bpf: Allow narrow loads with offset > 0

PATCH Applied Andrey Ignatov 2 participants 6 messages Nov 7, 2018 – Nov 11, 2018

Subsystems (6)

Networking Network drivers Kselftest BPF +2 more subsystems

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Andrey Ignatov

author

2018-11-07 11:15 UTC #0 lore raw

cover bpf: Allow narrow loads with offset > 0

This patch set adds support for narrow loads with offset > 0 to BPF verifier.

Patch 1 provides more details and is the main patch in the set. Patches 2 and 3 add new test cases to test_verifier and test_sock_addr selftests.

Andrey Ignatov (3):
bpf: Allow narrow loads with offset > 0 selftests/bpf: Test narrow loads with off > 0 in test_verifier selftests/bpf: Test narrow loads with off > 0 for bpf_sock_addr

 include/linux/filter.h                       | 16 +------
 kernel/bpf/verifier.c                        | 19 ++++++--
 tools/testing/selftests/bpf/test_sock_addr.c | 28 ++++++++++--
 tools/testing/selftests/bpf/test_verifier.c  | 48 ++++++++++++++++----
 4 files changed, 78 insertions(+), 33 deletions(-)

Signature · 1 line

2.17.1

Andrey Ignatov

author

2018-11-07 11:15 UTC #1 lore raw

PATCH 1/3 bpf: Allow narrow loads with offset > 0

Currently BPF verifier allows narrow loads for a context field only with offset zero. E.g. if there is a __u32 field then only the following loads are permitted:
* off=0, size=1 (narrow);
* off=0, size=2 (narrow);
* off=0, size=4 (full).

On the other hand LLVM can generate a load with offset different than zero that make sense from program logic point of view, but verifier doesn't accept it.

E.g. tools/testing/selftests/bpf/sendmsg4_prog.c has code:

  #define DST_IP4			0xC0A801FEU /* 192.168.1.254 */
  ...
  	if ((ctx->user_ip4 >> 24) == (bpf_htonl(DST_IP4) >> 24) &&

where ctx is struct bpf_sock_addr.

Some versions of LLVM can produce the following byte code for it:

       8:       71 12 07 00 00 00 00 00         r2 = *(u8 *)(r1 + 7)
       9:       67 02 00 00 18 00 00 00         r2 <<= 24
      10:       18 03 00 00 00 00 00 fe 00 00 00 00 00 00 00 00         r3 = 4261412864 ll
      12:       5d 32 07 00 00 00 00 00         if r2 != r3 goto +7 <LBB0_6>

where *(u8 *)(r1 + 7) means narrow load for ctx->user_ip4 with size=1 and offset=3 (7 - sizeof(ctx->user_family) = 3). This load is currently rejected by verifier.

Verifier code that rejects such loads is in bpf_ctx_narrow_access_ok() what means any is_valid_access implementation, that uses the function, works this way, e.g. bpf_skb_is_valid_access() for __sk_buff or sock_addr_is_valid_access() for bpf_sock_addr.

The patch makes such loads supported. Offset can be in [0; size_default) but has to be multiple of load size. E.g. for __u32 field the following loads are supported now:
* off=0, size=1 (narrow);
* off=1, size=1 (narrow);
* off=2, size=1 (narrow);
* off=3, size=1 (narrow);
* off=0, size=2 (narrow);
* off=2, size=2 (narrow);
* off=0, size=4 (full).

Reported-by Yonghong Song <[email protected]>

Signed-off-by Andrey Ignatov <[email protected]>

---
 include/linux/filter.h | 16 +---------------
 kernel/bpf/verifier.c  | 19 +++++++++++++++----
 2 files changed, 16 insertions(+), 19 deletions(-)

include/linux/filter.h +1 ~~-15~~

index de629b706d1d..cc17f5f32fbb 100644
@@ -668,24 +668,10 @@ static inline u32 bpf_ctx_off_adjust_machine(u32 size)
 	return size;
 }
 
