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 EB1C723D2A1; Sun, 12 Jul 2026 07:43:22 +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=1783842204; cv=none; b=UvBF2etO1XWEPxX8EpV5QtNYMFcU/HwRBci9CXhfD8ZraGi9iicyTrI/aBQMO2eOi6xq76qjFFPkmBmDMvG/vuw1hrbNRwJnAHu8W6ao3X2Jvh8NVDPsSdRV6d9DRMLOjO6LIiAVvTcXv34CNGPsi6gTL9dRHiiyzwj6g//+rPk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783842204; c=relaxed/simple; bh=UYx5fo7e+lxcyM59ISiOU13g5eIgoyjRrIVDtKL9jW0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=tuXmy22sChn1MFUedB9nsFgG5lJzP2pF2YF6WY0MhTaFZkVW2KP/AZv//83r3FTCeIPmE6xBJgMh5CRSqJJR+/cp4C1VVxHwInGb4QRW2MOyrr8mkHfo1k6kOGTJn71TrJd994zf6Z+kQ+6Q03GsAlo+yWnm0Iliq7Gcf/I9SJ8= 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=UM3JYBV9; 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="UM3JYBV9" 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 5BDC01682; Sun, 12 Jul 2026 00:43:17 -0700 (PDT) Received: from [10.163.129.143] (unknown [10.163.129.143]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 8ECF53F85F; Sun, 12 Jul 2026 00:43:15 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1783842201; bh=UYx5fo7e+lxcyM59ISiOU13g5eIgoyjRrIVDtKL9jW0=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=UM3JYBV9OTPunMhOREV5sC8dPLz7aJpnp0FpbpEaiJeHy/lYgBBaRn2JyJ/+vUS9b C7yMwQ1qtJP5l8lzEdfU1Sr3He1ud6P8jR573YExHwyIIZQC99cpw/T9zunBGIhJIL tkpnh2iCeXxOpqU6JlnwaGudOBuSmCDc3g6D2EAM= Message-ID: Date: Sun, 12 Jul 2026 13:13:12 +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 , 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 Cc: 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: <20260710-series-vmap-race-fix-v1-1-5b3794c113fe@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit [-----] > 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? > 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.