* [PATCH bpf-next 1/2] bpf: Teach verifier that trusted PTR_TO_BTF_ID pointers are non-NULL
@ 2023-06-02 15:01 David Vernet
2023-06-02 15:01 ` [PATCH bpf-next 2/2] selftests/bpf: Add test for non-NULLable PTR_TO_BTF_IDs David Vernet
2023-06-05 21:40 ` [PATCH bpf-next 1/2] bpf: Teach verifier that trusted PTR_TO_BTF_ID pointers are non-NULL patchwork-bot+netdevbpf
0 siblings, 2 replies; 4+ messages in thread
From: David Vernet @ 2023-06-02 15:01 UTC (permalink / raw)
To: bpf
Cc: ast, daniel, andrii, martin.lau, song, yhs, john.fastabend,
kpsingh, sdf, haoluo, jolsa, linux-kernel, kernel-team
In reg_type_not_null(), we currently assume that a pointer may be NULL
if it has the PTR_MAYBE_NULL modifier, or if it doesn't belong to one of
several base type of pointers that are never NULL-able. For example,
PTR_TO_CTX, PTR_TO_MAP_VALUE, etc.
It turns out that in some cases, PTR_TO_BTF_ID can never be NULL as
well, though we currently don't specify it. For example, if you had the
following program:
SEC("tc")
long example_refcnt_fail(void *ctx)
{
struct bpf_cpumask *mask1, *mask2;
mask1 = bpf_cpumask_create();
mask2 = bpf_cpumask_create();
if (!mask1 || !mask2)
goto error_release;
bpf_cpumask_test_cpu(0, (const struct cpumask *)mask1);
bpf_cpumask_test_cpu(0, (const struct cpumask *)mask2);
error_release:
if (mask1)
bpf_cpumask_release(mask1);
if (mask2)
bpf_cpumask_release(mask2);
return ret;
}
The verifier will incorrectly fail to load the program, thinking
(unintuitively) that we have a possibly-unreleased reference if the mask
is NULL, because we (correctly) don't issue a bpf_cpumask_release() on
the NULL path.
The reason the verifier gets confused is due to the fact that we don't
explicitly tell the verifier that trusted PTR_TO_BTF_ID pointers can
never be NULL. Basically, if we successfully get past the if check
(meaning both pointers go from ptr_or_null_bpf_cpumask to
ptr_bpf_cpumask), the verifier will correctly assume that the references
need to be dropped on any possible branch that leads to program exit.
However, it will _incorrectly_ think that the ptr == NULL branch is
possible, and will erroneously detect it as a branch on which we failed
to drop the reference.
The solution is of course to teach the verifier that trusted
PTR_TO_BTF_ID pointers can never be NULL, so that it doesn't incorrectly
think it's possible for the reference to be present on the ptr == NULL
branch.
A follow-on patch will add a selftest that verifies this behavior.
Signed-off-by: David Vernet <void@manifault.com>
---
kernel/bpf/verifier.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 086b2a14905b..63187ba223d5 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -197,6 +197,7 @@ static int ref_set_non_owning(struct bpf_verifier_env *env,
struct bpf_reg_state *reg);
static void specialize_kfunc(struct bpf_verifier_env *env,
u32 func_id, u16 offset, unsigned long *addr);
+static bool is_trusted_reg(const struct bpf_reg_state *reg);
static bool bpf_map_ptr_poisoned(const struct bpf_insn_aux_data *aux)
{
@@ -439,8 +440,11 @@ static bool type_may_be_null(u32 type)
return type & PTR_MAYBE_NULL;
}
-static bool reg_type_not_null(enum bpf_reg_type type)
+static bool reg_not_null(const struct bpf_reg_state *reg)
{
+ enum bpf_reg_type type;
+
+ type = reg->type;
if (type_may_be_null(type))
return false;
@@ -450,6 +454,7 @@ static bool reg_type_not_null(enum bpf_reg_type type)
type == PTR_TO_MAP_VALUE ||
type == PTR_TO_MAP_KEY ||
type == PTR_TO_SOCK_COMMON ||
+ (type == PTR_TO_BTF_ID && is_trusted_reg(reg)) ||
type == PTR_TO_MEM;
}
@@ -13157,7 +13162,7 @@ static int is_branch_taken(struct bpf_reg_state *reg, u64 val, u8 opcode,
bool is_jmp32)
{
if (__is_pointer_value(false, reg)) {
- if (!reg_type_not_null(reg->type))
+ if (!reg_not_null(reg))
return -1;
/* If pointer is valid tests against zero will fail so we can
--
2.40.1
^ permalink raw reply related [flat|nested] 4+ messages in thread
* [PATCH bpf-next 2/2] selftests/bpf: Add test for non-NULLable PTR_TO_BTF_IDs
2023-06-02 15:01 [PATCH bpf-next 1/2] bpf: Teach verifier that trusted PTR_TO_BTF_ID pointers are non-NULL David Vernet
@ 2023-06-02 15:01 ` David Vernet
2023-06-03 1:26 ` Stanislav Fomichev
2023-06-05 21:40 ` [PATCH bpf-next 1/2] bpf: Teach verifier that trusted PTR_TO_BTF_ID pointers are non-NULL patchwork-bot+netdevbpf
1 sibling, 1 reply; 4+ messages in thread
From: David Vernet @ 2023-06-02 15:01 UTC (permalink / raw)
To: bpf
Cc: ast, daniel, andrii, martin.lau, song, yhs, john.fastabend,
kpsingh, sdf, haoluo, jolsa, linux-kernel, kernel-team
In a recent patch, we taught the verifier that trusted PTR_TO_BTF_ID can
never be NULL. This prevents the verifier from incorrectly failing to
load certain programs where it gets confused and thinks a reference
isn't dropped because it incorrectly assumes that a branch exists in
which a NULL PTR_TO_BTF_ID pointer is never released.
