[PATCH bpf 1/2] bpf: Fix divide-by-zero in btf_struct_walk()

[PATCH bpf 1/2] bpf: Fix divide-by-zero in btf_struct_walk()

[Jiayuan Chen]

When an access goes past the struct and the last member is a flexible
array, btf_struct_walk() folds the offset back into a single element with
(off - moff) % t->size, but never checks that the element type has a size.

BTF takes an empty struct, so this in program BTF

	/* event could be empty */
	struct event {
 #ifdef HAVE_TIMESTAMP
		__u64 ts;
 #endif
	};

	struct batch {
		int nr;
		struct event events[];
	};

divides by zero at prog load time. Getting there needs a PTR_TO_BTF_ID that
is not MEM_ALLOC, e.g. a plain read of a local kptr stashed in a map from a
sleepable program.

Oops: divide error: 0000 [#1] SMP KASAN PTI
RIP: 0010:btf_struct_walk+0x53f/0x1570
Call Trace:
 <TASK>
 btf_struct_access+0x42a/0xcd0
 check_ptr_to_btf_access+0x4dc/0x1160
 check_mem_access+0x3a45/0x8740
 check_load_mem+0x36a/0xd10
 do_check_common+0x3ef0/0xb210
 bpf_check+0x6d3b/0x8580
 bpf_prog_load+0xf7c/0x2720
 __sys_bpf+0xa83/0x3690
 __x64_sys_bpf+0xc7/0x150
 x64_sys_call+0x1f3f/0x27e0
 do_syscall_64+0xe5/0x610
 entry_SYSCALL_64_after_hwframe+0x76/0x7e
 </TASK>

Reject a zero-sized element type. The fixed array path in the same function
already bails out on the same thing:

	btf_struct_walk()
	...
		/* skip empty array */
		if (moff == mtrue_end)
			continue;

		msize /= total_nelems;

Fixes: 9c5f8a1008a1 ("bpf: Support variable length array in tracing programs")
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 kernel/bpf/btf.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 31057c8f3a7c..1c5de50b9cab 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -7203,7 +7203,7 @@ static int btf_struct_walk(struct bpf_verifier_log *log, const struct btf *btf,
 		if (btf_type_is_int(t))
 			return WALK_SCALAR;
 
-		if (!btf_type_is_struct(t))
+		if (!btf_type_is_struct(t) || !t->size)
 			goto error;
 
 		off = (off - moff) % t->size;
-- 
2.43.0

[PATCH bpf 2/2] selftests/bpf: Test BTF walk into a flexible array of zero-sized elements

[Jiayuan Chen]

The program stashes a bpf_obj_new() object whose type ends with a flexible
array of empty structs, then reads it back as an untrusted kptr. Without
the previous patch this divides by zero in btf_struct_walk() instead of
being rejected.

  # ./test_progs -t verifier_btf_flex_array
  ...
  #602     verifier_btf_flex_array:OK
  Summary: 1/1 PASSED, 0 SKIPPED, 0/0 FAILED

Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 .../selftests/bpf/prog_tests/verifier.c       |  2 +
 .../bpf/progs/verifier_btf_flex_array.c       | 56 +++++++++++++++++++
 2 files changed, 58 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/verifier_btf_flex_array.c

diff --git a/tools/testing/selftests/bpf/prog_tests/verifier.c b/tools/testing/selftests/bpf/prog_tests/verifier.c
index f7f94ccebce2..33a5da37e3ad 100644
--- a/tools/testing/selftests/bpf/prog_tests/verifier.c
+++ b/tools/testing/selftests/bpf/prog_tests/verifier.c
@@ -24,6 +24,7 @@
 #include "verifier_bpf_trap.skel.h"
 #include "verifier_bswap.skel.h"
 #include "verifier_btf_ctx_access.skel.h"
+#include "verifier_btf_flex_array.skel.h"
 #include "verifier_btf_unreliable_prog.skel.h"
 #include "verifier_call_large_imm.skel.h"
 #include "verifier_cfg.skel.h"
@@ -188,6 +189,7 @@ void test_verifier_bpf_get_stack(void)        { RUN(verifier_bpf_get_stack); }
 void test_verifier_bpf_trap(void)             { RUN(verifier_bpf_trap); }
 void test_verifier_bswap(void)                { RUN(verifier_bswap); }
 void test_verifier_btf_ctx_access(void)       { RUN(verifier_btf_ctx_access); }
+void test_verifier_btf_flex_array(void)       { RUN(verifier_btf_flex_array); }
 void test_verifier_btf_unreliable_prog(void)  { RUN(verifier_btf_unreliable_prog); }
 void test_verifier_call_large_imm(void)       { RUN(verifier_call_large_imm); }
 void test_verifier_cfg(void)                  { RUN(verifier_cfg); }
diff --git a/tools/testing/selftests/bpf/progs/verifier_btf_flex_array.c b/tools/testing/selftests/bpf/progs/verifier_btf_flex_array.c
new file mode 100644
index 000000000000..59b84261f622
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/verifier_btf_flex_array.c
@@ -0,0 +1,56 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+
+#include "bpf_experimental.h"
+#include "bpf_misc.h"
+
+struct test_empty_event {};
+
+struct test_flex_batch {
+	int nr;
+	struct test_empty_event events[];
+};
+
+struct map_value {
+	struct test_flex_batch __kptr *batch;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__type(key, int);
+	__type(value, struct map_value);
+	__uint(max_entries, 1);
+} batches SEC(".maps");
+
+SEC("syscall")
+__description("btf walk into flexible array of zero-sized elements")
+__failure __msg("access beyond struct test_flex_batch at off 4 size 1")
+int stash_and_peek(void *ctx)
+{
+	struct test_flex_batch *b, *old;
+	struct map_value *v;
+	int key = 0;
+
+	v = bpf_map_lookup_elem(&batches, &key);
+	if (!v)
+		return 0;
+
+	b = bpf_obj_new(struct test_flex_batch);
+	if (!b)
+		return 0;
+	b->nr = 1;
+
+	old = bpf_kptr_xchg(&v->batch, b);
+	if (old)
+		bpf_obj_drop(old);
+
+	b = v->batch;
+	if (!b)
+		return 0;
+
+	return b->nr + *(char *)&b->events[0];
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.43.0

