Re: [PATCH v2 2/2] x86/cfi,bpf: Fix BPF JIT call

Peter Zijlstra
2023-12-08T10:30:04Z

On Thu, Dec 07, 2023 at 02:32:12PM -0800, Alexei Starovoitov wrote:
> On Thu, Dec 07, 2023 at 10:31:05AM +0100, Peter Zijlstra wrote:
> > On Wed, Dec 06, 2023 at 01:39:43PM -0800, Alexei Starovoitov wrote:
> >
> >
> > > All is ok until kCFI comes into picture.
> > > Here we probably need to teach arch_prepare_bpf_trampoline() to emit
> > > different __kcfi_typeid depending on kernel function proto,
> > > so that caller hash checking logic won't be tripped.
> > > I suspect that requires to reverse engineer an algorithm of computing kcfi from clang.
> > > other ideas?
> >
> > I was going to try and extend bpf_struct_ops with a pointer, this
> > pointer will point to a struct of the right type with all ops filled out
> > as stubs.
>
> Right. Something like this, but it's more nuanced.
>
> The bpf_struct_ops concept is a generic mechanism to provide bpf-based callback
> to any set of kernel callbacks.
>
> bpf tcp CC plugs into:
> struct tcp_congestion_ops {
> /* do new cwnd calculation (required) */
> void (*cong_avoid)(struct sock *sk, u32 ack, u32 acked);
>
> /* call before changing ca_state (optional) */
> void (*set_state)(struct sock *sk, u8 new_state);
>
> /* call when cwnd event occurs (optional) */
> void (*cwnd_event)(struct sock *sk, enum tcp_ca_event ev);
> ...
> };
>
> and from bpf side we don't touch tcp kernel bits at all.
> tcp stack doesn't know whether it's calling bpf based CC or builtin CC or CC provided by kernel module.
>
> bpf struct_ops mechanim is a zero cost extension to potentially any kernel mechanism
> that is already setup with callbacks. tcp_congestion_ops is one of them.
>
> The allowlisting of tcp_congestion_ops for bpf use is done in net/ipv4/bpf_tcp_ca.c via:
>
> struct bpf_struct_ops bpf_tcp_congestion_ops = {
> ...
> .reg = bpf_tcp_ca_reg,
> .unreg = bpf_tcp_ca_unreg,
> ...
> .name = "tcp_congestion_ops",
> };
> static int bpf_tcp_ca_reg(void *kdata)
> {
> return tcp_register_congestion_control(kdata);
> }
> and
> int tcp_register_congestion_control(struct tcp_congestion_ops *type);
> is a normal TCP CC registration routine that is used by all CCs.
>
> The bpf struct_ops infra prepares everything inside
> 'struct tcp_congestion_ops' that makes it indistinguishable from normal kernel CC,
> except kCFI part. sadly.
>
> The kCFI challenge is that clang may not generate any __cfi + __kcfi_typeid at all.
> Like if vmlinux doesn't have any TCP CCs built-in there will be no kCFI hashes
> in the kernel that represent a required hash to call cong_avoid, set_state, cwnd_event.

Right, I got that. So what I meant was something like the below
(compiled only).

By adding an actual ops struct, but filled with no-op stubs to
bpf_struct_ops, we can crib the CFI hash from those functions. They'll
never be called, but the compiler cannot tell and has to generate them.

The only problem I now have is the one XXX, I'm not entirely sure what
signature to use there.

---
Index: linux-2.6/include/linux/bpf.h
===================================================================
--- linux-2.6.orig/include/linux/bpf.h
+++ linux-2.6/include/linux/bpf.h
@@ -1060,6 +1060,17 @@ struct btf_func_model {
*/
#define BPF_TRAMP_F_TAIL_CALL_CTX BIT(7)

+/*
+ * Indicate the trampoline should be suitable to receive indirect calls;
+ * without this indirectly calling the generated code can result in #UD/#CP,
+ * depending on the CFI options.
+ *
+ * Used by bpf_struct_ops.
+ *
+ * Incompatible with FENTRY usage, overloads @func_addr argument.
+ */
+#define BPF_TRAMP_F_INDIRECT BIT(8)
+
/* Each call __bpf_prog_enter + call bpf_func + call __bpf_prog_exit is ~50
* bytes on x86.
*/
@@ -1695,6 +1706,7 @@ struct bpf_struct_ops {
struct btf_func_model func_models[BPF_STRUCT_OPS_MAX_NR_MEMBERS];
u32 type_id;
u32 value_id;
+ void *bpf_ops_stubs;
};

