[PATCH bpf-next v3 1/2] bpf: Fix sleepable check for tracing prog

[PATCH bpf-next v3 1/2] bpf: Fix sleepable check for tracing prog

[Leon Hwang]

When CONFIG_FUNCTION_ERROR_INJECTION is disabled, a sleepable tracing prog
is allowed to attach to '__x64_'-alike prefix symbols.

It is because the verifier does not verify whether the symbol is a kernel
function or a bpf prog. That said, a sleepable tracing prog is allowed to
attach to a bpf prog target whose name has '__x64_'-alike prefix.

For example, a sleepable fentry prog attaches to a '__x64_sys_nop' XDP
prog, and copies buffer from a user pointer with bpf_copy_from_user()
helper. After attaching the XDP prog to lo interface, the kernel BUG
could be triggered by 'ping -c 1 -W 1 127.0.0.1':

[    3.460756] BUG: sleeping function called from invalid context at kernel/bpf/trampoline.c:1324

Fix it by disallowing sleepable tracing prog always when its target btf
is not kernel's btf.

Fixes: 16d9c5660692 ("bpf: Always allow sleepable programs on syscalls")
Acked-by: Viktor Malik <vmalik@redhat.com>
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 kernel/bpf/verifier.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index b274004fccfd..7bb541e343b2 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -19019,6 +19019,9 @@ static int btf_id_allow_sleepable(u32 btf_id, unsigned long addr, const struct b
 
 	switch (prog->type) {
 	case BPF_PROG_TYPE_TRACING:
+		if (!btf_is_kernel(btf))
+			return -EINVAL;
+
 		t = btf_type_by_id(btf, btf_id);
 		if (!t)
 			return -EINVAL;
-- 
2.55.0

Re: [PATCH bpf-next v3 1/2] bpf: Fix sleepable check for tracing prog

[Andrii Nakryiko]

On Tue, Aug 4, 2026 at 7:57 AM Leon Hwang <leon.hwang@linux.dev> wrote:
>
> When CONFIG_FUNCTION_ERROR_INJECTION is disabled, a sleepable tracing prog
> is allowed to attach to '__x64_'-alike prefix symbols.
>
> It is because the verifier does not verify whether the symbol is a kernel
> function or a bpf prog. That said, a sleepable tracing prog is allowed to
> attach to a bpf prog target whose name has '__x64_'-alike prefix.
>
> For example, a sleepable fentry prog attaches to a '__x64_sys_nop' XDP
> prog, and copies buffer from a user pointer with bpf_copy_from_user()
> helper. After attaching the XDP prog to lo interface, the kernel BUG
> could be triggered by 'ping -c 1 -W 1 127.0.0.1':
>
> [    3.460756] BUG: sleeping function called from invalid context at kernel/bpf/trampoline.c:1324
>
> Fix it by disallowing sleepable tracing prog always when its target btf
> is not kernel's btf.
>
> Fixes: 16d9c5660692 ("bpf: Always allow sleepable programs on syscalls")
> Acked-by: Viktor Malik <vmalik@redhat.com>
> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
> ---
>  kernel/bpf/verifier.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index b274004fccfd..7bb541e343b2 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -19019,6 +19019,9 @@ static int btf_id_allow_sleepable(u32 btf_id, unsigned long addr, const struct b
>
>         switch (prog->type) {
>         case BPF_PROG_TYPE_TRACING:
> +               if (!btf_is_kernel(btf))
> +                       return -EINVAL;
> +

see sashiko reply, just move it outside of switch and disallow
sleepable for anything that is not kernel/module BTF, regardless of
program type

pw-bot: cr


>                 t = btf_type_by_id(btf, btf_id);
>                 if (!t)
>                         return -EINVAL;
> --
> 2.55.0
>

Re: [PATCH bpf-next v3 1/2] bpf: Fix sleepable check for tracing prog

[Leon Hwang]

On 5/8/26 07:39, Andrii Nakryiko wrote:
> On Tue, Aug 4, 2026 at 7:57 AM Leon Hwang <leon.hwang@linux.dev> wrote:
[...]
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index b274004fccfd..7bb541e343b2 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -19019,6 +19019,9 @@ static int btf_id_allow_sleepable(u32 btf_id, unsigned long addr, const struct b
>>
>>         switch (prog->type) {
>>         case BPF_PROG_TYPE_TRACING:
>> +               if (!btf_is_kernel(btf))
>> +                       return -EINVAL;
>> +
> 
> see sashiko reply, just move it outside of switch and disallow
> sleepable for anything that is not kernel/module BTF, regardless of
> program type
> 

Ack.

I think another Fixes tag is needed for the LSM case.

Thanks,
Leon

[PATCH bpf-next v3 2/2] selftests/bpf: Verify rejection of sleepable tracing prog

[Leon Hwang]

Add a test to verify that the sleepable tracing prog cannot attach to a
'__x64_sys' prefix prog target.

