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 D25E7310645 for ; Wed, 1 Jul 2026 05:42:23 +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=1782884546; cv=none; b=BzMpzqkxf76phMJ5NjXz6I4tSlp9bWabbdusPaEeuCNP2JNGNiGfIUoiBlTq0uYBDSiNm42DXPfiELYMQFxPSx3sUMkMLtObf6SefCqizY8GBdcwtR1BnwyfQGA/Uiagd3T/IJ3Sn1W4jQuF22uatu61Fk489w7vG0Fp6o/1SNI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782884546; c=relaxed/simple; bh=Hn1g9R5Z8hjvzBeDVB/yavX32Z5EEPqnLXM9LxfbYXs=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=CUBW/d6/72bDFYx6FVDgAvhL4m8G74NH9ysnr54OeyVTpVGgM5DImaDHdizSUtOhfkzH0wAinVxLnnnt/K4NjVC+ftsY6cyw+lGpMF//5rDRlsl+amKcOLluK5Wgd3xysBqWIp5JN9p8a/NH7ODH+WIEXEHGw2/r7DfUokk9WIU= 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=fjUXTQia; 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="fjUXTQia" 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 7AA8E2364; Tue, 30 Jun 2026 22:42:18 -0700 (PDT) Received: from [10.164.19.15] (unknown [10.164.19.15]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 1C7323F905; Tue, 30 Jun 2026 22:42:17 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1782884542; bh=Hn1g9R5Z8hjvzBeDVB/yavX32Z5EEPqnLXM9LxfbYXs=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=fjUXTQiaDXQglJl+QbVZ1KookWSK/G4jH25RrXefdflAm8CZr794+g3Q4iWKdI5U6 ecLhnqfBX6W+wI55G6PM8/w71uMaFj3z2LLCWYuhXWxUM0IUXUGEosOHXSz8GUS16/ gpvS10NvVNYdmrIg4OKNDJw7X73F7LCmumHHOLjw= Message-ID: Date: Wed, 1 Jul 2026 11:12:15 +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] mm: pgtable: free kernel page tables via RCU to fix ptdump UAF To: David Carlier , Andrew Morton Cc: David Hildenbrand , Lorenzo Stoakes , "Liam R . Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti , Dave Hansen , Lu Baolu , syzbot+fd95a72470f5a44e464c@syzkaller.appspotmail.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org References: <20260630041012.5975-1-devnexen@gmail.com> Content-Language: en-US From: Dev Jain In-Reply-To: <20260630041012.5975-1-devnexen@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 30/06/26 9:40 am, David Carlier wrote: > ptdump_walk_pgd() walks the kernel page tables under get_online_mems() and > mmap_write_lock(&init_mm). Neither lock stops vmalloc from freeing a kernel > PTE page underneath the walk. > > When vmap_try_huge_pmd() promotes a range to a huge PMD it collapses the > existing PTE table and frees it via pmd_free_pte_page(). On x86, riscv and > powerpc this runs without the init_mm mmap lock; only arm64 takes it, and not > on the block-split path. So ptdump can dereference a just-freed PTE page, > which is the use after free syzbot hit in ptdump_pte_entry(). > > The race is not new. ptdump walks the whole kernel address space, including > ranges other code is actively mapping, so it reads page tables it does not > own. 5ba2f0a15564 ("mm: introduce deferred freeing for kernel page tables") > only widened the window; the Fixes tag points there for that reason. > > Every other walker works on a range it owns and is the only one mutating it: > set_memory() on arm64/riscv/loongarch, the arm64 block-split path, the > openrisc DMA path and the hugetlb_vmemmap remap. Nothing frees those ranges > concurrently, so they cannot race and do not need RCU. ptdump is the only > walker that traverses ranges it does not own. > > Defer the free by an RCU grace period. pagetable_free_kernel() now frees via > call_rcu() in both the async and non-async configs. The async path still > flushes