This patch adds a testcase that verifies this cannot happen.
Signed-off-by: David Vernet <void@manifault.com>
---
.../selftests/bpf/prog_tests/cpumask.c | 1 +
.../selftests/bpf/progs/cpumask_success.c | 24 +++++++++++++++++++
2 files changed, 25 insertions(+)
diff --git a/tools/testing/selftests/bpf/prog_tests/cpumask.c b/tools/testing/selftests/bpf/prog_tests/cpumask.c
index cdf4acc18e4c..d89191440fb1 100644
--- a/tools/testing/selftests/bpf/prog_tests/cpumask.c
+++ b/tools/testing/selftests/bpf/prog_tests/cpumask.c
@@ -70,5 +70,6 @@ void test_cpumask(void)
verify_success(cpumask_success_testcases[i]);
}
+ RUN_TESTS(cpumask_success);
RUN_TESTS(cpumask_failure);
}
diff --git a/tools/testing/selftests/bpf/progs/cpumask_success.c b/tools/testing/selftests/bpf/progs/cpumask_success.c
index 2fcdd7f68ac7..602a88b03dbc 100644
--- a/tools/testing/selftests/bpf/progs/cpumask_success.c
+++ b/tools/testing/selftests/bpf/progs/cpumask_success.c
@@ -5,6 +5,7 @@
#include <bpf/bpf_tracing.h>
#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
#include "cpumask_common.h"
char _license[] SEC("license") = "GPL";
@@ -426,3 +427,26 @@ int BPF_PROG(test_global_mask_rcu, struct task_struct *task, u64 clone_flags)
return 0;
}
+
+SEC("tp_btf/task_newtask")
+__success
+int BPF_PROG(test_refcount_null_tracking, struct task_struct *task, u64 clone_flags)
+{
+ struct bpf_cpumask *mask1, *mask2;
+
+ mask1 = bpf_cpumask_create();
+ mask2 = bpf_cpumask_create();
+
+ if (!mask1 || !mask2)
+ goto free_masks_return;
+
+ bpf_cpumask_test_cpu(0, (const struct cpumask *)mask1);
+ bpf_cpumask_test_cpu(0, (const struct cpumask *)mask2);
+
+free_masks_return:
+ if (mask1)
+ bpf_cpumask_release(mask1);
+ if (mask2)
+ bpf_cpumask_release(mask2);
+ return 0;
+}
--
2.40.1
^ permalink raw reply related [flat|nested] 4+ messages in thread
* Re: [PATCH bpf-next 2/2] selftests/bpf: Add test for non-NULLable PTR_TO_BTF_IDs
2023-06-02 15:01 ` [PATCH bpf-next 2/2] selftests/bpf: Add test for non-NULLable PTR_TO_BTF_IDs David Vernet
@ 2023-06-03 1:26 ` Stanislav Fomichev
0 siblings, 0 replies; 4+ messages in thread
From: Stanislav Fomichev @ 2023-06-03 1:26 UTC (permalink / raw)
To: David Vernet
Cc: bpf, ast, daniel, andrii, martin.lau, song, yhs, john.fastabend,
kpsingh, haoluo, jolsa, linux-kernel, kernel-team
On 06/02, David Vernet wrote:
> In a recent patch, we taught the verifier that trusted PTR_TO_BTF_ID can
> never be NULL. This prevents the verifier from incorrectly failing to
> load certain programs where it gets confused and thinks a reference
> isn't dropped because it incorrectly assumes that a branch exists in
> which a NULL PTR_TO_BTF_ID pointer is never released.
>
> This patch adds a testcase that verifies this cannot happen.
>
> Signed-off-by: David Vernet <void@manifault.com>
Acked-by: Stanislav Fomichev <sdf@google.com>
I hope someone else can look at the actual change. It looks good to
me conceptually, but not sure what other parts it might affect.
