From: Kumar Kartikeya Dwivedi Date: Fri, 22 May 2026 17:22:13 +0000 (-1000) Subject: bpf: Recover arena kernel faults with scratch page X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=dc11a4dba2464e5144c318ffaf7fb16b1a5c74d6;p=thirdparty%2Flinux.git bpf: Recover arena kernel faults with scratch page BPF arena usage is becoming more prevalent, but kernel <-> BPF communication over arena memory is awkward today. Data has to be staged through a trusted kernel pointer with extra code and copying on the BPF side. While reads through arena pointers can use a fault-safe helper, writes don't have a good solution. The in-line alternative would need instruction emulation or asm fixup labels. Enable direct kernel-side reads and writes within GUARD_SZ / 2 of any handed-in arena pointer, without bounds checking. A per-arena scratch page is installed by the arch fault path into empty arena kernel PTEs - x86 from page_fault_oops() for not-present faults, arm64 from __do_kernel_fault() for translation faults, both after the existing exception-table and KFENCE handling. The faulting instruction retries and the access is also reported through the program's BPF stream, preserving error reporting. bpf_prog_find_from_stack() resolves the current BPF program (and its arena) from the kernel stack - no new bpf_run_ctx state is added. Recovery covers the 4 GiB arena plus the upper half-guard (GUARD_SZ / 2). The lower half-guard is excluded because well-behaved kfuncs only access forward from arena pointers. The kfunc-author contract - access at most GUARD_SZ / 2 past a handed-in pointer - is documented in Documentation/bpf/kfuncs.rst. The install is lock-free via ptep_try_set(). On race-loss the winning installer's PTE is already valid, so the access retry succeeds. The arena clear path uses ptep_get_and_clear() so installer and clearer race through atomic accessors. No flush_tlb_kernel_range() afterwards. Stale "not mapped" entries just cause one extra re-fault, cheaper than a global IPI on every install. Scratch exists only to keep the kernel from oopsing on an in-line arena access. Its presence at a PTE means the BPF program has already malfunctioned, and the violation is reported through the program's BPF stream. The only requirement for behavior on a scratched PTE is that the kernel doesn't crash. In particular, any user-side access through such a PTE may segfault. The shared scratch page is freed once during map destruction. BPF instruction faults continue to use the existing JIT exception-table path. This patch changes only the kernel-text fault path. No UAPI flag is added. The new behavior is the default. v2: Use ptep_get_and_clear() in apply_range_clear_cb(). (David) v3: Stub bpf_arena_handle_page_fault() for !CONFIG_BPF_SYSCALL. (lkp) Suggested-by: Alexei Starovoitov Signed-off-by: Kumar Kartikeya Dwivedi Signed-off-by: Tejun Heo Reviewed-by: Emil Tsalapatis Cc: David Hildenbrand Link: https://lore.kernel.org/r/20260522172219.1423324-3-tj@kernel.org Signed-off-by: Alexei Starovoitov --- diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst index 75e6c078e0e72..6d497e7209984 100644 --- a/Documentation/bpf/kfuncs.rst +++ b/Documentation/bpf/kfuncs.rst @@ -462,6 +462,20 @@ In order to accommodate such requirements, the verifier will enforce strict PTR_TO_BTF_ID type matching if two types have the exact same name, with one being suffixed with ``___init``. +2.8 Accessing arena memory through kfunc arguments +-------------------------------------------------- + +A read or write at any address inside an arena does not