[PATCH bpf-next v3] bpf: fix out-of-bounds write in bpf_bprintf_prepare with %pI4/%pI6

[PATCH bpf-next v3] bpf: fix out-of-bounds write in bpf_bprintf_prepare with %pI4/%pI6

[Ibrahim Zein]

In bpf_bprintf_prepare(), the bounds check for %pI4 and %pI6 format
specifiers uses sizeof_cur_ip (4 for IPv4, 16 for IPv6), which is the
raw byte count of the IP address. However, snprintf() returns the
length of the formatted string, not the raw bytes. For IPv4 this can
be up to 15 characters (255.255.255.255) and for IPv6 up to 39.

tmp_buf is then advanced by (err + 1) using the full string length,
which can push tmp_buf past tmp_buf_end. The next iteration's bounds
check underflows due to unsigned arithmetic and passes, allowing a
write past the end of the per-CPU bin_args buffer.

Fix this by checking against the maximum formatted string size:
16 bytes for IPv4 and 40 bytes for IPv6.

Fixes: 48cac3f4a96d ("bpf: Implement formatted output helpers with bstr_printf")
Signed-off-by: Ibrahim Zein <zeroxjacks@gmail.com>
---
 kernel/bpf/helpers.c                          |  2 +-
 .../bpf/prog_tests/test_snprintf_ip.c         | 54 +++++++++++++
 .../selftests/bpf/progs/test_snprintf_ip.c    | 78 +++++++++++++++++++
 3 files changed, 133 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/test_snprintf_ip.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_snprintf_ip.c

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index cb6d242bd..dcaa822ba 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -930,7 +930,7 @@ int bpf_bprintf_prepare(const char *fmt, u32 fmt_size, const u64 *raw_args,
 				goto nocopy_fmt;
 
