Merge branch 'bpf-support-private-stack-for-bpf-progs'

Yonghong Song says:

====================
bpf: Support private stack for bpf progs

The main motivation for private stack comes from nested scheduler in
sched-ext from Tejun. The basic idea is that
 - each cgroup will its own associated bpf program,
 - bpf program with parent cgroup will call bpf programs
   in immediate child cgroups.

Let us say we have the following cgroup hierarchy:
  root_cg (prog0):
    cg1 (prog1):
      cg11 (prog11):
        cg111 (prog111)
        cg112 (prog112)
      cg12 (prog12):
        cg121 (prog121)
        cg122 (prog122)
    cg2 (prog2):
      cg21 (prog21)
      cg22 (prog22)
      cg23 (prog23)

In the above example, prog0 will call a kfunc which will call prog1 and
prog2 to get sched info for cg1 and cg2 and then the information is
summarized and sent back to prog0. Similarly, prog11 and prog12 will be
invoked in the kfunc and the result will be summarized and sent back to
prog1, etc. The following illustrates a possible call sequence:
   ... -> bpf prog A -> kfunc -> ops.<callback_fn> (bpf prog B) ...

Currently, for each thread, the x86 kernel allocate 16KB stack. Each
bpf program (including its subprograms) has maximum 512B stack size to
avoid potential stack overflow. Nested bpf programs further increase the
risk of stack overflow. To avoid potential stack overflow caused by bpf
programs, this patch set supported private stack and bpf program stack
space is allocated during jit time. Using private stack for bpf progs
can reduce or avoid potential kernel stack overflow.

Currently private stack is applied to tracing programs like kprobe/uprobe,
perf_event, tracepoint, raw tracepoint and struct_ops progs.
Tracing progs enable private stack if any subprog stack size is more
than a threshold (i.e. 64B). Struct-ops progs enable private stack
based on particular struct op implementation which can enable private
stack before verification at per-insn level. Struct-ops progs have
the same treatment as tracing progs w.r.t when to enable private stack.

For all these progs, the kernel will do recursion check (no nesting for
per prog per cpu) to ensure that private stack won't be overwritten.
The bpf_prog_aux struct has a callback func recursion_detected() which
can be implemented by kernel subsystem to synchronously detect recursion,
report error, etc.

Only x86_64 arch supports private stack now. It can be extended to other
archs later. Please see each individual patch for details.

Change logs:
  v11 -> v12:
    - v11 link: https://lore.kernel.org/bpf/[email protected]/
    - Fix a bug where allocated percpu space is less than actual private stack.
    - Add guard memory (before and after actual prog stack) to detect potential
      underflow/overflow.
  v10 -> v11:
    - v10 link: https://lore.kernel.org/bpf/[email protected]/
    - Use two bool variables, priv_stack_requested (used by struct-ops only) and
      jits_use_priv_stack, in order to make code cleaner.
    - Set env->prog->aux->jits_use_priv_stack to true if any subprog uses private stack.
      This is for struct-ops use case to kick in recursion protection.
  v9 -> v10:
    - v9 link: https://lore.kernel.org/bpf/[email protected]/
    - Simplify handling async cbs by making those async cb related progs using normal
      kernel stack.
    - Do percpu allocation in jit instead of verifier.
  v8 -> v9:
    - v8 link: https://lore.kernel.org/bpf/[email protected]/
    - Use enum to express priv stack mode.
    - Use bits in bpf_subprog_info struct to do subprog recursion check between
      main/async and async subprogs.
    - Fix potential memory leak.
    - Rename recursion detection func from recursion_skipped() to recursion_detected().
  v7 -> v8:
    - v7 link: https://lore.kernel.org/bpf/[email protected]/
    - Add recursion_skipped() callback func to bpf_prog->aux structure such that if
      a recursion miss happened and bpf_prog->aux->recursion_skipped is not NULL, the
      callback fn will be called so the subsystem can do proper action based on their
      respective design.
  v6 -> v7:
    - v6 link: https://lore.kernel.org/bpf/[email protected]/
    - Going back to do private stack allocation per prog instead per subtree. This can
      simplify implementation and avoid verifier complexity.
    - Handle potential nested subprog run if async callback exists.
    - Use struct_ops->check_member() callback to set whether a particular struct-ops
      prog wants private stack or not.
  v5 -> v6:
    - v5 link: https://lore.kernel.org/bpf/[email protected]/
    - Instead of using (or not using) private stack at struct_ops level,
      each prog in struct_ops can decide whether to use private stack or not.
  v4 -> v5:
    - v4 link: https://lore.kernel.org/bpf/[email protected]/
    - Remove bpf_prog_call() related implementation.
    - Allow (opt-in) private stack for sched-ext progs.
  v3 -> v4:
    - v3 link: https://lore.kernel.org/bpf/[email protected]/
      There is a long discussion in the above v3 link trying to allow private
      stack to be used by kernel functions in order to simplify implementation.
      But unfortunately we didn't find a workable solution yet, so we return
      to the approach where private stack is only used by bpf programs.
    - Add bpf_prog_call() kfunc.
  v2 -> v3:
    - Instead of per-subprog private stack allocation, allocate private
      stacks at main prog or callback entry prog. Subprogs not main or callback
      progs will increment the inherited stack pointer to be their
      frame pointer.
    - Private stack allows each prog max stack size to be 512 bytes, intead
      of the whole prog hierarchy to be 512 bytes.
    - Add some tests.
====================

Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: Alexei Starovoitov

arch/x86/net/bpf_jit_comp.c

@@ -325,6 +325,22 @@ struct jit_context {
 /* Number of bytes that will be skipped on tailcall */
 #define X86_TAIL_CALL_OFFSET	(12 + ENDBR_INSN_SIZE)
 
+static void push_r9(u8 **pprog)
+{
+	u8 *prog = *pprog;
+
+	EMIT2(0x41, 0x51);   /* push r9 */
+	*pprog = prog;
+}
+
+static void pop_r9(u8 **pprog)
+{
+	u8 *prog = *pprog;
+
+	EMIT2(0x41, 0x59);   /* pop r9 */
+	*pprog = prog;
+}
+
 static void push_r12(u8 **pprog)
 {
 	u8 *prog = *pprog;
@@ -1404,6 +1420,24 @@ static void emit_shiftx(u8 **pprog, u32 dst_reg, u8 src_reg, bool is64, u8 op)
 	*pprog = prog;
 }
 
+static void emit_priv_frame_ptr(u8 **pprog, void __percpu *priv_frame_ptr)
+{
+	u8 *prog = *pprog;
+
+	/* movabs r9, priv_frame_ptr */
+	emit_mov_imm64(&prog, X86_REG_R9, (__force long) priv_frame_ptr >> 32,
+		       (u32) (__force long) priv_frame_ptr);
+
+#ifdef CONFIG_SMP
+	/* add <r9>, gs:[<off>] */
+	EMIT2(0x65, 0x4c);
+	EMIT3(0x03, 0x0c, 0x25);
+	EMIT((u32)(unsigned long)&this_cpu_off, 4);
+#endif
+
+	*pprog = prog;
+}
+
 #define INSN_SZ_DIFF (((addrs[i] - addrs[i - 1]) - (prog - temp)))
 
 #define __LOAD_TCC_PTR(off)			\
@@ -1412,6 +1446,10 @@ static void emit_shiftx(u8 **pprog, u32 dst_reg, u8 src_reg, bool is64, u8 op)
 #define LOAD_TAIL_CALL_CNT_PTR(stack)				\
 	__LOAD_TCC_PTR(BPF_TAIL_CALL_CNT_PTR_STACK_OFF(stack))
 
+/* Memory size/value to protect private stack overflow/underflow */
+#define PRIV_STACK_GUARD_SZ    8
+#define PRIV_STACK_GUARD_VAL   0xEB9F12345678eb9fULL
+
 static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, u8 *rw_image,
 		  int oldproglen, struct jit_context *ctx, bool jmp_padding)
 {
@@ -1421,18 +1459,28 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, u8 *rw_image
 	int insn_cnt = bpf_prog->len;
 	bool seen_exit = false;
 	u8 temp[BPF_MAX_INSN_SIZE + BPF_INSN_SAFETY];
+	void __percpu *priv_frame_ptr = NULL;
 	u64 arena_vm_start, user_vm_start;
+	void __percpu *priv_stack_ptr;
 	int i, excnt = 0;
 	int ilen, proglen = 0;
 	u8 *prog = temp;
+	u32 stack_depth;
 	int err;
 