#if defined(CONFIG_BPF_JIT) && defined(CONFIG_BPF_SYSCALL)
@@ -1708,6 +1720,7 @@ int bpf_struct_ops_map_sys_lookup_elem(s
int bpf_struct_ops_prepare_trampoline(struct bpf_tramp_links *tlinks,
struct bpf_tramp_link *link,
const struct btf_func_model *model,
+ void *stub_func,
void *image, void *image_end);
static inline bool bpf_try_module_get(const void *data, struct module *owner)
{
Index: linux-2.6/kernel/bpf/bpf_struct_ops.c
===================================================================
--- linux-2.6.orig/kernel/bpf/bpf_struct_ops.c
+++ linux-2.6/kernel/bpf/bpf_struct_ops.c
@@ -352,17 +352,16 @@ const struct bpf_link_ops bpf_struct_ops
int bpf_struct_ops_prepare_trampoline(struct bpf_tramp_links *tlinks,
struct bpf_tramp_link *link,
const struct btf_func_model *model,
- void *image, void *image_end)
+ void *stub_func, void *image, void *image_end)
{
- u32 flags;
+ u32 flags = BPF_TRAMP_F_INDIRECT;
int size;

tlinks[BPF_TRAMP_FENTRY].links[0] = link;
tlinks[BPF_TRAMP_FENTRY].nr_links = 1;
- /* BPF_TRAMP_F_RET_FENTRY_RET is only used by bpf_struct_ops,
- * and it must be used alone.
- */
- flags = model->ret_size > 0 ? BPF_TRAMP_F_RET_FENTRY_RET : 0;
+
+ if (model->ret_size > 0)
+ flags |= BPF_TRAMP_F_RET_FENTRY_RET;

size = arch_bpf_trampoline_size(model, flags, tlinks, NULL);
if (size < 0)
@@ -370,7 +369,7 @@ int bpf_struct_ops_prepare_trampoline(st
if (size > (unsigned long)image_end - (unsigned long)image)
return -E2BIG;
return arch_prepare_bpf_trampoline(NULL, image, image_end,
- model, flags, tlinks, NULL);
+ model, flags, tlinks, stub_func);
}

static long bpf_struct_ops_map_update_elem(struct bpf_map *map, void *key,
@@ -504,11 +503,12 @@ static long bpf_struct_ops_map_update_el

err = bpf_struct_ops_prepare_trampoline(tlinks, link,
&st_ops->func_models[i],
+ *(void **)(st_ops->bpf_ops_stubs + moff),
image, image_end);
if (err < 0)
goto reset_unlock;

- *(void **)(kdata + moff) = image;
+ *(void **)(kdata + moff) = image + cfi_get_offset();
image += err;

/* put prog_id to udata */
Index: linux-2.6/net/ipv4/bpf_tcp_ca.c
===================================================================
--- linux-2.6.orig/net/ipv4/bpf_tcp_ca.c
+++ linux-2.6/net/ipv4/bpf_tcp_ca.c
@@ -271,6 +271,74 @@ static int bpf_tcp_ca_validate(void *kda
return tcp_validate_congestion_control(kdata);
}

