Re: [PATCH bpf-next v3] bpf: Propagate stack bounds to registers in atomics w/ BPF_FETCH

[Yonghong Song]

On 2/2/21 5:50 AM, Brendan Jackman wrote:
> When BPF_FETCH is set, atomic instructions load a value from memory
> into a register. The current verifier code first checks via
> check_mem_access whether we can access the memory, and then checks
> via check_reg_arg whether we can write into the register.
>
> For loads, check_reg_arg has the side-effect of marking the
> register's value as unkonwn, and check_mem_access has the side effect
> of propagating bounds from memory to the register. This currently only
> takes effect for stack memory.
>
> Therefore with the current order, bounds information is thrown away,
> but by simply reversing the order of check_reg_arg
> vs. check_mem_access, we can instead propagate bounds smartly.
>
> A simple test is added with an infinite loop that can only be proved
> unreachable if this propagation is present. This is implemented both
> with C and directly in test_verifier using assembly.
>
> Suggested-by: John Fastabend <john.fastabend@xxxxxxxxx>
> Signed-off-by: Brendan Jackman <jackmanb@xxxxxxxxxx>

Ack with a nit below.

Acked-by: Yonghong Song <yhs@xxxxxx>

> ---
>
> Difference from v2->v3 [1]:
>
>   * Fixed missing ENABLE_ATOMICS_TESTS check.
>
> Difference from v1->v2:
>
>   * Reworked commit message to clarify this only affects stack memory
>   * Added the Suggested-by
>   * Added a C-based test.
>
> [1]: https://lore.kernel.org/bpf/CA+i-1C2ZWUbGxWJ8kAxbri9rBboyuMaVj_BBhg+2Zf_Su9BOJA@xxxxxxxxxxxxxx/T/#t
>
>   kernel/bpf/verifier.c                         | 32 +++++++++++--------
>   .../selftests/bpf/prog_tests/atomic_bounds.c  | 15 +++++++++
>   .../selftests/bpf/progs/atomic_bounds.c       | 24 ++++++++++++++
>   .../selftests/bpf/verifier/atomic_bounds.c    | 27 ++++++++++++++++
>   4 files changed, 84 insertions(+), 14 deletions(-)
>   create mode 100644 tools/testing/selftests/bpf/prog_tests/atomic_bounds.c
>   create mode 100644 tools/testing/selftests/bpf/progs/atomic_bounds.c
>   create mode 100644 tools/testing/selftests/bpf/verifier/atomic_bounds.c
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 972fc38eb62d..5e09632efddb 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -3665,9 +3665,26 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i
>   		return -EACCES;
>   	}
>
> +	if (insn->imm & BPF_FETCH) {
> +		if (insn->imm == BPF_CMPXCHG)
> +			load_reg = BPF_REG_0;
> +		else
> +			load_reg = insn->src_reg;
> +
> +		/* check and record load of old value */
> +		err = check_reg_arg(env, load_reg, DST_OP);
> +		if (err)
> +			return err;
> +	} else {
> +		/* This instruction accesses a memory location but doesn't
> +		 * actually load it into a register.
> +		 */
> +		load_reg = -1;
> +	}
> +
>   	/* check whether we can read the memory */
>   	err = check_mem_access(env, insn_idx, insn->dst_reg, insn->off,
> -			       BPF_SIZE(insn->code), BPF_READ, -1, true);
> +			       BPF_SIZE(insn->code), BPF_READ, load_reg, true);
>   	if (err)
>   		return err;
>
> @@ -3677,19 +3694,6 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i
>   	if (err)
>   		return err;
>
> -	if (!(insn->imm & BPF_FETCH))
> -		return 0;
> -
> -	if (insn->imm == BPF_CMPXCHG)
> -		load_reg = BPF_REG_0;
> -	else
> -		load_reg = insn->src_reg;
> -
> -	/* check and record load of old value */
> -	err = check_reg_arg(env, load_reg, DST_OP);
> -	if (err)
> -		return err;
> -
>   	return 0;
>   }
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/atomic_bounds.c b/tools/testing/selftests/bpf/prog_tests/atomic_bounds.c
> new file mode 100644
> index 000000000000..addf127068e4
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/atomic_bounds.c
> @@ -0,0 +1,15 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <test_progs.h>
> +
> +#include "atomic_bounds.skel.h"
> +
> +void test_atomic_bounds(void)
> +{
> +	struct atomic_bounds *skel;
> +	__u32 duration = 0;
> +
> +	skel = atomic_bounds__open_and_load();
> +	if (CHECK(!skel, "skel_load", "couldn't load program\n"))
> +		return;

You are missing
	atomic_bounds__destroy(skel);
here.

> +}
> diff --git a/tools/testing/selftests/bpf/progs/atomic_bounds.c b/tools/testing/selftests/bpf/progs/atomic_bounds.c
> new file mode 100644
> index 000000000000..e5fff7fc7f8f
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/atomic_bounds.c
> @@ -0,0 +1,24 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#include <linux/bpf.h>
> +#include <bpf/bpf_helpers.h>
> +#include <bpf/bpf_tracing.h>
> +#include <stdbool.h>
> +
> +#ifdef ENABLE_ATOMICS_TESTS
> +bool skip_tests __attribute((__section__(".data"))) = false;
> +#else
> +bool skip_tests = true;
> +#endif
> +
> +SEC("fentry/bpf_fentry_test1")
> +int BPF_PROG(sub, int x)
> +{
> +#ifdef ENABLE_ATOMICS_TESTS
> +	int a = 0;
> +	int b = __sync_fetch_and_add(&a, 1);
> +	/* b is certainly 0 here. Can the verifier tell? */
> +	while (b)
> +		continue;
> +#endif
> +	return 0;
> +}
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