+	stack_depth = bpf_prog->aux->stack_depth;
+	priv_stack_ptr = bpf_prog->aux->priv_stack_ptr;
+	if (priv_stack_ptr) {
+		priv_frame_ptr = priv_stack_ptr + PRIV_STACK_GUARD_SZ + round_up(stack_depth, 8);
+		stack_depth = 0;
+	}
+
 	arena_vm_start = bpf_arena_get_kern_vm_start(bpf_prog->aux->arena);
 	user_vm_start = bpf_arena_get_user_vm_start(bpf_prog->aux->arena);
 
 	detect_reg_usage(insn, insn_cnt, callee_regs_used);
 
-	emit_prologue(&prog, bpf_prog->aux->stack_depth,
+	emit_prologue(&prog, stack_depth,
 		      bpf_prog_was_classic(bpf_prog), tail_call_reachable,
 		      bpf_is_subprog(bpf_prog), bpf_prog->aux->exception_cb);
 	/* Exception callback will clobber callee regs for its own use, and
@@ -1454,6 +1502,9 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, u8 *rw_image
 		emit_mov_imm64(&prog, X86_REG_R12,
 			       arena_vm_start >> 32, (u32) arena_vm_start);
 
+	if (priv_frame_ptr)
+		emit_priv_frame_ptr(&prog, priv_frame_ptr);
+
 	ilen = prog - temp;
 	if (rw_image)
 		memcpy(rw_image + proglen, temp, ilen);
@@ -1473,6 +1524,14 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, u8 *rw_image
 		u8 *func;
 		int nops;
 
+		if (priv_frame_ptr) {
+			if (src_reg == BPF_REG_FP)
+				src_reg = X86_REG_R9;
+
+			if (dst_reg == BPF_REG_FP)
+				dst_reg = X86_REG_R9;
+		}
+
 		switch (insn->code) {
 			/* ALU */
 		case BPF_ALU | BPF_ADD | BPF_X:
@@ -2128,14 +2187,20 @@ st:			if (is_imm8(insn->off))
 
 			func = (u8 *) __bpf_call_base + imm32;
 			if (tail_call_reachable) {
-				LOAD_TAIL_CALL_CNT_PTR(bpf_prog->aux->stack_depth);
+				LOAD_TAIL_CALL_CNT_PTR(stack_depth);
 				ip += 7;
 			}
 			if (!imm32)
 				return -EINVAL;
+			if (priv_frame_ptr) {
+				push_r9(&prog);
+				ip += 2;
+			}
 			ip += x86_call_depth_emit_accounting(&prog, func, ip);
 			if (emit_call(&prog, func, ip))
 				return -EINVAL;
+			if (priv_frame_ptr)
+				pop_r9(&prog);
 			break;
 		}
 
@@ -2145,13 +2210,13 @@ st:			if (is_imm8(insn->off))
 							  &bpf_prog->aux->poke_tab[imm32 - 1],
 							  &prog, image + addrs[i - 1],
 							  callee_regs_used,
-							  bpf_prog->aux->stack_depth,
+							  stack_depth,
 							  ctx);
 			else
 				emit_bpf_tail_call_indirect(bpf_prog,
 							    &prog,
 							    callee_regs_used,
-							    bpf_prog->aux->stack_depth,
+							    stack_depth,
 							    image + addrs[i - 1],
 							    ctx);
 			break;
@@ -3303,6 +3368,42 @@ int arch_prepare_bpf_dispatcher(void *image, void *buf, s64 *funcs, int num_func
 	return emit_bpf_dispatcher(&prog, 0, num_funcs - 1, funcs, image, buf);
 }
 
