arena_vm_fault() allocated the page while holding arena->spinlock, so it could only use the non-blocking allocator. Once the memcg is at memory.max that allocation just fails, the fault turns into VM_FAULT_SIGSEGV, and the process gets a SIGSEGV on a perfectly valid arena address. Hitting memory.max is routine (e.g. page cache from reading a big file), so this kills innocent processes.
Preallocate the page before taking the lock, like do_anonymous_page() does, so the allocation can sleep and go through reclaim and the OOM path, and return VM_FAULT_OOM on failure so the memcg OOM handler runs instead of a fake segfault. Also tidy up the error labels. Signed-off-by: Jiayuan Chen <[email protected]> --- kernel/bpf/arena.c | 83 +++++++++++++++++++++++++++++++++++----------- 1 file changed, 63 insertions(+), 20 deletions(-) diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c index 34f023a537fe..22a41e3c53b8 100644 --- a/kernel/bpf/arena.c +++ b/kernel/bpf/arena.c @@ -481,7 +481,8 @@ static vm_fault_t arena_vm_fault(struct vm_fault *vmf) struct bpf_map *map = vmf->vma->vm_file->private_data; struct bpf_arena *arena = container_of(map, struct bpf_arena, map); struct mem_cgroup *new_memcg, *old_memcg; - struct page *page; + struct page *page, *new_page = NULL; + vm_fault_t fault_ret; long kbase, kaddr; unsigned long flags; int ret; @@ -489,55 +490,97 @@ static vm_fault_t arena_vm_fault(struct vm_fault *vmf) kbase = bpf_arena_get_kern_vm_start(arena); kaddr = kbase + (u32)(vmf->address); - if (raw_res_spin_lock_irqsave(&arena->spinlock, flags)) + page = vmalloc_to_page((void *)kaddr); + if (!page) { + /* + * Preallocate outside the lock so the allocation can sleep and go + * through reclaim (both memcg and global), the way do_anonymous_page() + * does. Under arena->spinlock only the non-blocking allocator is + * available, which never reclaims. + * + * This has to be the sleepable variant: VM_FAULT_OOM below is only + * meaningful if the OOM machinery was actually engaged. A failure + * from the non-blocking allocator engages nothing, so the fault + * would be retried forever. + */ + bpf_map_memcg_enter(&arena->map, &old_memcg, &new_memcg); + new_page = bpf_map_alloc_page_sleepable(map, NUMA_NO_NODE); + bpf_map_memcg_exit(old_memcg, new_memcg); + if (!new_page) + return VM_FAULT_OOM; + } + + if (raw_res_spin_lock_irqsave(&arena->spinlock, flags)) { /* Make a reasonable effort to address impossible case */ - return VM_FAULT_RETRY; + fault_ret = VM_FAULT_RETRY; + goto out_err; + } page = vmalloc_to_page((void *)kaddr); if (page) { - if (page == arena->scratch_page) + if (page == arena->scratch_page) { /* BPF triggered scratch here; don't lazy-alloc over it */ - goto out_sigsegv; + fault_ret = VM_FAULT_SIGSEGV; + goto out_err_locked; + } /* already have a page vmap-ed */ goto out; } + /* + * The lockless probe was racy: it saw a page, so nothing was + * preallocated, but the re-check under the lock finds it gone - a + * concurrent free must have run in between. There is nothing to + * install and we cannot allocate under the lock, so retry the fault + * and preallocate next time. + */ + if (!new_page) { + fault_ret = VM_FAULT_RETRY; + goto out_err_locked; + } + bpf_map_memcg_enter(&arena->map, &old_memcg, &new_memcg); - if (arena->map.map_flags & BPF_F_SEGV_ON_FAULT) + 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_sigsegv_memcg; + fault_ret = VM_FAULT_SIGSEGV; + goto out_err_locked_memcg; + } ret = range_tree_clear(&arena->rt, vmf->pgoff, 1); - if (ret) - goto out_sigsegv_memcg; - - struct apply_range_data data = { .arena = arena, .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_sigsegv_memcg; + fault_ret = VM_FAULT_OOM; + goto out_err_locked_memcg; } + struct apply_range_data data = { .arena = arena, .pages = &new_page, .i = 0 }; 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_sigsegv_memcg; + fault_ret = VM_FAULT_SIGSEGV; + goto out_err_locked_memcg; } flush_vmap_cache(kaddr, PAGE_SIZE); bpf_map_memcg_exit(old_memcg, new_memcg); + /* new_page was consumed */ + page = new_page; + new_page = NULL; out: page_ref_add(page, 1); raw_res_spin_unlock_irqrestore(&arena->spinlock, flags); + if (new_page) + free_pages_nolock(new_page, 0); vmf->page = page; return 0; -out_sigsegv_memcg: + +out_err_locked_memcg: bpf_map_memcg_exit(old_memcg, new_memcg); -out_sigsegv: +out_err_locked: raw_res_spin_unlock_irqrestore(&arena->spinlock, flags); - return VM_FAULT_SIGSEGV; +out_err: + if (new_page) + free_pages_nolock(new_page, 0); + return fault_ret; } static const struct vm_operations_struct arena_vm_ops = { -- 2.43.0