-static inline bool bpf_ctx_narrow_align_ok(u32 off, u32 size_access,
-					   u32 size_default)
-{
-	size_default = bpf_ctx_off_adjust_machine(size_default);
-	size_access  = bpf_ctx_off_adjust_machine(size_access);
-
-#ifdef __LITTLE_ENDIAN
-	return (off & (size_default - 1)) == 0;
-#else
-	return (off & (size_default - 1)) + size_access == size_default;
-#endif
-}
-
 static inline bool
 bpf_ctx_narrow_access_ok(u32 off, u32 size, u32 size_default)
 {
-	return bpf_ctx_narrow_align_ok(off, size, size_default) &&
-	       size <= size_default && (size & (size - 1)) == 0;
+	return size <= size_default && (size & (size - 1)) == 0;
 }
 
 #define bpf_classic_proglen(fprog) (fprog->len * sizeof(fprog->filter[0]))

kernel/bpf/verifier.c +15 -4

index 1971ca325fb4..fa592502568e 100644
@@ -5803,9 +5803,9 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
 		 * we will apply proper mask to the result.
 		 */
 		is_narrower_load = size < ctx_field_size;
+		u32 size_default = bpf_ctx_off_adjust_machine(ctx_field_size);
+		u32 off = insn->off;
 		if (is_narrower_load) {
-			u32 size_default = bpf_ctx_off_adjust_machine(ctx_field_size);
-			u32 off = insn->off;
 			u8 size_code;
 
 			if (type == BPF_WRITE) {

@@ -5833,12 +5833,23 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
 		}
 
 		if (is_narrower_load && size < target_size) {
-			if (ctx_field_size <= 4)
+			u8 shift = (off & (size_default - 1)) * 8;
+
+			if (ctx_field_size <= 4) {
+				if (shift)
+					insn_buf[cnt++] = BPF_ALU32_IMM(BPF_RSH,
+									insn->dst_reg,
+									shift);
 				insn_buf[cnt++] = BPF_ALU32_IMM(BPF_AND, insn->dst_reg,
 								(1 << size * 8) - 1);
-			else
+			} else {
+				if (shift)
+					insn_buf[cnt++] = BPF_ALU64_IMM(BPF_RSH,
+									insn->dst_reg,
+									shift);
 				insn_buf[cnt++] = BPF_ALU64_IMM(BPF_AND, insn->dst_reg,
 								(1 << size * 8) - 1);
+			}
 		}
 
 		new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);

Signature · 1 line

2.17.1

Andrey Ignatov

author

2018-11-07 11:35 UTC #2 lore raw

PATCH 2/3 selftests/bpf: Test narrow loads with off > 0 in test_verifier

In reply to Andrey Ignatov

Test the following narrow loads in test_verifier for context __sk_buff:
* off=1, size=1 - ok;
* off=2, size=1 - ok;
* off=3, size=1 - ok;
* off=0, size=2 - ok;
* off=1, size=2 - fail;
* off=0, size=2 - ok;
* off=3, size=2 - fail.

Signed-off-by Andrey Ignatov <[email protected]>

---
 tools/testing/selftests/bpf/test_verifier.c | 48 ++++++++++++++++-----
 1 file changed, 38 insertions(+), 10 deletions(-)

tools/testing/selftests/bpf/test_verifier.c +38 ~~-10~~

index 6f61df62f690..54d16fbdef8b 100644
@@ -2026,29 +2026,27 @@ static struct bpf_test tests[] = {
 		.result = ACCEPT,
 	},
 	{
-		"check skb->hash byte load not permitted 1",
+		"check skb->hash byte load permitted 1",
 		.insns = {
 			BPF_MOV64_IMM(BPF_REG_0, 0),
 			BPF_LDX_MEM(BPF_B, BPF_REG_0, BPF_REG_1,
 				    offsetof(struct __sk_buff, hash) + 1),
 			BPF_EXIT_INSN(),
 		},
-		.errstr = "invalid bpf_context access",
-		.result = REJECT,
+		.result = ACCEPT,
 	},
 	{
-		"check skb->hash byte load not permitted 2",
+		"check skb->hash byte load permitted 2",
 		.insns = {
 			BPF_MOV64_IMM(BPF_REG_0, 0),
 			BPF_LDX_MEM(BPF_B, BPF_REG_0, BPF_REG_1,
 				    offsetof(struct __sk_buff, hash) + 2),
 			BPF_EXIT_INSN(),
 		},
-		.errstr = "invalid bpf_context access",
-		.result = REJECT,
+		.result = ACCEPT,
 	},
 	{
-		"check skb->hash byte load not permitted 3",
+		"check skb->hash byte load permitted 3",
 		.insns = {
 			BPF_MOV64_IMM(BPF_REG_0, 0),
 #if __BYTE_ORDER == __LITTLE_ENDIAN

