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[188.141.5.72]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-493e006c47dsm11447725e9.1.2026.07.06.13.31.30 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 06 Jul 2026 13:31:31 -0700 (PDT) From: David Carlier To: Andrew Morton Cc: Dev Jain , 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 , Madhavan Srinivasan , Michael Ellerman , Nicholas Piggin , Christophe Leroy , Ritesh Harjani , syzbot+fd95a72470f5a44e464c@syzkaller.appspotmail.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, David Carlier Subject: [PATCH v8] mm: pgtable: free kernel page tables via RCU to fix ptdump UAF Date: Mon, 6 Jul 2026 21:31:27 +0100 Message-ID: <20260706203128.162335-1-devnexen@gmail.com> X-Mailer: git-send-email 2.53.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit ptdump_walk_pgd() walks the kernel page tables under get_online_mems(). That does not stop 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. Commit 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. x86, arm64 and riscv reach pagetable_free_kernel() for the collapsed PTE page (the ptdesc carries PT_kernel), so the deferral covers them. powerpc uses its own fragment allocator: pte_free_kernel() there frees the page synchronously via pte_fragment_free() and the ptdesc is not flagged kernel, so defer the kernel case in pte_fragment_free() as well. arm64 also takes init_mm.mmap_lock around the free under a static key; that is now redundant with the RCU deferral but left in place. On the read side walk_page_range_debug() walks the init_mm range in bounded chunks, taking rcu_read_lock() around each chunk and calling cond_resched() between them. It uses the lockless walker as the mmap lock is no longer held, and pgd_addr_end() to bound each chunk without overflowing at the top of the address space. 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. Stop taking mmap_write_lock() in ptdump_walk_pgd() for init_mm. It never guarded against this free -- most architectures free the collapsed PTE table without it -- and RCU now provides the synchronisation. efi_mm and current->mm page tables are not RCU-freed, so they keep the mmap write lock. ptdump callbacks run under RCU within a chunk, so they must not sleep. The arch note_page() and effective_prot() callbacks only format into the preallocated seq_file buffer; the only GFP_KERNEL marker setup runs before the walk, and cond_resched() happens between chunks, outside the read lock. 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 --- v8: fix four issues raised by the Gemini review of v7 (relayed by Andrew). - the init_mm walk called walk_kernel_page_table_range(), which asserts init_mm.mmap_lock -- but that lock is now dropped; use the lockless walker instead. - only stop taking mmap_write_lock() for init_mm; efi_mm and current->mm are not RCU-freed and keep it. - bound each chunk with pgd_addr_end() so the last chunk does not overflow at the top of the address space. - powerpc frees kernel PTE pages synchronously via pte_fragment_free() and never sets PT_kernel, so it bypassed the RCU deferral; defer the kernel case there too. v7: no code change; add version tag and per-revision changelog (Dev). v6: chunk the init_mm walk in walk_page_range_debug() and take rcu_read_lock() per chunk (reverting v5's caller-side lock + assert) so the read section stays bounded on large kernel address spaces and can cond_resched() between chunks; drop the now-redundant mmap_write_lock() in ptdump_walk_pgd(). 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; add a walk_kernel_page_table_range_rcu() helper. v3: take rcu_read_lock() in the init_mm branch of walk_page_range_debug(). v2: use call_rcu() instead of synchronize_rcu(). arch/powerpc/mm/pgtable-frag.c | 7 ++++++- include/linux/mm.h | 7 ------- mm/pagewalk.c | 36 +++++++++++++++++++++++++++++----- mm/pgtable-generic.c | 22 ++++++++++++++++++++- mm/ptdump.c | 11 +++++++++-- 5 files changed, 67 insertions(+), 16 deletions(-) diff --git a/arch/powerpc/mm/pgtable-frag.c b/arch/powerpc/mm/pgtable-frag.c index ae742564a3d5..1e1e88f831f7 100644 --- a/arch/powerpc/mm/pgtable-frag.c +++ b/arch/powerpc/mm/pgtable-frag.c @@ -123,7 +123,12 @@ void pte_fragment_free(unsigned long *table, int kernel) BUG_ON(atomic_read(&ptdesc->pt_frag_refcount) <= 0); if (atomic_dec_and_test(&ptdesc->pt_frag_refcount)) { - if (kernel || !folio_test_clear_active(ptdesc_folio(ptdesc))) + /* + * Kernel page tables may be walked locklessly under RCU by + * ptdump, so defer their free by a grace period too, like the + * lockless-GUP case below for user tables. + */ + if (!kernel && !folio_test_clear_active(ptdesc_folio(ptdesc))) pte_free_now(&ptdesc->pt_rcu_head); else call_rcu(&ptdesc->pt_rcu_head, pte_free_now); 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..fc2fe014ac8c 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 +692,35 @@ 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) { + unsigned long addr = start; + + /* + * Walk in bounded chunks so the RCU read lock is never held + * across the whole kernel address space. A kernel page table + * freed via pagetable_free_kernel() stays valid until the walk + * that may have observed it drops the lock; releasing the lock + * between chunks is safe as no page table pointer is held + * across the gap. The mmap lock is not held, so use the + * lockless walker; RCU, not the lock, keeps the table alive. + */ + while (addr < end) { + unsigned long next = pgd_addr_end(addr, end); + int err; + + rcu_read_lock(); + err = walk_kernel_page_table_range_lockless(addr, next, ops, + pgd, private); + rcu_read_unlock(); + if (err) + return err; + + addr = next; + cond_resched(); + } + return 0; + } + 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..537be9995e1a 100644 --- a/mm/ptdump.c +++ b/mm/ptdump.c @@ -177,13 +177,20 @@ void ptdump_walk_pgd(struct ptdump_state *st, struct mm_struct *mm, pgd_t *pgd) const struct ptdump_range *range = st->range; get_online_mems(); - mmap_write_lock(mm); + /* + * init_mm is walked locklessly under RCU by walk_page_range_debug(). + * efi_mm / current->mm page tables are not RCU-freed, so hold the + * mmap write lock to keep them stable against concurrent teardown. + */ + if (mm != &init_mm) + mmap_write_lock(mm); while (range->start != range->end) { walk_page_range_debug(mm, range->start, range->end, &ptdump_ops, pgd, st); range++; } - mmap_write_unlock(mm); + if (mm != &init_mm) + mmap_write_unlock(mm); put_online_mems(); /* Flush out the last page */ -- 2.53.0