 			sizeof_cur_ip = (fmt[i] == '4') ? 4 : 16;
-			if (tmp_buf_end - tmp_buf < sizeof_cur_ip) {
+			if (tmp_buf_end - tmp_buf < (size_t)((fmt[i] == '4') ? 16 : 40)) {
 				err = -ENOSPC;
 				goto out;
 			}
diff --git a/tools/testing/selftests/bpf/prog_tests/test_snprintf_ip.c b/tools/testing/selftests/bpf/prog_tests/test_snprintf_ip.c
new file mode 100644
index 000000000..5b000d6d1
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/test_snprintf_ip.c
@@ -0,0 +1,54 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Ibrahim Zein <zeroxjacks@gmail.com> */
+#include <test_progs.h>
+#include "test_snprintf_ip.skel.h"
+
+#define EXP_IP4_OUT  "192.168.1.1"
+#define EXP_IP4_RET  sizeof(EXP_IP4_OUT)
+
+#define EXP_WORST_IP4_OUT  "255.255.255.255"
+#define EXP_WORST_IP4_RET  sizeof(EXP_WORST_IP4_OUT)
+
+#define EXP_IP6_OUT  "ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff"
+#define EXP_IP6_RET  sizeof(EXP_IP6_OUT)
+
+void test_snprintf_ip(void)
+{
+	struct test_snprintf_ip *skel;
+
+	skel = test_snprintf_ip__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel_open"))
+		return;
+
+	skel->bss->pid = getpid();
+
+	if (!ASSERT_OK(test_snprintf_ip__attach(skel), "skel_attach"))
+		goto cleanup;
+
+	/* trigger tracepoint */
+	usleep(1);
+
+	/* Test 1: normal IPv4 */
+	ASSERT_STREQ(skel->bss->ip4_out, EXP_IP4_OUT, "ip4_out");
+	ASSERT_EQ(skel->bss->ip4_ret, EXP_IP4_RET, "ip4_ret");
+
+	/* Test 2: normal IPv6 */
+	ASSERT_STREQ(skel->bss->ip6_out, EXP_IP6_OUT, "ip6_out");
+	ASSERT_EQ(skel->bss->ip6_ret, EXP_IP6_RET, "ip6_ret");
+
+	/* Test 3: worst-case IPv4 "255.255.255.255" */
+	ASSERT_STREQ(skel->bss->worst_ip4_out, EXP_WORST_IP4_OUT,
+		     "worst_ip4_out");
+	ASSERT_EQ(skel->bss->worst_ip4_ret, EXP_WORST_IP4_RET,
+		  "worst_ip4_ret");
+
+	/*
+	 * Test 4: near-overflow scenario.
+	 * Before fix: tmp_buf overflows, kernel may crash or corrupt memory.
+	 * After fix: returns valid length without overflow.
+	 */
+	ASSERT_GE(skel->bss->near_overflow_ret, 0, "near_overflow_ret");
+
+cleanup:
+	test_snprintf_ip__destroy(skel);
+}
diff --git a/tools/testing/selftests/bpf/progs/test_snprintf_ip.c b/tools/testing/selftests/bpf/progs/test_snprintf_ip.c
new file mode 100644
index 000000000..a372b35c3
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_snprintf_ip.c
@@ -0,0 +1,78 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Ibrahim Zein <zeroxjacks@gmail.com> */
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+/*
+ * Test that bpf_snprintf() with %pI4/%pI6 does not overflow the
+ * internal bin_args buffer when the buffer is nearly full.
+ *
+ * The bug: sizeof_cur_ip (4 for IPv4) was used for the bounds check,
+ * but snprintf() returns the full formatted string length (up to 15
+ * for "255.255.255.255"), pushing tmp_buf past tmp_buf_end.
+ */
+
+/* Output buffers */
+char ip4_out[64] = {};
+long ip4_ret = 0;
+
+char ip6_out[128] = {};
+long ip6_ret = 0;
+
+/* Test %pI4 with worst-case IP (255.255.255.255) */
+char worst_ip4_out[64] = {};
+long worst_ip4_ret = 0;
+
+/* Return value for the near-overflow test */
+long near_overflow_ret = 0;
+
+__u32 pid = 0;
+
+SEC("raw_tp/sys_enter")
+int handler(const void *ctx)
+{
+	/* Worst-case IPv4: "255.255.255.255" = 15 chars */
+	const __u8 ip4_worst[] = {255, 255, 255, 255};
+	/* Normal IPv4 */
+	const __u8 ip4_normal[] = {192, 168, 1, 1};
+	/* IPv6 worst-case */
+	const __u8 ip6_worst[] = {0xff, 0xff, 0xff, 0xff,
+				   0xff, 0xff, 0xff, 0xff,
+				   0xff, 0xff, 0xff, 0xff,
+				   0xff, 0xff, 0xff, 0xff};
+
+	if ((int)bpf_get_current_pid_tgid() != pid)
+		return 0;
+
+	/* Test 1: normal %pI4 usage */
+	ip4_ret = BPF_SNPRINTF(ip4_out, sizeof(ip4_out),
+			       "%pI4", &ip4_normal);
+
+	/* Test 2: normal %pI6 usage */
+	ip6_ret = BPF_SNPRINTF(ip6_out, sizeof(ip6_out),
+			       "%pI6", &ip6_worst);
+
+	/* Test 3: worst-case %pI4 "255.255.255.255" */
+	worst_ip4_ret = BPF_SNPRINTF(worst_ip4_out,
+				     sizeof(worst_ip4_out),
+				     "%pI4", &ip4_worst);
+
+	/*
+	 * Test 4: near-overflow scenario.
+	 * Fill bin_args with scalar args (%d), then use %pI4.
+	 * Before the fix: tmp_buf overflows by 8 bytes.
+	 * After the fix: correctly returns -ENOSPC or fits exactly.
+	 *
+	 * We use fewer args here since BPF_SNPRINTF macro has a limit,
+	 * but this validates the format string parsing path.
+	 */
+	/* 11 x %d + 1 x %pI4 = 12 args (BPF_SNPRINTF max) */
+	near_overflow_ret = BPF_SNPRINTF(NULL, 0,
+					  "%d %d %d %d %d %d %d %d %d %d %d %pI4",
+					  1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,
+					  &ip4_worst);
+
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.52.0