oops the kernel. +Unallocated arena pages are lazily backed by a scratch page and the +access is reported through the program's BPF stream as an error. Only +the BPF program's correctness is affected; the kernel itself remains +intact. + +The arena is followed by a ``GUARD_SZ / 2`` (32 KiB) guard region that +is also covered by this recovery. A kfunc handed an arena pointer may +therefore access up to ``GUARD_SZ / 2`` past it without bounds-checking +against the arena. Larger accesses must verify the range explicitly. + .. _BPF_kfunc_lifecycle_expectations: 3. kfunc lifecycle expectations diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c index 0f3c5c7ca0547..b4290d16ff92b 100644 --- a/arch/arm64/mm/fault.c +++ b/arch/arm64/mm/fault.c @@ -9,6 +9,7 @@ #include #include +#include #include #include #include @@ -436,9 +437,12 @@ static void __do_kernel_fault(unsigned long addr, unsigned long esr, } else if (is_pkvm_stage2_abort(esr)) { msg = "access to hypervisor-protected memory"; } else { - if (esr_fsc_is_translation_fault(esr) && - kfence_handle_page_fault(addr, esr & ESR_ELx_WNR, regs)) - return; + if (esr_fsc_is_translation_fault(esr)) { + if (kfence_handle_page_fault(addr, esr & ESR_ELx_WNR, regs)) + return; + if (bpf_arena_handle_page_fault(addr, esr & ESR_ELx_WNR, regs->pc)) + return; + } msg = "paging request"; } diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c index 63de8e8684f23..7ea6a9362173a 100644 --- a/arch/x86/mm/fault.c +++ b/arch/x86/mm/fault.c @@ -8,6 +8,7 @@ #include /* task_stack_*(), ... */ #include /* oops_begin/end, ... */ #include /* max_low_pfn */ +#include /* bpf_arena_handle_page_fault */ #include /* kfence_handle_page_fault */ #include /* NOKPROBE_SYMBOL, ... */ #include /* kmmio_handler, ... */ @@ -688,10 +689,13 @@ page_fault_oops(struct pt_regs *regs, unsigned long error_code, if (IS_ENABLED(CONFIG_EFI)) efi_crash_gracefully_on_page_fault(address, regs); - /* Only not-present faults should be handled by KFENCE. */ - if (!(error_code & X86_PF_PROT) && - kfence_handle_page_fault(address, error_code & X86_PF_WRITE, regs)) - return; + /* Only not-present faults should be handled by KFENCE or BPF arena. */ + if (!(error_code & X86_PF_PROT)) { + if (kfence_handle_page_fault(address, error_code & X86_PF_WRITE, regs)) + return; + if (bpf_arena_handle_page_fault(address, error_code & X86_PF_WRITE, regs->ip)) + return; + } oops: /* diff --git a/include/linux/bpf.h b/include/linux/bpf.h index 01e2039648928..bb4261a5df649 100644 --- a/include/linux/bpf.h +++ b/include/linux/bpf.h @@ -6,6 +6,7 @@ #include #include +#include #include #include diff --git a/include/linux/bpf_defs.h b/include/linux/bpf_defs.h new file mode 100644 index 0000000000000..2185cd3966d43 --- /dev/null +++ b/include/linux/bpf_defs.h @@ -0,0 +1,19 @@ +/* SPDX-License-Identifier: GPL-2.0-or-later */ +/* + * Subset of bpf.h declarations, split out so files that need only these + * declarations can avoid bpf.h's full include cost. + */ +#ifndef _LINUX_BPF_DEFS_H +#define _LINUX_BPF_DEFS_H + +#ifdef CONFIG_BPF_SYSCALL +bool bpf_arena_handle_page_fault(unsigned long addr, bool is_write, unsigned long fault_ip); +#else +static inline bool bpf_arena_handle_page_fault(unsigned long addr, bool is_write, + unsigned long fault_ip) +{ + return false; +} +#endif + +#endif /* _LINUX_BPF_DEFS_H */ diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c index 49a8f7b1beef5..2da2c275cff67 100644 --- a/kernel/bpf/arena.c +++ b/kernel/bpf/arena.c @@ -53,6 +53,7 @@ struct bpf_arena { u64 user_vm_start; u64 user_vm_end; struct vm_struct *kern_vm; + struct page *scratch_page; struct range_tree rt; /* protects rt */ rqspinlock_t spinlock; @@ -118,6 +119,11 @@ struct apply_range_data { int i; }; +struct clear_range_data { + struct llist_head *free_pages; + struct page *scratch_page; +}; + static int apply_range_set_cb(pte_t *pte, unsigned long addr, void *data) { struct apply_range_data *d = data; @@ -144,33 +150,59 @@ static void flush_vmap_cache(unsigned long start, unsigned long size) flush_cache_vmap(start, start + size); } -static int apply_range_clear_cb(pte_t *pte, unsigned long addr, void *free_pages) +static int apply_range_clear_cb(pte_t *pte, unsigned long addr, void *data) { + struct clear_range_data *d = data; pte_t old_pte; struct page *page; - /* sanity check */ - old_pte = ptep_get(pte); + /* + * Pairs with ptep_try_set() in the kernel-fault scratch installer. + * Both sides must be atomic. + */ + old_pte = ptep_get_and_clear(&init_mm, addr, pte); if (pte_none(old_pte) || !pte_present(old_pte)) - return 0; /* nothing to do */ + return 0; page = pte_page(old_pte); if (WARN_ON_ONCE(!page)) return -EINVAL; - pte_clear(&init_mm, addr, pte); + /* + * Skip the per-arena scratch page. A kernel fault on an unallocated uaddr + * scratches its PTE. A later bpf_arena_free_pages() over that range walks + * here. Without the skip, scratch_page would be freed. + */ + if (page == d->scratch_page) + return 0; + + __llist_add(&page->pcp_llist, d->free_pages); + return 0; +} - /* Add page to the list so it is freed later */ - if (free_pages) - __llist_add(&page->pcp_llist, free_pages); +static int apply_range_set_scratch_cb(pte_t *pte, unsigned long addr, void *data) +{ + struct page *scratch_page = data; + if (!pte_none(ptep_get(pte))) + return 0; + /* + * Best-effort install. ptep_try_set() returns false only if another + * installer (real allocation or concurrent fault) won the cmpxchg. + * Their PTE is already valid, so the access retry succeeds. + * + * No flush_tlb_kernel_range() needed. Stale "not mapped" entries just + * cause one extra re-fault through this same path. + */ + ptep_try_set(pte, mk_pte(scratch_page, PAGE_KERNEL)); return 0; } static int populate_pgtable_except_pte(struct bpf_arena *arena) { + /* Populate intermediates for the recovery range (4 GiB + upper half-guard). */ return apply_to_page_range(&init_mm, bpf_arena_get_kern_vm_start(arena), - KERN_VM_SZ - GUARD_SZ, apply_range_set_cb, NULL); + SZ_4G + GUARD_SZ / 2, apply_range_set_cb, NULL); } static struct bpf_map *arena_map_alloc(union bpf_attr *attr) @@ -221,22 +253,29 @@ static struct bpf_map *arena_map_alloc(union bpf_attr *attr) init_irq_work(&arena->free_irq, arena_free_irq); INIT_WORK(&arena->free_work, arena_free_worker); bpf_map_init_from_attr(&arena->map, attr); + + err = bpf_map_alloc_pages(&arena->map, NUMA_NO_NODE, 1, &arena->scratch_page); + if (err) + goto err_free_arena; + range_tree_init(&arena->rt); err = range_tree_set(&arena->rt, 0, attr->max_entries); - if (err) { - bpf_map_area_free(arena); - goto err; - } + if (err) + goto err_free_scratch; mutex_init(&arena->lock); raw_res_spin_lock_init(&arena->spinlock); err = populate_pgtable_except_pte(arena); - if (err) { - range_tree_destroy(&arena->rt); - bpf_map_area_free(arena); - goto err; - } + if (err) + goto err_destroy_rt; return &arena->map; + +err_destroy_rt: + range_tree_destroy(&arena->rt); +err_free_scratch: + __free_page(arena->scratch_page); +err_free_arena: + bpf_map_area_free(arena); err: free_vm_area(kern_vm); return ERR_PTR(err); @@ -244,6 +283,7 @@ err: static int existing_page_cb(pte_t *ptep, unsigned long addr, void *data) { + struct bpf_arena *arena = data; struct page *page; pte_t pte; @@ -251,6 +291,12 @@ static int existing_page_cb(pte_t *ptep, unsigned long addr, void *data) if (!pte_present(pte)) /* sanity check */ return 0; page = pte_page(pte); + /* + * Skip the scratch page. The walk is page-table-driven, not range-tree-driven, + * so it can visit scratch PTEs at uaddrs the BPF program never allocated. + */ + if (page == arena->scratch_page) + return 0; /* * We do not update pte here: * 1. Nobody should be accessing bpf_arena's range outside of a kernel bug @@ -286,9 +332,10 @@ static void arena_map_free(struct bpf_map *map) * free those pages. */ apply_to_existing_page_range(&init_mm, bpf_arena_get_kern_vm_start(arena), - KERN_VM_SZ - GUARD_SZ, existing_page_cb, NULL); + SZ_4G + GUARD_SZ / 2, existing_page_cb, arena); free_vm_area(arena->kern_vm); range_tree_destroy(&arena->rt); + __free_page(arena->scratch_page); bpf_map_area_free(arena); } @@ -384,33 +431,37 @@ static vm_fault_t arena_vm_fault(struct vm_fault *vmf) return VM_FAULT_RETRY; page = vmalloc_to_page((void *)kaddr); - if (page) + if (page) { + if (page == arena->scratch_page) + /* BPF triggered scratch here; don't lazy-alloc over it */ + goto out_sigsegv; /* already have a page vmap-ed */ goto out; + } bpf_map_memcg_enter(&arena->map, &old_memcg, &new_memcg); if (arena->map.map_flags & BPF_F_SEGV_ON_FAULT) /* User space requested to segfault when page is not allocated by bpf prog */ - goto out_unlock_sigsegv; + goto out_sigsegv_memcg; ret = range_tree_clear(&arena->rt, vmf->pgoff, 1); if (ret) - goto out_unlock_sigsegv; + goto out_sigsegv_memcg; struct apply_range_data data = { .pages = &page, .i = 0 }; /* Account into memcg of the process that created bpf_arena */ ret = bpf_map_alloc_pages(map, NUMA_NO_NODE, 1, &page); if (ret) { range_tree_set(&arena->rt, vmf->pgoff, 1); - goto out_unlock_sigsegv; + goto out_sigsegv_memcg; } ret = apply_to_page_range(&init_mm, kaddr, PAGE_SIZE, apply_range_set_cb, &data); if (ret) { range_tree_set(&arena->rt, vmf->pgoff, 1); free_pages_nolock(page, 0); - goto out_unlock_sigsegv; + goto out_sigsegv_memcg; } flush_vmap_cache(kaddr, PAGE_SIZE); bpf_map_memcg_exit(old_memcg, new_memcg); @@ -419,8 +470,9 @@ out: raw_res_spin_unlock_irqrestore(&arena->spinlock, flags); vmf->page = page; return 0; -out_unlock_sigsegv: +out_sigsegv_memcg: bpf_map_memcg_exit(old_memcg, new_memcg); +out_sigsegv: raw_res_spin_unlock_irqrestore(&arena->spinlock, flags); return VM_FAULT_SIGSEGV; } @@ -685,6 +737,7 @@ static void arena_free_pages(struct bpf_arena *arena, long uaddr, long page_cnt, struct llist_head free_pages; struct llist_node *pos, *t; struct arena_free_span *s; + struct clear_range_data cdata; unsigned long flags; int ret = 0; @@ -713,9 +766,11 @@ static void arena_free_pages(struct bpf_arena *arena, long uaddr, long page_cnt, range_tree_set(&arena->rt, pgoff, page_cnt); init_llist_head(&free_pages); + cdata.free_pages = &free_pages; + cdata.scratch_page = arena->scratch_page; /* clear ptes and collect struct pages */ apply_to_existing_page_range(&init_mm, kaddr, page_cnt << PAGE_SHIFT, - apply_range_clear_cb, &free_pages); + apply_range_clear_cb, &cdata); /* drop the lock to do the tlb flush and zap pages */ raw_res_spin_unlock_irqrestore(&arena->spinlock, flags); @@ -805,6 +860,7 @@ static void arena_free_worker(struct work_struct *work) struct arena_free_span *s; u64 arena_vm_start, user_vm_start; struct llist_head free_pages; + struct clear_range_data cdata; struct page *page; unsigned long