+static const char *bpf_get_prog_name(struct bpf_prog *prog)
+{
+	if (prog->aux->ksym.prog)
+		return prog->aux->ksym.name;
+	return prog->aux->name;
+}
+
+static void priv_stack_init_guard(void __percpu *priv_stack_ptr, int alloc_size)
+{
+	int cpu, underflow_idx = (alloc_size - PRIV_STACK_GUARD_SZ) >> 3;
+	u64 *stack_ptr;
+
+	for_each_possible_cpu(cpu) {
+		stack_ptr = per_cpu_ptr(priv_stack_ptr, cpu);
+		stack_ptr[0] = PRIV_STACK_GUARD_VAL;
+		stack_ptr[underflow_idx] = PRIV_STACK_GUARD_VAL;
+	}
+}
+
+static void priv_stack_check_guard(void __percpu *priv_stack_ptr, int alloc_size,
+				   struct bpf_prog *prog)
+{
+	int cpu, underflow_idx = (alloc_size - PRIV_STACK_GUARD_SZ) >> 3;
+	u64 *stack_ptr;
+
+	for_each_possible_cpu(cpu) {
+		stack_ptr = per_cpu_ptr(priv_stack_ptr, cpu);
+		if (stack_ptr[0] != PRIV_STACK_GUARD_VAL ||
+		    stack_ptr[underflow_idx] != PRIV_STACK_GUARD_VAL) {
+			pr_err("BPF private stack overflow/underflow detected for prog %sx\n",
+			       bpf_get_prog_name(prog));
+			break;
+		}
+	}
+}
+
 struct x64_jit_data {
 	struct bpf_binary_header *rw_header;
 	struct bpf_binary_header *header;
@@ -3320,7 +3421,9 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 	struct bpf_binary_header *rw_header = NULL;
 	struct bpf_binary_header *header = NULL;
 	struct bpf_prog *tmp, *orig_prog = prog;
+	void __percpu *priv_stack_ptr = NULL;
 	struct x64_jit_data *jit_data;
+	int priv_stack_alloc_sz;
 	int proglen, oldproglen = 0;
 	struct jit_context ctx = {};
 	bool tmp_blinded = false;
@@ -3356,6 +3459,23 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 		}
 		prog->aux->jit_data = jit_data;
 	}
+	priv_stack_ptr = prog->aux->priv_stack_ptr;
+	if (!priv_stack_ptr && prog->aux->jits_use_priv_stack) {
+		/* Allocate actual private stack size with verifier-calculated
+		 * stack size plus two memory guards to protect overflow and
+		 * underflow.
+		 */
+		priv_stack_alloc_sz = round_up(prog->aux->stack_depth, 8) +
+				      2 * PRIV_STACK_GUARD_SZ;
+		priv_stack_ptr = __alloc_percpu_gfp(priv_stack_alloc_sz, 8, GFP_KERNEL);
+		if (!priv_stack_ptr) {
+			prog = orig_prog;
+			goto out_priv_stack;
+		}
+
+		priv_stack_init_guard(priv_stack_ptr, priv_stack_alloc_sz);
+		prog->aux->priv_stack_ptr = priv_stack_ptr;
+	}
 	addrs = jit_data->addrs;
 	if (addrs) {
 		ctx = jit_data->ctx;
@@ -3491,6 +3611,11 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 			bpf_prog_fill_jited_linfo(prog, addrs + 1);
 out_addrs:
 		kvfree(addrs);
+		if (!image && priv_stack_ptr) {
+			free_percpu(priv_stack_ptr);
+			prog->aux->priv_stack_ptr = NULL;
+		}
+out_priv_stack:
 		kfree(jit_data);
 		prog->aux->jit_data = NULL;
 	}
@@ -3529,6 +3654,8 @@ void bpf_jit_free(struct bpf_prog *prog)
 	if (prog->jited) {
 		struct x64_jit_data *jit_data = prog->aux->jit_data;
 		struct bpf_binary_header *hdr;
+		void __percpu *priv_stack_ptr;
+		int priv_stack_alloc_sz;
 
 		/*
 		 * If we fail the final pass of JIT (from jit_subprogs),
@@ -3544,6 +3671,13 @@ void bpf_jit_free(struct bpf_prog *prog)
 		prog->bpf_func = (void *)prog->bpf_func - cfi_get_offset();
 		hdr = bpf_jit_binary_pack_hdr(prog);
 		bpf_jit_binary_pack_free(hdr, NULL);
+		priv_stack_ptr = prog->aux->priv_stack_ptr;
+		if (priv_stack_ptr) {
+			priv_stack_alloc_sz = round_up(prog->aux->stack_depth, 8) +
+					      2 * PRIV_STACK_GUARD_SZ;
+			priv_stack_check_guard(priv_stack_ptr, priv_stack_alloc_sz, prog);
+			free_percpu(prog->aux->priv_stack_ptr);
+		}
 		WARN_ON_ONCE(!bpf_prog_kallsyms_verify_off(prog));
 	}
 
@@ -3559,6 +3693,11 @@ bool bpf_jit_supports_exceptions(void)
 	return IS_ENABLED(CONFIG_UNWINDER_ORC);
 }
 
