[PATCH net 0/2] bpf: fix verifier min/max handling in BPF_SUB

[PATCH net 0/2] bpf: fix verifier min/max handling in BPF_SUB

[Edward Cree]

I managed to come up with a test for the swapped bounds in BPF_SUB, so here
 it is along with a patch that fixes it, separated out from my 'rewrite
 everything' series so it can go to -stable.

Edward Cree (2):
  selftests/bpf: subtraction bounds test
  bpf/verifier: fix min/max handling in BPF_SUB

 kernel/bpf/verifier.c                       | 21 +++++++++++++++------
 tools/testing/selftests/bpf/test_verifier.c | 28 ++++++++++++++++++++++++++++
 2 files changed, 43 insertions(+), 6 deletions(-)

[PATCH net 1/2] selftests/bpf: subtraction bounds test

[Edward Cree]

There is a bug in the verifier's handling of BPF_SUB: [a,b] - [c,d] yields
 was [a-c, b-d] rather than the correct [a-d, b-c].  So here is a test
 which, with the bogus handling, will produce ranges of [0,0] and thus
 allowed accesses; whereas the correct handling will give a range of
 [-255, 255] (and hence the right-shift will give a range of [0, 255]) and
 the accesses will be rejected.

Signed-off-by: Edward Cree <ecree@solarflare.com>
---
 tools/testing/selftests/bpf/test_verifier.c | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/tools/testing/selftests/bpf/test_verifier.c b/tools/testing/selftests/bpf/test_verifier.c
index af7d173..addea82 100644
--- a/tools/testing/selftests/bpf/test_verifier.c
+++ b/tools/testing/selftests/bpf/test_verifier.c
@@ -5980,6 +5980,34 @@ static struct bpf_test tests[] = {
 		.result = REJECT,
 		.result_unpriv = REJECT,
 	},
+	{
+		"subtraction bounds (map value)",
+		.insns = {
+			BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
+			BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
+			BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
+			BPF_LD_MAP_FD(BPF_REG_1, 0),
+			BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0,
+				     BPF_FUNC_map_lookup_elem),
+			BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 9),
+			BPF_LDX_MEM(BPF_B, BPF_REG_1, BPF_REG_0, 0),
+			BPF_JMP_IMM(BPF_JGT, BPF_REG_1, 0xff, 7),
+			BPF_LDX_MEM(BPF_B, BPF_REG_3, BPF_REG_0, 1),
+			BPF_JMP_IMM(BPF_JGT, BPF_REG_3, 0xff, 5),
+			BPF_ALU64_REG(BPF_SUB, BPF_REG_1, BPF_REG_3),
+			BPF_ALU64_IMM(BPF_RSH, BPF_REG_1, 56),
+			BPF_ALU64_REG(BPF_ADD, BPF_REG_0, BPF_REG_1),
+			BPF_LDX_MEM(BPF_B, BPF_REG_0, BPF_REG_0, 0),
+			BPF_EXIT_INSN(),
+			BPF_MOV64_IMM(BPF_REG_0, 0),
+			BPF_EXIT_INSN(),
+		},
+		.fixup_map1 = { 3 },
+		.errstr_unpriv = "R0 pointer arithmetic prohibited",
+		.errstr = "R0 min value is negative, either use unsigned index or do a if (index >=0) check.",
+		.result = REJECT,
+		.result_unpriv = REJECT,
+	},
 };
 
 static int probe_filter_length(const struct bpf_insn *fp)

[PATCH net 2/2] bpf/verifier: fix min/max handling in BPF_SUB

[Edward Cree]

We have to subtract the src max from the dst min, and vice-versa, since
 (e.g.) the smallest result comes from the largest subtrahend.

Fixes: 484611357c19 ("bpf: allow access into map value arrays")
Signed-off-by: Edward Cree <ecree@solarflare.com>
---
 kernel/bpf/verifier.c | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index af9e84a..664d939 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1865,10 +1865,12 @@ static void adjust_reg_min_max_vals(struct bpf_verifier_env *env,
 	 * do our normal operations to the register, we need to set the values
 	 * to the min/max since they are undefined.
 	 */
-	if (min_val == BPF_REGISTER_MIN_RANGE)
-		dst_reg->min_value = BPF_REGISTER_MIN_RANGE;
-	if (max_val == BPF_REGISTER_MAX_RANGE)
-		dst_reg->max_value = BPF_REGISTER_MAX_RANGE;
+	if (opcode != BPF_SUB) {
+		if (min_val == BPF_REGISTER_MIN_RANGE)
+			dst_reg->min_value = BPF_REGISTER_MIN_RANGE;
+		if (max_val == BPF_REGISTER_MAX_RANGE)
+			dst_reg->max_value = BPF_REGISTER_MAX_RANGE;
+	}
 
