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(-)
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 <[email protected]>
---
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)
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 <[email protected]>
---
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:
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 <[email protected]>
Acked-by: Daniel Borkmann <[email protected]>
> 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)
>
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 <[email protected]>
LGTM, thanks for the fix!
Acked-by: Daniel Borkmann <[email protected]>
> ---
> 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:
>
Edward Cree via iovisor-dev <[email protected]> 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
> [email protected]
> https://lists.iovisor.org/mailman/listinfo/iovisor-dev
Thanks for separating it.
Nadav
From: Edward Cree <[email protected]>
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.