@@ -2060,8 +2058,7 @@ static struct bpf_test tests[] = {
 #endif
 			BPF_EXIT_INSN(),
 		},
-		.errstr = "invalid bpf_context access",
-		.result = REJECT,
+		.result = ACCEPT,
 	},
 	{
 		"check cb access: byte, wrong type",

@@ -2173,7 +2170,7 @@ static struct bpf_test tests[] = {
 		.result = ACCEPT,
 	},
 	{
-		"check skb->hash half load not permitted",
+		"check skb->hash half load permitted 2",
 		.insns = {
 			BPF_MOV64_IMM(BPF_REG_0, 0),
 #if __BYTE_ORDER == __LITTLE_ENDIAN

@@ -2182,6 +2179,37 @@ static struct bpf_test tests[] = {
 #else
 			BPF_LDX_MEM(BPF_H, BPF_REG_0, BPF_REG_1,
 				    offsetof(struct __sk_buff, hash)),
+#endif
+			BPF_EXIT_INSN(),
+		},
+		.result = ACCEPT,
+	},
+	{
+		"check skb->hash half load not permitted, unaligned 1",
+		.insns = {
+			BPF_MOV64_IMM(BPF_REG_0, 0),
+#if __BYTE_ORDER == __LITTLE_ENDIAN
+			BPF_LDX_MEM(BPF_H, BPF_REG_0, BPF_REG_1,
+				    offsetof(struct __sk_buff, hash) + 1),
+#else
+			BPF_LDX_MEM(BPF_H, BPF_REG_0, BPF_REG_1,
+				    offsetof(struct __sk_buff, hash) + 3),
+#endif
+			BPF_EXIT_INSN(),
+		},
+		.errstr = "invalid bpf_context access",
+		.result = REJECT,
+	},
+	{
+		"check skb->hash half load not permitted, unaligned 3",
+		.insns = {
+			BPF_MOV64_IMM(BPF_REG_0, 0),
+#if __BYTE_ORDER == __LITTLE_ENDIAN
+			BPF_LDX_MEM(BPF_H, BPF_REG_0, BPF_REG_1,
+				    offsetof(struct __sk_buff, hash) + 3),
+#else
+			BPF_LDX_MEM(BPF_H, BPF_REG_0, BPF_REG_1,
+				    offsetof(struct __sk_buff, hash) + 1),
 #endif
 			BPF_EXIT_INSN(),
 		},

Signature · 1 line

2.17.1

Andrey Ignatov

author

2018-11-07 11:50 UTC #3 lore raw

PATCH 3/3 selftests/bpf: Test narrow loads with off > 0 for bpf_sock_addr

In reply to Andrey Ignatov

Add more test cases for context bpf_sock_addr to test narrow loads with offset > 0 for ctx->user_ip4 field (__u32):
* off=1, size=1;
* off=2, size=1;
* off=3, size=1;
* off=2, size=2.

Signed-off-by Andrey Ignatov <[email protected]>

---
 tools/testing/selftests/bpf/test_sock_addr.c | 28 +++++++++++++++++---
 1 file changed, 24 insertions(+), 4 deletions(-)

tools/testing/selftests/bpf/test_sock_addr.c +24 -4

index aeeb76a54d63..73b7493d4120 100644
@@ -574,24 +574,44 @@ static int bind4_prog_load(const struct sock_addr_test *test)
 		/* if (sk.family == AF_INET && */
 		BPF_LDX_MEM(BPF_W, BPF_REG_7, BPF_REG_6,
 			    offsetof(struct bpf_sock_addr, family)),
-		BPF_JMP_IMM(BPF_JNE, BPF_REG_7, AF_INET, 16),
+		BPF_JMP_IMM(BPF_JNE, BPF_REG_7, AF_INET, 24),
 