> ---
> .../selftests/bpf/prog_tests/cpumask.c | 1 +
> .../selftests/bpf/progs/cpumask_success.c | 24 +++++++++++++++++++
> 2 files changed, 25 insertions(+)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/cpumask.c b/tools/testing/selftests/bpf/prog_tests/cpumask.c
> index cdf4acc18e4c..d89191440fb1 100644
> --- a/tools/testing/selftests/bpf/prog_tests/cpumask.c
> +++ b/tools/testing/selftests/bpf/prog_tests/cpumask.c
> @@ -70,5 +70,6 @@ void test_cpumask(void)
> verify_success(cpumask_success_testcases[i]);
> }
>
> + RUN_TESTS(cpumask_success);
> RUN_TESTS(cpumask_failure);
> }
> diff --git a/tools/testing/selftests/bpf/progs/cpumask_success.c b/tools/testing/selftests/bpf/progs/cpumask_success.c
> index 2fcdd7f68ac7..602a88b03dbc 100644
> --- a/tools/testing/selftests/bpf/progs/cpumask_success.c
> +++ b/tools/testing/selftests/bpf/progs/cpumask_success.c
> @@ -5,6 +5,7 @@
> #include <bpf/bpf_tracing.h>
> #include <bpf/bpf_helpers.h>
>
> +#include "bpf_misc.h"
> #include "cpumask_common.h"
>
> char _license[] SEC("license") = "GPL";
> @@ -426,3 +427,26 @@ int BPF_PROG(test_global_mask_rcu, struct task_struct *task, u64 clone_flags)
>
> return 0;
> }
> +
> +SEC("tp_btf/task_newtask")
> +__success
> +int BPF_PROG(test_refcount_null_tracking, struct task_struct *task, u64 clone_flags)
> +{
> + struct bpf_cpumask *mask1, *mask2;
> +
> + mask1 = bpf_cpumask_create();
> + mask2 = bpf_cpumask_create();
> +
> + if (!mask1 || !mask2)
> + goto free_masks_return;
> +
> + bpf_cpumask_test_cpu(0, (const struct cpumask *)mask1);
> + bpf_cpumask_test_cpu(0, (const struct cpumask *)mask2);
> +
> +free_masks_return:
> + if (mask1)
> + bpf_cpumask_release(mask1);
> + if (mask2)
> + bpf_cpumask_release(mask2);
> + return 0;
> +}
> --
> 2.40.1
>
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH bpf-next 1/2] bpf: Teach verifier that trusted PTR_TO_BTF_ID pointers are non-NULL
2023-06-02 15:01 [PATCH bpf-next 1/2] bpf: Teach verifier that trusted PTR_TO_BTF_ID pointers are non-NULL David Vernet
2023-06-02 15:01 ` [PATCH bpf-next 2/2] selftests/bpf: Add test for non-NULLable PTR_TO_BTF_IDs David Vernet
@ 2023-06-05 21:40 ` patchwork-bot+netdevbpf
1 sibling, 0 replies; 4+ messages in thread
From: patchwork-bot+netdevbpf @ 2023-06-05 21:40 UTC (permalink / raw)
To: David Vernet
Cc: bpf, ast, daniel, andrii, martin.lau, song, yhs, john.fastabend,
kpsingh, sdf, haoluo, jolsa, linux-kernel, kernel-team
Hello:
This series was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <ast@kernel.org>:
On Fri, 2 Jun 2023 10:01:11 -0500 you wrote:
> In reg_type_not_null(), we currently assume that a pointer may be NULL
> if it has the PTR_MAYBE_NULL modifier, or if it doesn't belong to one of
> several base type of pointers that are never NULL-able. For example,
> PTR_TO_CTX, PTR_TO_MAP_VALUE, etc.
>
> It turns out that in some cases, PTR_TO_BTF_ID can never be NULL as
> well, though we currently don't specify it. For example, if you had the
> following program:
>
> [...]
Here is the summary with links:
- [bpf-next,1/2] bpf: Teach verifier that trusted PTR_TO_BTF_ID pointers are non-NULL
https://git.kernel.org/bpf/bpf-next/c/51302c951c8f
- [bpf-next,2/2] selftests/bpf: Add test for non-NULLable PTR_TO_BTF_IDs
https://git.kernel.org/bpf/bpf-next/c/f904c67876c4
You are awesome, thank you!
--
Deet-doot-dot, I am a bot.
https://korg.docs.kernel.org/patchwork/pwbot.html
^ permalink raw reply [flat|nested] 4+ messages in thread
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2023-06-02 15:01 [PATCH bpf-next 1/2] bpf: Teach verifier that trusted PTR_TO_BTF_ID pointers are non-NULL David Vernet
2023-06-02 15:01 ` [PATCH bpf-next 2/2] selftests/bpf: Add test for non-NULLable PTR_TO_BTF_IDs David Vernet
2023-06-03 1:26 ` Stanislav Fomichev
2023-06-05 21:40 ` [PATCH bpf-next 1/2] bpf: Teach verifier that trusted PTR_TO_BTF_ID pointers are non-NULL patchwork-bot+netdevbpf
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