Re: [PATCH bpf 2/2] selftests/bpf: Test BTF walk into a flexible array of zero-sized elements

[Eduard Zingerman]

On Thu, 2026-09-10 at 20:22 +0800, Jiayuan Chen wrote:
> The program stashes a bpf_obj_new() object whose type ends with a flexible
> array of empty structs, then reads it back as an untrusted kptr. Without
> the previous patch this divides by zero in btf_struct_walk() instead of
> being rejected.
> 
>   # ./test_progs -t verifier_btf_flex_array
>   ...
>   #602     verifier_btf_flex_array:OK
>   Summary: 1/1 PASSED, 0 SKIPPED, 0/0 FAILED
> 
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> ---

Please simplify test case as follows:

SEC("syscall")
__failure __msg("access beyond struct test_flex_batch at off 4 size 1")
int stash_and_peek(void *ctx)
{
        struct map_value *v;
        int key = 0;

        v = bpf_map_lookup_elem(&batches, &key);
        if (!v || !v->batch)
                return 0;

        return *(char *)&v->batch->events[0];
}

Also, aren't there already test cases for flex arrays somewhere?
Probably no need to add a new file.

...

Re: [PATCH bpf 1/2] bpf: Fix divide-by-zero in btf_struct_walk()

[Eduard Zingerman]

On Thu, 2026-09-10 at 20:22 +0800, Jiayuan Chen wrote:
> When an access goes past the struct and the last member is a flexible
> array, btf_struct_walk() folds the offset back into a single element with
> (off - moff) % t->size, but never checks that the element type has a size.
> 
> BTF takes an empty struct, so this in program BTF
> 
> 	/* event could be empty */
> 	struct event {
>  #ifdef HAVE_TIMESTAMP
> 		__u64 ts;
>  #endif
> 	};
> 
> 	struct batch {
> 		int nr;
> 		struct event events[];
> 	};
> 
> divides by zero at prog load time. Getting there needs a PTR_TO_BTF_ID that
> is not MEM_ALLOC, e.g. a plain read of a local kptr stashed in a map from a
> sleepable program.
> 
> Oops: divide error: 0000 [#1] SMP KASAN PTI
> RIP: 0010:btf_struct_walk+0x53f/0x1570
> Call Trace:
>  <TASK>
>  btf_struct_access+0x42a/0xcd0
>  check_ptr_to_btf_access+0x4dc/0x1160
>  check_mem_access+0x3a45/0x8740
>  check_load_mem+0x36a/0xd10
>  do_check_common+0x3ef0/0xb210
>  bpf_check+0x6d3b/0x8580
>  bpf_prog_load+0xf7c/0x2720
>  __sys_bpf+0xa83/0x3690
>  __x64_sys_bpf+0xc7/0x150
>  x64_sys_call+0x1f3f/0x27e0
>  do_syscall_64+0xe5/0x610
>  entry_SYSCALL_64_after_hwframe+0x76/0x7e
>  </TASK>
> 
> Reject a zero-sized element type. The fixed array path in the same function
> already bails out on the same thing:
> 
> 	btf_struct_walk()
> 	...
> 		/* skip empty array */
> 		if (moff == mtrue_end)
> 			continue;
> 
> 		msize /= total_nelems;
> 
> Fixes: 9c5f8a1008a1 ("bpf: Support variable length array in tracing programs")
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> ---

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

Re: [PATCH bpf 1/2] bpf: Fix divide-by-zero in btf_struct_walk()

[patchwork-bot+netdevbpf]

Hello:

This series was applied to bpf/bpf.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Thu, 10 Sep 2026 20:22:55 +0800 you wrote:
> When an access goes past the struct and the last member is a flexible
> array, btf_struct_walk() folds the offset back into a single element with
> (off - moff) % t->size, but never checks that the element type has a size.
> 
> BTF takes an empty struct, so this in program BTF
> 
> 	/* event could be empty */
> 	struct event {
>  #ifdef HAVE_TIMESTAMP
> 		__u64 ts;
>  #endif
> 	};
> 
> [...]

Here is the summary with links:
  - [bpf,1/2] bpf: Fix divide-by-zero in btf_struct_walk()
    https://git.kernel.org/bpf/bpf/c/b0b3dc665296
  - [bpf,2/2] selftests/bpf: Test BTF walk into a flexible array of zero-sized elements
    https://git.kernel.org/bpf/bpf/c/f77d21245710

You are awesome, thank you!
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