[PATCH] ebpf: verify the output of the JIT

From: Tycho Andersen
Date: Tue Apr 04 2017 - 18:10:10 EST

The goal of this patch is to protect the JIT against an attacker with a
write-in-memory primitive. The JIT allocates a buffer which will eventually
be marked +x, so we need to make sure that what was written to this buffer
is what was intended.

We acheive this by building a hash of the instruction buffer as
instructions are emittted and then comparing that to a hash at the end of
the JIT compile after the buffer has been marked read-only.

Signed-off-by: Tycho Andersen <tycho@xxxxxxxxxx>
CC: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
CC: Alexei Starovoitov <ast@xxxxxxxxxx>
CC: Kees Cook <keescook@xxxxxxxxxxxx>
CC: MickaÃl SalaÃn <mic@xxxxxxxxxxx>
---
arch/x86/Kconfig | 11 ++++
arch/x86/net/bpf_jit_comp.c | 147 ++++++++++++++++++++++++++++++++++++++++----
2 files changed, 147 insertions(+), 11 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index cc98d5a..7b2db2c 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2789,6 +2789,17 @@ config X86_DMA_REMAP

source "net/Kconfig"

+config EBPF_JIT_HASH_OUTPUT
+ def_bool y
+ depends on HAVE_EBPF_JIT
+ depends on BPF_JIT
+ select CRYPTO_SHA256
+ ---help---
+ Enables a double check of the JIT's output after it is marked read-only to
+ ensure that it matches what the JIT generated.
+
+ Note, only applies when /proc/sys/net/core/bpf_jit_harden > 0.
+
source "drivers/Kconfig"

source "drivers/firmware/Kconfig"
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 32322ce..be1271e 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -13,9 +13,15 @@
#include <linux/if_vlan.h>
#include <asm/cacheflush.h>
#include <linux/bpf.h>
+#include <linux/crypto.h>
+#include <crypto/hash.h>

int bpf_jit_enable __read_mostly;

+#ifdef CONFIG_EBPF_JIT_HASH_OUTPUT
+struct crypto_shash *tfm __read_mostly;
+#endif
+
/*
* assembly code in arch/x86/net/bpf_jit.S
*/
@@ -25,7 +31,8 @@ extern u8 sk_load_byte_positive_offset[];
extern u8 sk_load_word_negative_offset[], sk_load_half_negative_offset[];
extern u8 sk_load_byte_negative_offset[];

-static u8 *emit_code(u8 *ptr, u32 bytes, unsigned int len)
+static u8 *emit_code(u8 *ptr, u32 bytes, unsigned int len,
+ struct shash_desc *hash)
{
if (len == 1)
*ptr = bytes;
@@ -35,11 +42,15 @@ static u8 *emit_code(u8 *ptr, u32 bytes, unsigned int len)
*(u32 *)ptr = bytes;
barrier();
}
+
+ if (IS_ENABLED(CONFIG_EBPF_JIT_HASH_OUTPUT) && hash)
+ crypto_shash_update(hash, (u8 *) &bytes, len);
+
return ptr + len;
}

#define EMIT(bytes, len) \
- do { prog = emit_code(prog, bytes, len); cnt += len; } while (0)
+ do { prog = emit_code(prog, bytes, len, hash); cnt += len; } while (0)

#define EMIT1(b1) EMIT(b1, 1)
#define EMIT2(b1, b2) EMIT((b1) + ((b2) << 8), 2)
@@ -206,7 +217,7 @@ struct jit_context {
/* emit x64 prologue code for BPF program and check it's size.
* bpf_tail_call helper will skip it while jumping into another program
*/
-static void emit_prologue(u8 **pprog)
+static void emit_prologue(u8 **pprog, struct shash_desc *hash)
{
u8 *prog = *pprog;
int cnt = 0;
@@ -264,7 +275,7 @@ static void emit_prologue(u8 **pprog)
* goto *(prog->bpf_func + prologue_size);
* out:
*/
-static void emit_bpf_tail_call(u8 **pprog)
+static void emit_bpf_tail_call(u8 **pprog, struct shash_desc *hash)
{
u8 *prog = *pprog;
int label1, label2, label3;
@@ -328,7 +339,7 @@ static void emit_bpf_tail_call(u8 **pprog)
}


-static void emit_load_skb_data_hlen(u8 **pprog)
+static void emit_load_skb_data_hlen(u8 **pprog, struct shash_desc *hash)
{
u8 *prog = *pprog;
int cnt = 0;
@@ -348,7 +359,8 @@ static void emit_load_skb_data_hlen(u8 **pprog)
}

static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
- int oldproglen, struct jit_context *ctx)
+ int oldproglen, struct jit_context *ctx,
+ struct shash_desc *hash)
{
struct bpf_insn *insn = bpf_prog->insnsi;
int insn_cnt = bpf_prog->len;
@@ -360,10 +372,10 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
int proglen = 0;
u8 *prog = temp;