 	switch (opcode) {
 	case BPF_ADD:
@@ -1879,10 +1881,17 @@ static void adjust_reg_min_max_vals(struct bpf_verifier_env *env,
 		dst_reg->min_align = min(src_align, dst_align);
 		break;
 	case BPF_SUB:
+		/* If one of our values was at the end of our ranges, then the
+		 * _opposite_ value in the dst_reg goes to the end of our range.
+		 */
+		if (min_val == BPF_REGISTER_MIN_RANGE)
+			dst_reg->max_value = BPF_REGISTER_MAX_RANGE;
+		if (max_val == BPF_REGISTER_MAX_RANGE)
+			dst_reg->min_value = BPF_REGISTER_MIN_RANGE;
 		if (dst_reg->min_value != BPF_REGISTER_MIN_RANGE)
-			dst_reg->min_value -= min_val;
+			dst_reg->min_value -= max_val;
 		if (dst_reg->max_value != BPF_REGISTER_MAX_RANGE)
-			dst_reg->max_value -= max_val;
+			dst_reg->max_value -= min_val;
 		dst_reg->min_align = min(src_align, dst_align);
 		break;
 	case BPF_MUL:

Re: [PATCH net 1/2] selftests/bpf: subtraction bounds test

[Daniel Borkmann]

On 07/21/2017 03:36 PM, Edward Cree wrote:
> There is a bug in the verifier's handling of BPF_SUB: [a,b] - [c,d] yields
>   was [a-c, b-d] rather than the correct [a-d, b-c].  So here is a test
>   which, with the bogus handling, will produce ranges of [0,0] and thus
>   allowed accesses; whereas the correct handling will give a range of
>   [-255, 255] (and hence the right-shift will give a range of [0, 255]) and
>   the accesses will be rejected.
>
> Signed-off-by: Edward Cree <ecree@solarflare.com>

Acked-by: Daniel Borkmann <daniel@iogearbox.net>

>   tools/testing/selftests/bpf/test_verifier.c | 28 ++++++++++++++++++++++++++++
>   1 file changed, 28 insertions(+)
>
> diff --git a/tools/testing/selftests/bpf/test_verifier.c b/tools/testing/selftests/bpf/test_verifier.c
> index af7d173..addea82 100644
> --- a/tools/testing/selftests/bpf/test_verifier.c
> +++ b/tools/testing/selftests/bpf/test_verifier.c
> @@ -5980,6 +5980,34 @@ static struct bpf_test tests[] = {
>   		.result = REJECT,
>   		.result_unpriv = REJECT,
>   	},
> +	{
> +		"subtraction bounds (map value)",
> +		.insns = {
> +			BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
> +			BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
> +			BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
> +			BPF_LD_MAP_FD(BPF_REG_1, 0),
> +			BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0,
> +				     BPF_FUNC_map_lookup_elem),
> +			BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 9),
> +			BPF_LDX_MEM(BPF_B, BPF_REG_1, BPF_REG_0, 0),
> +			BPF_JMP_IMM(BPF_JGT, BPF_REG_1, 0xff, 7),
> +			BPF_LDX_MEM(BPF_B, BPF_REG_3, BPF_REG_0, 1),
> +			BPF_JMP_IMM(BPF_JGT, BPF_REG_3, 0xff, 5),
> +			BPF_ALU64_REG(BPF_SUB, BPF_REG_1, BPF_REG_3),
> +			BPF_ALU64_IMM(BPF_RSH, BPF_REG_1, 56),
> +			BPF_ALU64_REG(BPF_ADD, BPF_REG_0, BPF_REG_1),
> +			BPF_LDX_MEM(BPF_B, BPF_REG_0, BPF_REG_0, 0),
> +			BPF_EXIT_INSN(),
> +			BPF_MOV64_IMM(BPF_REG_0, 0),
> +			BPF_EXIT_INSN(),
> +		},
> +		.fixup_map1 = { 3 },
> +		.errstr_unpriv = "R0 pointer arithmetic prohibited",
> +		.errstr = "R0 min value is negative, either use unsigned index or do a if (index >=0) check.",
> +		.result = REJECT,
> +		.result_unpriv = REJECT,
> +	},
>   };
>
>   static int probe_filter_length(const struct bpf_insn *fp)
>

Re: [PATCH net 2/2] bpf/verifier: fix min/max handling in BPF_SUB

[Daniel Borkmann]

On 07/21/2017 03:37 PM, Edward Cree wrote:
> We have to subtract the src max from the dst min, and vice-versa, since
>   (e.g.) the smallest result comes from the largest subtrahend.
>
> Fixes: 484611357c19 ("bpf: allow access into map value arrays")
> Signed-off-by: Edward Cree <ecree@solarflare.com>

LGTM, thanks for the fix!