+bool bpf_jit_supports_private_stack(void)
+{
+	return true;
+}
+
 void arch_bpf_stack_walk(bool (*consume_fn)(void *cookie, u64 ip, u64 sp, u64 bp), void *cookie)
 {
 #if defined(CONFIG_UNWINDER_ORC)

include/linux/bpf.h

@@ -1507,6 +1507,7 @@ struct bpf_prog_aux {
 	u32 max_rdwr_access;
 	struct btf *attach_btf;
 	const struct bpf_ctx_arg_aux *ctx_arg_info;
+	void __percpu *priv_stack_ptr;
 	struct mutex dst_mutex; /* protects dst_* pointers below, *after* prog becomes visible */
 	struct bpf_prog *dst_prog;
 	struct bpf_trampoline *dst_trampoline;
@@ -1523,9 +1524,12 @@ struct bpf_prog_aux {
 	bool exception_cb;
 	bool exception_boundary;
 	bool is_extended; /* true if extended by freplace program */
+	bool jits_use_priv_stack;
+	bool priv_stack_requested;
 	u64 prog_array_member_cnt; /* counts how many times as member of prog_array */
 	struct mutex ext_mutex; /* mutex for is_extended and prog_array_member_cnt */
 	struct bpf_arena *arena;
+	void (*recursion_detected)(struct bpf_prog *prog); /* callback if recursion is detected */
 	/* BTF_KIND_FUNC_PROTO for valid attach_btf_id */
 	const struct btf_type *attach_func_proto;
 	/* function name for valid attach_btf_id */

include/linux/bpf_verifier.h

@@ -633,6 +633,12 @@ struct bpf_subprog_arg_info {
 	};
 };
 
+enum priv_stack_mode {
+	PRIV_STACK_UNKNOWN,
+	NO_PRIV_STACK,
+	PRIV_STACK_ADAPTIVE,
+};
+
 struct bpf_subprog_info {
 	/* 'start' has to be the first field otherwise find_subprog() won't work */
 	u32 start; /* insn idx of function entry point */
@@ -653,6 +659,7 @@ struct bpf_subprog_info {
 	/* true if bpf_fastcall stack region is used by functions that can't be inlined */
 	bool keep_fastcall_stack: 1;
 
+	enum priv_stack_mode priv_stack_mode;
 	u8 arg_cnt;
 	struct bpf_subprog_arg_info args[MAX_BPF_FUNC_REG_ARGS];
 };
@@ -872,6 +879,7 @@ static inline bool bpf_prog_check_recur(const struct bpf_prog *prog)
 	case BPF_PROG_TYPE_TRACING:
 		return prog->expected_attach_type != BPF_TRACE_ITER;
 	case BPF_PROG_TYPE_STRUCT_OPS:
+		return prog->aux->jits_use_priv_stack;
 	case BPF_PROG_TYPE_LSM:
 		return false;
 	default:

include/linux/filter.h

@@ -1119,6 +1119,7 @@ bool bpf_jit_supports_exceptions(void);
 bool bpf_jit_supports_ptr_xchg(void);
 bool bpf_jit_supports_arena(void);
 bool bpf_jit_supports_insn(struct bpf_insn *insn, bool in_arena);
+bool bpf_jit_supports_private_stack(void);
 u64 bpf_arch_uaddress_limit(void);
 void arch_bpf_stack_walk(bool (*consume_fn)(void *cookie, u64 ip, u64 sp, u64 bp), void *cookie);
 bool bpf_helper_changes_pkt_data(void *func);

kernel/bpf/core.c

@@ -3045,6 +3045,11 @@ bool __weak bpf_jit_supports_exceptions(void)
 	return false;
 }
 
+bool __weak bpf_jit_supports_private_stack(void)
+{
+	return false;
+}
+
 void __weak arch_bpf_stack_walk(bool (*consume_fn)(void *cookie, u64 ip, u64 sp, u64 bp), void *cookie)
 {
 }

kernel/bpf/trampoline.c

@@ -899,6 +899,8 @@ static u64 notrace __bpf_prog_enter_recur(struct bpf_prog *prog, struct bpf_tram
 
 	if (unlikely(this_cpu_inc_return(*(prog->active)) != 1)) {
 		bpf_prog_inc_misses_counter(prog);
+		if (prog->aux->recursion_detected)
+			prog->aux->recursion_detected(prog);
 		return 0;
 	}
 	return bpf_prog_start_time();
@@ -975,6 +977,8 @@ u64 notrace __bpf_prog_enter_sleepable_recur(struct bpf_prog *prog,
 
 	if (unlikely(this_cpu_inc_return(*(prog->active)) != 1)) {
 		bpf_prog_inc_misses_counter(prog);
+		if (prog->aux->recursion_detected)
+			prog->aux->recursion_detected(prog);
 		return 0;
 	}
 	return bpf_prog_start_time();