- emit_prologue(&prog);
+ emit_prologue(&prog, hash);

if (seen_ld_abs)
- emit_load_skb_data_hlen(&prog);
+ emit_load_skb_data_hlen(&prog, hash);

for (i = 0; i < insn_cnt; i++, insn++) {
const s32 imm32 = insn->imm;
@@ -875,7 +887,7 @@ xadd: if (is_imm8(insn->off))
if (seen_ld_abs) {
if (reload_skb_data) {
EMIT1(0x5F); /* pop %rdi */
- emit_load_skb_data_hlen(&prog);
+ emit_load_skb_data_hlen(&prog, hash);
} else {
EMIT2(0x41, 0x59); /* pop %r9 */
EMIT2(0x41, 0x5A); /* pop %r10 */
@@ -884,7 +896,7 @@ xadd: if (is_imm8(insn->off))
break;

case BPF_JMP | BPF_CALL | BPF_X:
- emit_bpf_tail_call(&prog);
+ emit_bpf_tail_call(&prog, hash);
break;

/* cond jump */
@@ -1085,6 +1097,106 @@ xadd: if (is_imm8(insn->off))
return proglen;
}

+#ifdef CONFIG_EBPF_JIT_HASH_OUTPUT
+static struct shash_desc *bpf_alloc_hash_desc(void)
+{
+ struct shash_desc *hash;
+ int sz = sizeof(struct shash_desc) + crypto_shash_descsize(tfm);
+
+ hash = kzalloc(sz, GFP_KERNEL);
+ if (hash)
+ hash->tfm = tfm;
+ return hash;
+}
+
+static int init_hash(struct shash_desc **hash, u32 *nonce)
+{
+ if (!bpf_jit_harden)
+ return 0;
+
+ *nonce = get_random_int();
+
+ if (!tfm) {
+ tfm = crypto_alloc_shash("sha256", 0, 0);
+ if (IS_ERR(tfm))
+ return PTR_ERR(tfm);
+ }
+
+ if (!*hash) {
+ *hash = bpf_alloc_hash_desc();
+ if (!*hash)
+ return -ENOMEM;
+ }
+
+ if (crypto_shash_init(*hash) < 0)
+ return -1;
+
+ return crypto_shash_update(*hash, (u8 *) nonce, sizeof(*nonce));
+}
+
+static bool check_jit_hash(u8 *buf, u32 len, struct shash_desc *out_d,
+ u32 nonce)
+{
+ struct shash_desc *check_d;
+ void *out, *check;
+ unsigned int sz;
+ bool match = false;
+
+ if (!out_d)
+ return 0;
+
+ BUG_ON(out_d->tfm != tfm);
+
+ sz = crypto_shash_digestsize(out_d->tfm);
+ out = kzalloc(2 * sz, GFP_KERNEL);
+ if (!out)
+ return false;
+
+ if (crypto_shash_final(out_d, out) < 0) {
+ kfree(out);
+ return false;
+ }
+
+ check_d = bpf_alloc_hash_desc();
+ if (!check_d) {
+ kfree(out);
+ return false;
+ }
+
+ if (crypto_shash_init(check_d) < 0)
+ goto out;
+
+ if (crypto_shash_update(check_d, (u8 *) &nonce, sizeof(nonce)) < 0)
+ goto out;
+
+ if (crypto_shash_update(check_d, buf, len) < 0)
+ goto out;
+
+ check = out + sz;
+ if (crypto_shash_final(check_d, check) < 0)
+ goto out;
+
+ if (!memcmp(out, check, sz))
+ match = true;
+
+out:
+ kfree(out);
+ kfree(check_d);
+ return match;
+}
+#else
+static int init_hash(struct shash_desc **hash, u32 *nonce)
+{
+ return 0;
+}
+
+static bool check_jit_hash(u8 *buf, u32 len, struct shash_desc *out_d,
+ u32 nonce)
+{
+ return true;
+}
+#endif
+
struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
{
struct bpf_binary_header *header = NULL;
@@ -1096,6 +1208,8 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
int *addrs;
int pass;
int i;
+ struct shash_desc *hash = NULL;
+ u32 nonce;

if (!bpf_jit_enable)
return orig_prog;
@@ -1132,7 +1246,15 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
* pass to emit the final image
*/
for (pass = 0; pass < 10 || image; pass++) {
- proglen = do_jit(prog, addrs, image, oldproglen, &ctx);
+ if (init_hash(&hash, &nonce) < 0) {
+ image = NULL;
+ if (header)
+ bpf_jit_binary_free(header);
+ prog = orig_prog;
+ goto out_addrs;
+ }
+
+ proglen = do_jit(prog, addrs, image, oldproglen, &ctx, hash);
if (proglen <= 0) {
image = NULL;
if (header)
@@ -1166,6 +1288,8 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
if (image) {
bpf_flush_icache(header, image + proglen);
bpf_jit_binary_lock_ro(header);
+ if (!check_jit_hash(image, proglen, hash, nonce))
+ BUG();
prog->bpf_func = (void *)image;
prog->jited = 1;
} else {
@@ -1174,6 +1298,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)

out_addrs:
kfree(addrs);
+ kfree(hash);
out:
if (tmp_blinded)
bpf_jit_prog_release_other(prog, prog == orig_prog ?
--
2.9.3