full_uaddr; long kaddr, page_cnt, pgoff; @@ -818,6 +874,8 @@ static void arena_free_worker(struct work_struct *work) bpf_map_memcg_enter(&arena->map, &old_memcg, &new_memcg); init_llist_head(&free_pages); + cdata.free_pages = &free_pages; + cdata.scratch_page = arena->scratch_page; arena_vm_start = bpf_arena_get_kern_vm_start(arena); user_vm_start = bpf_arena_get_user_vm_start(arena); @@ -830,7 +888,7 @@ static void arena_free_worker(struct work_struct *work) /* clear ptes and collect pages in free_pages llist */ apply_to_existing_page_range(&init_mm, kaddr, page_cnt << PAGE_SHIFT, - apply_range_clear_cb, &free_pages); + apply_range_clear_cb, &cdata); range_tree_set(&arena->rt, pgoff, page_cnt); } @@ -945,23 +1003,12 @@ static int __init kfunc_init(void) } late_initcall(kfunc_init); -void bpf_prog_report_arena_violation(bool write, unsigned long addr, unsigned long fault_ip) +static void __bpf_prog_report_arena_violation(struct bpf_prog *prog, bool write, + unsigned long addr, unsigned long fault_ip) { struct bpf_stream_stage ss; - struct bpf_prog *prog; u64 user_vm_start; - /* - * The RCU read lock is held to safely traverse the latch tree, but we - * don't need its protection when accessing the prog, since it will not - * disappear while we are handling the fault. - */ - rcu_read_lock(); - prog = bpf_prog_ksym_find(fault_ip); - rcu_read_unlock(); - if (!prog) - return; - /* Use main prog for stream access */ prog = prog->aux->main_prog_aux->prog; @@ -974,3 +1021,53 @@ void bpf_prog_report_arena_violation(bool write, unsigned long addr, unsigned lo bpf_stream_dump_stack(ss); })); } + +bool bpf_arena_handle_page_fault(unsigned long addr, bool is_write, unsigned long fault_ip) +{ + struct bpf_arena *arena; + struct bpf_prog *prog; + unsigned long kbase; + unsigned long page_addr = addr & PAGE_MASK; + + prog = bpf_prog_find_from_stack(); + if (!prog) + return false; + + arena = prog->aux->arena; + /* a prog not using arena may be on stack, so arena can be NULL */ + if (!arena) + return false; + + kbase = bpf_arena_get_kern_vm_start(arena); + + /* + * Recovery covers the 4 GiB mappable band plus the upper half-guard. + * Lower guard is unreachable from kfuncs; an address there indicates + * a different bug class - leave it to the regular kernel oops path. + */ + if (page_addr < kbase || page_addr >= kbase + SZ_4G + GUARD_SZ / 2) + return false; + + apply_to_page_range(&init_mm, page_addr, PAGE_SIZE, + apply_range_set_scratch_cb, arena->scratch_page); + flush_vmap_cache(page_addr, PAGE_SIZE); + __bpf_prog_report_arena_violation(prog, is_write, page_addr - kbase, fault_ip); + return true; +} + +void bpf_prog_report_arena_violation(bool write, unsigned long addr, unsigned long fault_ip) +{ + struct bpf_prog *prog; + + /* + * The RCU read lock is held to safely traverse the latch tree, but we + * don't need its protection when accessing the prog, since it will not + * disappear while we are handling the fault. + */ + rcu_read_lock(); + prog = bpf_prog_ksym_find(fault_ip); + rcu_read_unlock(); + if (!prog) + return; + __bpf_prog_report_arena_violation(prog, write, addr, fault_ip); +} diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c index 8b018ff488750..fc3ee67486cec 100644 --- a/kernel/bpf/core.c +++ b/kernel/bpf/core.c @@ -3350,6 +3350,11 @@ __weak u64 bpf_arena_get_kern_vm_start(struct bpf_arena *arena) { return 0; } +__weak bool bpf_arena_handle_page_fault(unsigned long addr, bool is_write, + unsigned long fault_ip) +{ + return false; +} #ifdef CONFIG_BPF_SYSCALL static int __init bpf_global_ma_init(void)