ >>> #include >>> #include >>> #include >>> +#include >>> >>> #define CREATE_TRACE_POINTS >>> #include >>> @@ -158,10 +159,25 @@ static int vmap_try_huge_pmd(pmd_t *pmd, unsigned long addr, unsigned long end, >>> if (!IS_ALIGNED(phys_addr, PMD_SIZE)) >>> return 0; >>> >>> - if (pmd_present(*pmd) && !pmd_free_pte_page(pmd, addr)) >>> - return 0; >>> + if (!pmd_present(*pmd)) >>> + return pmd_set_huge(pmd, phys_addr, prot); >>> >>> - return pmd_set_huge(pmd, phys_addr, prot); >>> + /* >>> + * Kernel page table walkers either walk ranges they own exclusively >>> + * using the mmap lock for mutual exclusion, or hold the mmap write lock >>> + * on init_mm (ptdump being the motivating case). >>> + * >>> + * Therefore, acquire the mmap read lock to prevent use-after-free when >>> + * freeing page tables. >>> + */ >>> +#ifndef CONFIG_ARM64 >>> + scoped_cond_guard(mmap_read_lock_try, return 0, &init_mm) >>> +#endif >>> + { >>> + if (!pmd_free_pte_page(pmd, addr)) >>> + return 0; >>> + return pmd_set_huge(pmd, phys_addr, prot); >>> + } >>> } >>> >> Note that we do not need to take the lock around pmd_set_huge - we don't >> care if ptdump observes a temporarily cleared pmd entry. So how about keeping >> this outside the guard block. Otherwise right now we have an inconsistency: >> for !pmd_present() we do pmd_set_huge() without locking, but for pmd_present() >> we do pmd_set_huge() with locking. > > As I said in the commit message I'm intentionally taking the lock around all of > it so a concurrent ptdump sees either the leaf entry or the non-leaf entry. > > By doing that we can easiliy avoid the situation where a ptdump gives you > inconsistent output and it makes more sense logically. > > So this is the opposite of inconsistent - if !pmd_present() the ptdump may > observe the _genuine_ state of there not being an entry before. With > pmd_present() it either observes what was or what became, not something > inbetween :) Yeah okay fair enough. > >> >> > > Thanks, Lorenzo