+static u32 bpf_tcp_ca_ssthresh(struct sock *sk)
+{
+ return 0;
+}
+
+static void bpf_tcp_ca_cong_avoid(struct sock *sk, u32 ack, u32 acked)
+{
+}
+
+static void bpf_tcp_ca_set_state(struct sock *sk, u8 new_state)
+{
+}
+
+static void bpf_tcp_ca_cwnd_event(struct sock *sk, enum tcp_ca_event ev)
+{
+}
+
+static void bpf_tcp_ca_in_ack_event(struct sock *sk, u32 flags)
+{
+}
+
+static void bpf_tcp_ca_pkts_acked(struct sock *sk, const struct ack_sample *sample)
+{
+}
+
+static u32 bpf_tcp_ca_min_tso_segs(struct sock *sk)
+{
+ return 0;
+}
+
+static void bpf_tcp_ca_cong_control(struct sock *sk, const struct rate_sample *rs)
+{
+}
+
+static u32 bpf_tcp_ca_undo_cwnd(struct sock *sk)
+{
+ return 0;
+}
+
+static u32 bpf_tcp_ca_sndbuf_expand(struct sock *sk)
+{
+ return 0;
+}
+
+static void __bpf_tcp_ca_init(struct sock *sk)
+{
+}
+
+static void __bpf_tcp_ca_release(struct sock *sk)
+{
+}
+
+static struct tcp_congestion_ops __bpf_ops_tcp_congestion_ops = {
+ .ssthresh = bpf_tcp_ca_ssthresh,
+ .cong_avoid = bpf_tcp_ca_cong_avoid,
+ .set_state = bpf_tcp_ca_set_state,
+ .cwnd_event = bpf_tcp_ca_cwnd_event,
+ .in_ack_event = bpf_tcp_ca_in_ack_event,
+ .pkts_acked = bpf_tcp_ca_pkts_acked,
+ .min_tso_segs = bpf_tcp_ca_min_tso_segs,
+ .cong_control = bpf_tcp_ca_cong_control,
+ .undo_cwnd = bpf_tcp_ca_undo_cwnd,
+ .sndbuf_expand = bpf_tcp_ca_sndbuf_expand,
+
+ .init = __bpf_tcp_ca_init,
+ .release = __bpf_tcp_ca_release,
+};
+
struct bpf_struct_ops bpf_tcp_congestion_ops = {
.verifier_ops = &bpf_tcp_ca_verifier_ops,
.reg = bpf_tcp_ca_reg,
@@ -281,6 +349,7 @@ struct bpf_struct_ops bpf_tcp_congestion
.init = bpf_tcp_ca_init,
.validate = bpf_tcp_ca_validate,
.name = "tcp_congestion_ops",
+ .bpf_ops_stubs = &__bpf_ops_tcp_congestion_ops,
};

static int __init bpf_tcp_ca_kfunc_init(void)
Index: linux-2.6/arch/x86/include/asm/cfi.h
===================================================================
--- linux-2.6.orig/arch/x86/include/asm/cfi.h
+++ linux-2.6/arch/x86/include/asm/cfi.h
@@ -123,6 +123,8 @@ static inline cfi_get_offset(void)
}
#define cfi_get_offset cfi_get_offset

+extern u32 cfi_get_func_hash(void *func);
+
#else
static inline enum bug_trap_type handle_cfi_failure(struct pt_regs *regs)
{
@@ -130,6 +132,10 @@ static inline enum bug_trap_type handle_
}
#define cfi_bpf_hash 0U
#define cfi_bpf_subprog_hash 0U
+static inline u32 cfi_get_func_hash(void *func)
+{
+ return 0;
+}
#endif /* CONFIG_CFI_CLANG */

#endif /* _ASM_X86_CFI_H */
Index: linux-2.6/arch/x86/kernel/alternative.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/alternative.c
+++ linux-2.6/arch/x86/kernel/alternative.c
@@ -883,6 +883,28 @@ asm (
" .size cfi_bpf_subprog_hash, 4 \n"
" .popsection \n"
);
+
+u32 cfi_get_func_hash(void *func)
+{
+ u32 hash;
+
+ func -= cfi_get_offset();
+ switch (cfi_mode) {
+ case CFI_FINEIBT:
+ func += 7;
+ break;
+ case CFI_KCFI:
+ func += 1;
+ break;
+ default:
+ return 0;
+ }
+
+ if (get_kernel_nofault(hash, func))
+ return 0;
+
+ return hash;
+}
#endif

#ifdef CONFIG_FINEIBT
Index: linux-2.6/net/bpf/bpf_dummy_struct_ops.c
===================================================================
--- linux-2.6.orig/net/bpf/bpf_dummy_struct_ops.c
+++ linux-2.6/net/bpf/bpf_dummy_struct_ops.c
@@ -62,7 +62,7 @@ static int dummy_ops_copy_args(struct bp

static int dummy_ops_call_op(void *image, struct bpf_dummy_ops_test_args *args)
{
- dummy_ops_test_ret_fn test = (void *)image;
+ dummy_ops_test_ret_fn test = (void *)image + cfi_get_offset();
struct bpf_dummy_ops_state *state = NULL;