 		/*     (sk.type == SOCK_DGRAM || sk.type == SOCK_STREAM) && */
 		BPF_LDX_MEM(BPF_W, BPF_REG_7, BPF_REG_6,
 			    offsetof(struct bpf_sock_addr, type)),
 		BPF_JMP_IMM(BPF_JNE, BPF_REG_7, SOCK_DGRAM, 1),
 		BPF_JMP_A(1),
-		BPF_JMP_IMM(BPF_JNE, BPF_REG_7, SOCK_STREAM, 12),
+		BPF_JMP_IMM(BPF_JNE, BPF_REG_7, SOCK_STREAM, 20),
 
 		/*     1st_byte_of_user_ip4 == expected && */
 		BPF_LDX_MEM(BPF_B, BPF_REG_7, BPF_REG_6,
 			    offsetof(struct bpf_sock_addr, user_ip4)),
-		BPF_JMP_IMM(BPF_JNE, BPF_REG_7, ip4.u4_addr8[0], 10),
+		BPF_JMP_IMM(BPF_JNE, BPF_REG_7, ip4.u4_addr8[0], 18),
+
+		/*     2nd_byte_of_user_ip4 == expected && */
+		BPF_LDX_MEM(BPF_B, BPF_REG_7, BPF_REG_6,
+			    offsetof(struct bpf_sock_addr, user_ip4) + 1),
+		BPF_JMP_IMM(BPF_JNE, BPF_REG_7, ip4.u4_addr8[1], 16),
+
+		/*     3rd_byte_of_user_ip4 == expected && */
+		BPF_LDX_MEM(BPF_B, BPF_REG_7, BPF_REG_6,
+			    offsetof(struct bpf_sock_addr, user_ip4) + 2),
+		BPF_JMP_IMM(BPF_JNE, BPF_REG_7, ip4.u4_addr8[2], 14),
+
+		/*     4th_byte_of_user_ip4 == expected && */
+		BPF_LDX_MEM(BPF_B, BPF_REG_7, BPF_REG_6,
+			    offsetof(struct bpf_sock_addr, user_ip4) + 3),
+		BPF_JMP_IMM(BPF_JNE, BPF_REG_7, ip4.u4_addr8[3], 12),
 
 		/*     1st_half_of_user_ip4 == expected && */
 		BPF_LDX_MEM(BPF_H, BPF_REG_7, BPF_REG_6,
 			    offsetof(struct bpf_sock_addr, user_ip4)),
-		BPF_JMP_IMM(BPF_JNE, BPF_REG_7, ip4.u4_addr16[0], 8),
+		BPF_JMP_IMM(BPF_JNE, BPF_REG_7, ip4.u4_addr16[0], 10),
+
+		/*     2nd_half_of_user_ip4 == expected && */
+		BPF_LDX_MEM(BPF_H, BPF_REG_7, BPF_REG_6,
+			    offsetof(struct bpf_sock_addr, user_ip4) + 2),
+		BPF_JMP_IMM(BPF_JNE, BPF_REG_7, ip4.u4_addr16[1], 8),
 
 		/*     whole_user_ip4 == expected) { */
 		BPF_LDX_MEM(BPF_W, BPF_REG_7, BPF_REG_6,

Signature · 1 line

2.17.1

Alexei Starovoitov

Applied

2018-11-11 04:59 UTC #4 lore raw

In reply to Andrey Ignatov

Quoted 6 lines · Andrey Ignatov

This patch set adds support for narrow loads with offset > 0 to BPF verifier.
Patch 1 provides more details and is the main patch in the set. Patches 2 and 3 add new test cases to test_verifier and test_sock_addr selftests.

Applied, Thanks

Alexei Starovoitov

2018-11-11 09:25 UTC #5 lore raw

Quoted 10 lines · Alexei Starovoitov

Quoted 6 lines · Andrey Ignatov
This patch set adds support for narrow loads with offset > 0 to BPF verifier.
Patch 1 provides more details and is the main patch in the set. Patches 2 and 3 add new test cases to test_verifier and test_sock_addr selftests.
Applied, Thanks

reverted.
please fix compiler warning and respin.