Re: [PATCH bpf-next v3] bpf: fix out-of-bounds write in bpf_bprintf_prepare with %pI4/%pI6

[Paul Chaignon]

On Thu, Mar 19, 2026 at 12:49:00PM +0000, Ibrahim Zein wrote:
> In bpf_bprintf_prepare(), the bounds check for %pI4 and %pI6 format
> specifiers uses sizeof_cur_ip (4 for IPv4, 16 for IPv6), which is the
> raw byte count of the IP address. However, snprintf() returns the
> length of the formatted string, not the raw bytes. For IPv4 this can
> be up to 15 characters (255.255.255.255) and for IPv6 up to 39.
> 
> tmp_buf is then advanced by (err + 1) using the full string length,
> which can push tmp_buf past tmp_buf_end. The next iteration's bounds
> check underflows due to unsigned arithmetic and passes, allowing a
> write past the end of the per-CPU bin_args buffer.
> 
> Fix this by checking against the maximum formatted string size:
> 16 bytes for IPv4 and 40 bytes for IPv6.
> 
> Fixes: 48cac3f4a96d ("bpf: Implement formatted output helpers with bstr_printf")
> Signed-off-by: Ibrahim Zein <zeroxjacks@gmail.com>
> ---

Please include a changelog whenever sending a new version. That's
typically the first thing I look at, to get an idea of the current
state.

>  kernel/bpf/helpers.c                          |  2 +-
>  .../bpf/prog_tests/test_snprintf_ip.c         | 54 +++++++++++++
>  .../selftests/bpf/progs/test_snprintf_ip.c    | 78 +++++++++++++++++++
>  3 files changed, 133 insertions(+), 1 deletion(-)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/test_snprintf_ip.c
>  create mode 100644 tools/testing/selftests/bpf/progs/test_snprintf_ip.c
> 
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index cb6d242bd..dcaa822ba 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -930,7 +930,7 @@ int bpf_bprintf_prepare(const char *fmt, u32 fmt_size, const u64 *raw_args,
>  				goto nocopy_fmt;
>  
>  			sizeof_cur_ip = (fmt[i] == '4') ? 4 : 16;
> -			if (tmp_buf_end - tmp_buf < sizeof_cur_ip) {
> +			if (tmp_buf_end - tmp_buf < (size_t)((fmt[i] == '4') ? 16 : 40)) {

Maybe this code could be improved, but I don't think it's going to cause
an out-of-bounds write as the write into tmp_buf is done with snprintf.

>  				err = -ENOSPC;
>  				goto out;
>  			}
> diff --git a/tools/testing/selftests/bpf/prog_tests/test_snprintf_ip.c b/tools/testing/selftests/bpf/prog_tests/test_snprintf_ip.c
> new file mode 100644
> index 000000000..5b000d6d1
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/test_snprintf_ip.c

This new test is always passing.

In addition, when adding selftests that cover your changes, it's
preferred to introduce them in a new patch.

[...]

Re: [PATCH bpf-next v3] bpf: fix out-of-bounds write in bpf_bprintf_prepare with %pI4/%pI6

[Ibrahim Zein]

Hi Paul,

Apologies for dropping the list from CC, I will keep everyone in copy.

Regarding the (size_t) cast: agreed, I will remove it in v4.

Regarding the test: you are right. BPF_SNPRINTF is limited to 12
arguments, which is not enough to fill 512 bytes of bin_args with
scalar args alone to trigger the overflow path.

Would it be acceptable to keep the test as a regression test for
correct %pI4/%pI6 formatting, or should I remove it entirely?

Best regards,
Ibrahim Zein