/* state needs to be NULL if args[0] is 0 */
@@ -119,6 +119,7 @@ int bpf_struct_ops_test_run(struct bpf_p
op_idx = prog->expected_attach_type;
err = bpf_struct_ops_prepare_trampoline(tlinks, link,
&st_ops->func_models[op_idx],
+ /* XXX */ NULL,
image, image + PAGE_SIZE);
if (err < 0)
goto out;
@@ -219,6 +220,28 @@ static void bpf_dummy_unreg(void *kdata)
{
}

+static int bpf_dummy_test_1(struct bpf_dummy_ops_state *cb)
+{
+ return 0;
+}
+
+static int bpf_dummy_test_2(struct bpf_dummy_ops_state *cb, int a1, unsigned short a2,
+ char a3, unsigned long a4)
+{
+ return 0;
+}
+
+static int bpf_dummy_test_sleepable(struct bpf_dummy_ops_state *cb)
+{
+ return 0;
+}
+
+static struct bpf_dummy_ops __bpf_bpf_dummy_ops = {
+ .test_1 = bpf_dummy_test_1,
+ .test_2 = bpf_dummy_test_2,
+ .test_sleepable = bpf_dummy_test_sleepable,
+};
+
struct bpf_struct_ops bpf_bpf_dummy_ops = {
.verifier_ops = &bpf_dummy_verifier_ops,
.init = bpf_dummy_init,
@@ -227,4 +250,5 @@ struct bpf_struct_ops bpf_bpf_dummy_ops
.reg = bpf_dummy_reg,
.unreg = bpf_dummy_unreg,
.name = "bpf_dummy_ops",
+ .bpf_ops_stubs = &__bpf_bpf_dummy_ops,
};
Index: linux-2.6/arch/x86/net/bpf_jit_comp.c
===================================================================
--- linux-2.6.orig/arch/x86/net/bpf_jit_comp.c
+++ linux-2.6/arch/x86/net/bpf_jit_comp.c
@@ -312,9 +312,8 @@ static void pop_callee_regs(u8 **pprog,
* in arch/x86/kernel/alternative.c
*/

-static void emit_fineibt(u8 **pprog, bool is_subprog)
+static void emit_fineibt(u8 **pprog, u32 hash)
{
- u32 hash = is_subprog ? cfi_bpf_subprog_hash : cfi_bpf_hash;
u8 *prog = *pprog;

EMIT_ENDBR();
@@ -327,9 +326,8 @@ static void emit_fineibt(u8 **pprog, boo
*pprog = prog;
}

-static void emit_kcfi(u8 **pprog, bool is_subprog)
+static void emit_kcfi(u8 **pprog, u32 hash)
{
- u32 hash = is_subprog ? cfi_bpf_subprog_hash : cfi_bpf_hash;
u8 *prog = *pprog;

EMIT1_off32(0xb8, hash); /* movl $hash, %eax */
@@ -351,17 +349,17 @@ static void emit_kcfi(u8 **pprog, bool i
*pprog = prog;
}

-static void emit_cfi(u8 **pprog, bool is_subprog)
+static void emit_cfi(u8 **pprog, u32 hash)
{
u8 *prog = *pprog;

switch (cfi_mode) {
case CFI_FINEIBT:
- emit_fineibt(&prog, is_subprog);
+ emit_fineibt(&prog, hash);
break;

case CFI_KCFI:
- emit_kcfi(&prog, is_subprog);
+ emit_kcfi(&prog, hash);
break;

default:
@@ -383,7 +381,7 @@ static void emit_prologue(u8 **pprog, u3
{
u8 *prog = *pprog;

- emit_cfi(&prog, is_subprog);
+ emit_cfi(&prog, is_subprog ? cfi_bpf_subprog_hash : cfi_bpf_hash);
/* BPF trampoline can be made to work without these nops,
* but let's waste 5 bytes for now and optimize later
*/
@@ -2596,20 +2594,27 @@ static int __arch_prepare_bpf_trampoline

prog = rw_image;