the TLB first, then queues the per-page RCU free. The page stays > valid until any walk that may have observed it drops its RCU read lock. > > On the read side ptdump_walk_pgd() takes the RCU read lock around the walk, > and walk_page_range_debug() asserts it with RCU_LOCKDEP_WARN() for the > init_mm case rather than taking it, matching pagewalk.c convention. A walker > either sees the cleared PMD and skips, or keeps the page alive until it drops > the lock. The owned-range walkers are unchanged. > > ptdump callbacks now run under RCU, so they must not sleep. The arch > note_page() and effective_prot() callbacks only format into the preallocated > seq_file buffer, and the walker does not call cond_resched(); the only > GFP_KERNEL marker setup runs before the walk. > > Fixes: 5ba2f0a15564 ("mm: introduce deferred freeing for kernel page tables") > Reported-by: syzbot+fd95a72470f5a44e464c@syzkaller.appspotmail.com > Closes: https://lore.kernel.org/all/6a287988.39669fcc.33b062.00a0.GAE@google.com/T/ > Assisted-by: Claude:claude-opus-4-8 > Signed-off-by: David Carlier > --- Thanks. Is my assessment correct that this patch deems commit fa93b45fd397 ("arm64: Enable vmalloc-huge with ptdump") redundant? (in which case I can send a partial revert for that) > v5: reframe changelog around the pre-existing race and range ownership; > correct the mmap-lock description (arm64 is the exception, not x86); > move rcu_read_lock() into ptdump_walk_pgd() and assert it in > walk_page_range_debug(); drop walk_kernel_page_table_range_rcu(); fix the > pgtable-generic.c comment; document the no-sleep audit of the callbacks. > v4: defer the free in both the async and non async configs, not just > the async one. Move the walk under a named > walk_kernel_page_table_range_rcu() helper instead of open coding > rcu_read_lock() in walk_page_range_debug(). > v3: take rcu_read_lock() in the init_mm branch of > walk_page_range_debug() rather than inside the lockless walker, > which the arm64 split paths also use with GFP_PGTABLE_KERNEL and > can sleep. > v2: use call_rcu() instead of synchronize_rcu(). > --- > include/linux/mm.h | 7 ------- > mm/pagewalk.c | 14 +++++++++----- > mm/pgtable-generic.c | 22 +++++++++++++++++++++- > mm/ptdump.c | 2 ++ > 4 files changed, 32 insertions(+), 13 deletions(-) > > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 485df9c2dbdd..79408a17a1b0 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -3695,14 +3695,7 @@ static inline void __pagetable_free(struct ptdesc *pt) > __free_pages(page, compound_order(page)); > } > > -#ifdef CONFIG_ASYNC_KERNEL_PGTABLE_FREE > void pagetable_free_kernel(struct ptdesc *pt); > -#else > -static inline void pagetable_free_kernel(struct ptdesc *pt) > -{ > - __pagetable_free(pt); > -} > -#endif > /** > * pagetable_free - Free pagetables > * @pt: The page table descriptor > diff --git a/mm/pagewalk.c b/mm/pagewalk.c > index 3ae2586ff45b..c0be87580989 100644 > --- a/mm/pagewalk.c > +++ b/mm/pagewalk.c > @@ -620,7 +620,7 @@ int walk_page_range(struct mm_struct *mm, unsigned long start, > * Note: Be careful to walk the kernel pages tables, the caller may be need to > * take other effective approaches (mmap lock may be insufficient) to prevent > * the intermediate kernel page tables belonging to the specified address range > - * from being freed (e.g. memory hot-remove). > + * from being freed (e.g. memory hot-remove, vmap huge page promotion). > */ > int walk_kernel_page_table_range(unsigned