Acked-by: Daniel Borkmann <daniel@iogearbox.net>

> ---
>   kernel/bpf/verifier.c | 21 +++++++++++++++------
>   1 file changed, 15 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index af9e84a..664d939 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -1865,10 +1865,12 @@ static void adjust_reg_min_max_vals(struct bpf_verifier_env *env,
>   	 * do our normal operations to the register, we need to set the values
>   	 * to the min/max since they are undefined.
>   	 */
> -	if (min_val == BPF_REGISTER_MIN_RANGE)
> -		dst_reg->min_value = BPF_REGISTER_MIN_RANGE;
> -	if (max_val == BPF_REGISTER_MAX_RANGE)
> -		dst_reg->max_value = BPF_REGISTER_MAX_RANGE;
> +	if (opcode != BPF_SUB) {
> +		if (min_val == BPF_REGISTER_MIN_RANGE)
> +			dst_reg->min_value = BPF_REGISTER_MIN_RANGE;
> +		if (max_val == BPF_REGISTER_MAX_RANGE)
> +			dst_reg->max_value = BPF_REGISTER_MAX_RANGE;
> +	}
>
>   	switch (opcode) {
>   	case BPF_ADD:
> @@ -1879,10 +1881,17 @@ static void adjust_reg_min_max_vals(struct bpf_verifier_env *env,
>   		dst_reg->min_align = min(src_align, dst_align);
>   		break;
>   	case BPF_SUB:
> +		/* If one of our values was at the end of our ranges, then the
> +		 * _opposite_ value in the dst_reg goes to the end of our range.
> +		 */
> +		if (min_val == BPF_REGISTER_MIN_RANGE)
> +			dst_reg->max_value = BPF_REGISTER_MAX_RANGE;
> +		if (max_val == BPF_REGISTER_MAX_RANGE)
> +			dst_reg->min_value = BPF_REGISTER_MIN_RANGE;
>   		if (dst_reg->min_value != BPF_REGISTER_MIN_RANGE)
> -			dst_reg->min_value -= min_val;
> +			dst_reg->min_value -= max_val;
>   		if (dst_reg->max_value != BPF_REGISTER_MAX_RANGE)
> -			dst_reg->max_value -= max_val;
> +			dst_reg->max_value -= min_val;
>   		dst_reg->min_align = min(src_align, dst_align);
>   		break;
>   	case BPF_MUL:
>

Re: [iovisor-dev] [PATCH net 0/2] bpf: fix verifier min/max handling in BPF_SUB

[Nadav Amit]

Edward Cree via iovisor-dev <iovisor-dev@lists.iovisor.org> wrote:

> I managed to come up with a test for the swapped bounds in BPF_SUB, so here
> it is along with a patch that fixes it, separated out from my 'rewrite
> everything' series so it can go to -stable.
> 
> Edward Cree (2):
>  selftests/bpf: subtraction bounds test
>  bpf/verifier: fix min/max handling in BPF_SUB
> 
> kernel/bpf/verifier.c                       | 21 +++++++++++++++------
> tools/testing/selftests/bpf/test_verifier.c | 28 ++++++++++++++++++++++++++++
> 2 files changed, 43 insertions(+), 6 deletions(-)
> 
> _______________________________________________
> iovisor-dev mailing list
> iovisor-dev@lists.iovisor.org
> https://lists.iovisor.org/mailman/listinfo/iovisor-dev

Thanks for separating it.

Nadav

Re: [PATCH net 0/2] bpf: fix verifier min/max handling in BPF_SUB

[David Miller]

From: Edward Cree <ecree@solarflare.com>
Date: Fri, 21 Jul 2017 14:35:17 +0100

> I managed to come up with a test for the swapped bounds in BPF_SUB, so here
>  it is along with a patch that fixes it, separated out from my 'rewrite
>  everything' series so it can go to -stable.

Series applied and queued up for -stable, thanks.