- EMIT_ENDBR();
- /*
- * This is the direct-call trampoline, as such it needs accounting
- * for the __fentry__ call.
- */
- x86_call_depth_emit_accounting(&prog, NULL);
+ if (flags & BPF_TRAMP_F_INDIRECT) {
+ /*
+ * Indirect call for bpf_struct_ops
+ */
+ emit_cfi(&prog, cfi_get_func_hash(func_addr));
+ } else {
+ /*
+ * Direct-call fentry stub, as such it needs accounting for the
+ * __fentry__ call.
+ */
+ x86_call_depth_emit_accounting(&prog, NULL);
+ }
EMIT1(0x55); /* push rbp */
EMIT3(0x48, 0x89, 0xE5); /* mov rbp, rsp */
- if (!is_imm8(stack_size))
+ if (!is_imm8(stack_size)) {
/* sub rsp, stack_size */
EMIT3_off32(0x48, 0x81, 0xEC, stack_size);
- else
+ } else {
/* sub rsp, stack_size */
EMIT4(0x48, 0x83, 0xEC, stack_size);
+ }
if (flags & BPF_TRAMP_F_TAIL_CALL_CTX)
EMIT1(0x50); /* push rax */
/* mov QWORD PTR [rbp - rbx_off], rbx */
@@ -2643,10 +2648,11 @@ static int __arch_prepare_bpf_trampoline
}
}

- if (fentry->nr_links)
+ if (fentry->nr_links) {
if (invoke_bpf(m, &prog, fentry, regs_off, run_ctx_off,
flags & BPF_TRAMP_F_RET_FENTRY_RET, image, rw_image))
return -EINVAL;
+ }

if (fmod_ret->nr_links) {
branches = kcalloc(fmod_ret->nr_links, sizeof(u8 *),
@@ -2665,11 +2671,12 @@ static int __arch_prepare_bpf_trampoline
restore_regs(m, &prog, regs_off);
save_args(m, &prog, arg_stack_off, true);

- if (flags & BPF_TRAMP_F_TAIL_CALL_CTX)
+ if (flags & BPF_TRAMP_F_TAIL_CALL_CTX) {
/* Before calling the original function, restore the
* tail_call_cnt from stack to rax.
*/
RESTORE_TAIL_CALL_CNT(stack_size);
+ }

if (flags & BPF_TRAMP_F_ORIG_STACK) {
emit_ldx(&prog, BPF_DW, BPF_REG_6, BPF_REG_FP, 8);
@@ -2698,17 +2705,19 @@ static int __arch_prepare_bpf_trampoline
/* Update the branches saved in invoke_bpf_mod_ret with the
* aligned address of do_fexit.
*/
- for (i = 0; i < fmod_ret->nr_links; i++)
+ for (i = 0; i < fmod_ret->nr_links; i++) {
emit_cond_near_jump(&branches[i], image + (prog - (u8 *)rw_image),
image + (branches[i] - (u8 *)rw_image), X86_JNE);
+ }
}

- if (fexit->nr_links)
+ if (fexit->nr_links) {
if (invoke_bpf(m, &prog, fexit, regs_off, run_ctx_off,
false, image, rw_image)) {
ret = -EINVAL;
goto cleanup;
}
+ }

if (flags & BPF_TRAMP_F_RESTORE_REGS)
restore_regs(m, &prog, regs_off);
@@ -2725,11 +2734,12 @@ static int __arch_prepare_bpf_trampoline
ret = -EINVAL;
goto cleanup;
}
- } else if (flags & BPF_TRAMP_F_TAIL_CALL_CTX)
+ } else if (flags & BPF_TRAMP_F_TAIL_CALL_CTX) {
/* Before running the original function, restore the
* tail_call_cnt from stack to rax.
*/
RESTORE_TAIL_CALL_CNT(stack_size);
+ }

/* restore return value of orig_call or fentry prog back into RAX */
if (save_ret)
@@ -2737,9 +2747,10 @@ static int __arch_prepare_bpf_trampoline

emit_ldx(&prog, BPF_DW, BPF_REG_6, BPF_REG_FP, -rbx_off);
EMIT1(0xC9); /* leave */
- if (flags & BPF_TRAMP_F_SKIP_FRAME)
+ if (flags & BPF_TRAMP_F_SKIP_FRAME) {
/* skip our return address and return to parent */
EMIT4(0x48, 0x83, 0xC4, 8); /* add rsp, 8 */
+ }
emit_return(&prog, image + (prog - (u8 *)rw_image));
/* Make sure the trampoline generation logic doesn't overflow */
if (WARN_ON_ONCE(prog > (u8 *)rw_image_end - BPF_INSN_SAFETY)) {