// SPDX-License-Identifier: GPL-2.0 /* * Shadow Call Stack support. * * Copyright (C) 2019 Google LLC */ #include <linux/cpuhotplug.h> #include <linux/kasan.h> #include <linux/mm.h> #include <linux/scs.h> #include <linux/vmalloc.h> #include <linux/vmstat.h> static void __scs_account(void *s, int account) { struct page *scs_page = vmalloc_to_page(s); mod_node_page_state(page_pgdat(scs_page), NR_KERNEL_SCS_KB, account * (SCS_SIZE / SZ_1K)); } /* Matches NR_CACHED_STACKS for VMAP_STACK */ #define NR_CACHED_SCS 2 static DEFINE_PER_CPU(void *, scs_cache[NR_CACHED_SCS]); static void *__scs_alloc(int node) { int i; void *s; for (i = 0; i < NR_CACHED_SCS; i++) { s = this_cpu_xchg(scs_cache[i], NULL); if (s) { kasan_unpoison_vmalloc(s, SCS_SIZE); memset(s, 0, SCS_SIZE); return s; } } return __vmalloc_node_range(SCS_SIZE, 1, VMALLOC_START, VMALLOC_END, GFP_SCS, PAGE_KERNEL, 0, node, __builtin_return_address(0)); } void *scs_alloc(int node) { void *s; s = __scs_alloc(node); if (!s) return NULL; *__scs_magic(s) = SCS_END_MAGIC; /* * Poison the allocation to catch unintentional accesses to * the shadow stack when KASAN is enabled. */ kasan_poison_vmalloc(s, SCS_SIZE); __scs_account(s, 1); return s; } void scs_free(void *s) { int i; __scs_account(s, -1); /* * We cannot sleep as this can be called in interrupt context, * so use this_cpu_cmpxchg to update the cache, and vfree_atomic * to free the stack. */ for (i = 0; i < NR_CACHED_SCS; i++) if (this_cpu_cmpxchg(scs_cache[i], 0, s) == NULL) return; kasan_unpoison_vmalloc(s, SCS_SIZE); vfree_atomic(s); } static int scs_cleanup(unsigned int cpu) { int i; void **cache = per_cpu_ptr(scs_cache, cpu); for (i = 0; i < NR_CACHED_SCS; i++) { vfree(cache[i]); cache[i] = NULL; } return 0; } void __init scs_init(void) { cpuhp_setup_state(CPUHP_BP_PREPARE_DYN, "scs:scs_cache", NULL, scs_cleanup); } int scs_prepare(struct task_struct *tsk, int node) { void *s = scs_alloc(node); if (!s) return -ENOMEM; task_scs(tsk) = task_scs_sp(tsk) = s; return 0; } static void scs_check_usage(struct task_struct *tsk) { static unsigned long highest; unsigned long *p, prev, curr = highest, used = 0; if (!IS_ENABLED(CONFIG_DEBUG_STACK_USAGE)) return; for (p = task_scs(tsk); p < __scs_magic(tsk); ++p) { if (!READ_ONCE_NOCHECK(*p)) break; used += sizeof(*p); } while (used > curr) { prev = cmpxchg_relaxed(&highest, curr, used); if (prev == curr) { pr_info("%s (%d): highest shadow stack usage: %lu bytes\n", tsk->comm, task_pid_nr(tsk), used); break; } curr = prev; } } void scs_release(struct task_struct *tsk) { void *s = task_scs(tsk); if (!s) return; WARN(task_scs_end_corrupted(tsk), "corrupted shadow stack detected when freeing task\n"); scs_check_usage(tsk); scs_free(s); }