long start, unsigned long end, > const struct mm_walk_ops *ops, pgd_t *pgd, void *private) > @@ -643,7 +643,7 @@ int walk_kernel_page_table_range(unsigned long start, unsigned long end, > * Use this function to walk the kernel page tables locklessly. It should be > * guaranteed that the caller has exclusive access over the range they are > * operating on - that there should be no concurrent access, for example, > - * changing permissions for vmalloc objects. > + * changing permissions for vmalloc objects, or vmap huge page promotion. > */ > int walk_kernel_page_table_range_lockless(unsigned long start, unsigned long end, > const struct mm_walk_ops *ops, pgd_t *pgd, void *private) > @@ -692,9 +692,13 @@ int walk_page_range_debug(struct mm_struct *mm, unsigned long start, > }; > > /* For convenience, we allow traversal of kernel mappings. */ > - if (mm == &init_mm) > - return walk_kernel_page_table_range(start, end, ops, > - pgd, private); > + if (mm == &init_mm) { > + RCU_LOCKDEP_WARN(!rcu_read_lock_held(), > + "RCU read lock must be held across kernel page table walk"); > + return walk_kernel_page_table_range(start, end, ops, pgd, > + private); > + } > + > if (start >= end || !walk.mm) > return -EINVAL; > if (!check_ops_safe(ops)) > diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c > index b91b1a98029c..7a32e4821957 100644 > --- a/mm/pgtable-generic.c > +++ b/mm/pgtable-generic.c > @@ -410,6 +410,13 @@ pte_t *pte_offset_map_lock(struct mm_struct *mm, pmd_t *pmd, > goto again; > } > > +static void kernel_pgtable_free_rcu(struct rcu_head *head) > +{ > + struct ptdesc *pt = container_of(head, struct ptdesc, pt_rcu_head); > + > + __pagetable_free(pt); > +} > + > #ifdef CONFIG_ASYNC_KERNEL_PGTABLE_FREE > static void kernel_pgtable_work_func(struct work_struct *work); > > @@ -434,8 +441,15 @@ static void kernel_pgtable_work_func(struct work_struct *work) > spin_unlock(&kernel_pgtable_work.lock); > > iommu_sva_invalidate_kva_range(PAGE_OFFSET, TLB_FLUSH_ALL); > + > + /* > + * Debug walkers (ptdump) may walk ranges they do not own and race this > + * free, so they walk under rcu_read_lock(). Free after a grace period: > + * a walker either already saw the cleared PMD, or keeps the page alive > + * until it drops the RCU lock. > + */ > list_for_each_entry_safe(pt, next, &page_list, pt_list) > - __pagetable_free(pt); > + call_rcu(&pt->pt_rcu_head, kernel_pgtable_free_rcu); > } > > void pagetable_free_kernel(struct ptdesc *pt) > @@ -446,4 +460,10 @@ void pagetable_free_kernel(struct ptdesc *pt) > > schedule_work(&kernel_pgtable_work.work); > } > +#else > +void pagetable_free_kernel(struct ptdesc *pt) > +{ > + /* Defer the free by a grace period; see kernel_pgtable_work_func(). */ > + call_rcu(&pt->pt_rcu_head, kernel_pgtable_free_rcu); > +} > #endif > diff --git a/mm/ptdump.c b/mm/ptdump.c > index 973020000096..50cd96a33dfd 100644 > --- a/mm/ptdump.c > +++ b/mm/ptdump.c > @@ -178,11 +178,13 @@ void ptdump_walk_pgd(struct ptdump_state *st, struct mm_struct *mm, pgd_t *pgd) > > get_online_mems(); > mmap_write_lock(mm); mmap write lock is not required anymore no? > + rcu_read_lock(); Is there a possibility of RCU stall here? For example a system with lots of physical memory will have a huge linear map. > while (range->start != range->end) { > walk_page_range_debug(mm, range->start, range->end, > &ptdump_ops, pgd, st); > range++; > } > + rcu_read_unlock(); > mmap_write_unlock(mm); > put_online_mems(); >