When CONFIG_FUNCTION_ERROR_INJECTION is disabled, without the fix, the
test would trigger the BUG:

[    3.460756] BUG: sleeping function called from invalid context at kernel/bpf/trampoline.c:1324

Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 .../selftests/bpf/prog_tests/fexit_bpf2bpf.c  | 57 +++++++++++++++++++
 .../selftests/bpf/progs/fentry_sleepable.c    | 18 ++++++
 tools/testing/selftests/bpf/progs/xdp_dummy.c |  6 ++
 3 files changed, 81 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/fentry_sleepable.c

diff --git a/tools/testing/selftests/bpf/prog_tests/fexit_bpf2bpf.c b/tools/testing/selftests/bpf/prog_tests/fexit_bpf2bpf.c
index 4a87d7163c8c..2523c07a16c6 100644
--- a/tools/testing/selftests/bpf/prog_tests/fexit_bpf2bpf.c
+++ b/tools/testing/selftests/bpf/prog_tests/fexit_bpf2bpf.c
@@ -5,6 +5,7 @@
 #include <bpf/btf.h>
 #include "bind4_prog.skel.h"
 #include "freplace_progmap.skel.h"
+#include "fentry_sleepable.skel.h"
 #include "xdp_dummy.skel.h"
 
 typedef int (*test_cb)(struct bpf_object *obj);
@@ -576,6 +577,60 @@ static void test_func_replace_progmap(void)
 	freplace_progmap__destroy(skel);
 }
 
+static void test_sleepable_fentry_to_xdp(void)
+{
+	struct fentry_sleepable *skel = NULL;
+	struct xdp_dummy *skel_xdp = NULL;
+	int ifindex, prog_fd, err;
+	char buff[64] = {};
+
+#ifndef __x86_64__
+	test__skip();
+	return;
+#endif
+
+	ifindex = if_nametoindex("lo");
+	if (!ASSERT_GT(ifindex, 0, "if_nametoindex"))
+		return;
+
+	skel_xdp = xdp_dummy__open_and_load();
+	if (!ASSERT_OK_PTR(skel_xdp, "xdp_dummy__open_and_load"))
+		return;
+
+	skel = fentry_sleepable__open();
+	if (!ASSERT_OK_PTR(skel, "fentry_sleepable__open"))
+		goto out;
+
+	skel->bss->user_ptr = buff;
+
+	prog_fd = bpf_program__fd(skel_xdp->progs.__x64_sys_nop);
+	err = bpf_program__set_attach_target(skel->progs.fentry_xdp, prog_fd, "__x64_sys_nop");
+	if (!ASSERT_OK(err, "bpf_program__set_attach_target"))
+		goto out;
+
+	err = fentry_sleepable__load(skel);
+	ASSERT_ERR(err, "fentry_sleepable__load");
+	if (err)
+		goto out;
+
+	skel->links.fentry_xdp = bpf_program__attach_trace(skel->progs.fentry_xdp);
+	if (!ASSERT_OK_PTR(skel->links.fentry_xdp, "bpf_program__attach_trace"))
+		goto out;
+
+	skel_xdp->links.__x64_sys_nop = bpf_program__attach_xdp(skel_xdp->progs.__x64_sys_nop,
+								ifindex);
+	if (!ASSERT_OK_PTR(skel_xdp->links.__x64_sys_nop, "bpf_program__attach_xdp"))
+		goto out;
+
+	err = system("ping -q -c 1 -W 1 127.0.0.1 > /dev/null");
+	ASSERT_OK(err, "ping");
+	ASSERT_ERR(skel->bss->retval, "retval");
+
+out:
+	fentry_sleepable__destroy(skel);
+	xdp_dummy__destroy(skel_xdp);
+}
+
 /* NOTE: affect other tests, must run in serial mode */
 void serial_test_fexit_bpf2bpf(void)
 {
@@ -607,4 +662,6 @@ void serial_test_fexit_bpf2bpf(void)
 		test_func_replace_int_with_void();
 	if (test__start_subtest("freplace_void"))
 		test_func_replace_void();
+	if (test__start_subtest("sleepable_fentry_to_xdp"))
+		test_sleepable_fentry_to_xdp();
 }
diff --git a/tools/testing/selftests/bpf/progs/fentry_sleepable.c b/tools/testing/selftests/bpf/progs/fentry_sleepable.c
new file mode 100644
index 000000000000..8c0fc691d329
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/fentry_sleepable.c
@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+char LICENSE[] SEC("license") = "GPL";
+
+void *user_ptr;
+int retval;
+
+SEC("fentry.s")
+int BPF_PROG(fentry_xdp)
+{
+	char buff[64];
+
+	retval = bpf_copy_from_user(buff, sizeof(buff), user_ptr);
+	return 0;
+}
diff --git a/tools/testing/selftests/bpf/progs/xdp_dummy.c b/tools/testing/selftests/bpf/progs/xdp_dummy.c
index d988b2e0cee8..5f1e0771021d 100644
--- a/tools/testing/selftests/bpf/progs/xdp_dummy.c
+++ b/tools/testing/selftests/bpf/progs/xdp_dummy.c
@@ -10,4 +10,10 @@ int xdp_dummy_prog(struct xdp_md *ctx)
 	return XDP_PASS;
 }
 
+SEC("xdp")
+int __x64_sys_nop(struct xdp_md *ctx)
+{
+	return XDP_PASS;
+}
+
 char _license[] SEC("license") = "GPL";
-- 
2.55.0