[PATCH bpf v3 0/2] bpf: enhance validation of pointer formatting

[PATCH bpf v3 0/2] bpf: enhance validation of pointer formatting

[Ilya Shchipletsov]

This patch series enhances validation of pointer formatting to prevent same
exact issue happening again, as it happen before in [0] and happened now.

[0]: https://lkml.kernel.org/netdev/85a08645-453b-78ad-e401-55d2894fa64a@iogearbox.net/T/

Co-developed-by: Nikita Marushkin <hfggklm@gmail.com>
Signed-off-by: Nikita Marushkin <hfggklm@gmail.com>
Signed-off-by: Ilya Shchipletsov <rabbelkin@mail.ru>
---
Changes in v3:
- Resend due to odd "Changes Requested" status in Patchwork, although
  there've been no additional comments posted on mailing lists explaining
  that status.
- Add Yonghong's and Florent's ACKs.
- Link to v2: https://lore.kernel.org/bpf/20241028195343.2104-1-rabbelkin@mail.ru/
Changes in v2:
- Added Reported-by for syzbot [Florent Revest]
- Added negative tests for snprintf [Florent Revest]
- Moved comment to first 'if' statement [Yonghong Song]
- Link to v1: https://lore.kernel.org/bpf/9679a031-3858-4fef-bb8e-1cf436696095@mail.ru/
---
Ilya Shchipletsov (2):
  bpf: fix %p% runtime check in bpf_bprintf_prepare
  selftests/bpf: Add test cases for various pointer specifiers

 kernel/bpf/helpers.c                              | 13 +++++++++----
 tools/testing/selftests/bpf/prog_tests/snprintf.c | 15 +++++++++++++++
 2 files changed, 24 insertions(+), 4 deletions(-)
-- 
2.43.0

[PATCH bpf v3 1/2] bpf: fix %p% runtime check in bpf_bprintf_prepare

[Ilya Shchipletsov]

Fuzzing reports a warning in format_decode()

Please remove unsupported %� in format string
WARNING: CPU: 0 PID: 5091 at lib/vsprintf.c:2680 format_decode+0x1193/0x1bb0 lib/vsprintf.c:2680
Modules linked in:
CPU: 0 PID: 5091 Comm: syz-executor879 Not tainted 6.10.0-rc1-syzkaller-00021-ge0cce98fe279 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/02/2024
RIP: 0010:format_decode+0x1193/0x1bb0 lib/vsprintf.c:2680
Call Trace:
 <TASK>
 bstr_printf+0x137/0x1210 lib/vsprintf.c:3253
 ____bpf_trace_printk kernel/trace/bpf_trace.c:390 [inline]
 bpf_trace_printk+0x1a1/0x230 kernel/trace/bpf_trace.c:375
 bpf_prog_21da1b68f62e1237+0x36/0x41
 bpf_dispatcher_nop_func include/linux/bpf.h:1243 [inline]
 __bpf_prog_run include/linux/filter.h:691 [inline]
 bpf_prog_run include/linux/filter.h:698 [inline]
 bpf_test_run+0x40b/0x910 net/bpf/test_run.c:425
 bpf_prog_test_run_skb+0xafa/0x13a0 net/bpf/test_run.c:1066
 bpf_prog_test_run+0x33c/0x3b0 kernel/bpf/syscall.c:4291
 __sys_bpf+0x48d/0x810 kernel/bpf/syscall.c:5705
 __do_sys_bpf kernel/bpf/syscall.c:5794 [inline]
 __se_sys_bpf kernel/bpf/syscall.c:5792 [inline]
 __x64_sys_bpf+0x7c/0x90 kernel/bpf/syscall.c:5792
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

The problem occurs when trying to pass %p% at the end of format string,
which would result in skipping last % and passing invalid format string
down to format_decode() that would cause warning because of invalid
character after %.

Fix issue by advancing pointer only if next char is format modifier.
If next char is null/space/punct, then just accept formatting as is,
without advancing the pointer.

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.

Reported-by: syzbot+e2c932aec5c8a6e1d31c@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=e2c932aec5c8a6e1d31c
Fixes: 48cac3f4a96d ("bpf: Implement formatted output helpers with bstr_printf")
Co-developed-by: Nikita Marushkin <hfggklm@gmail.com>
Signed-off-by: Nikita Marushkin <hfggklm@gmail.com>
Signed-off-by: Ilya Shchipletsov <rabbelkin@mail.ru>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Florent Revest <revest@chromium.org>
---
 kernel/bpf/helpers.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 1a43d06eab28..3efa8b04999a 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -889,10 +889,8 @@ int bpf_bprintf_prepare(char *fmt, u32 fmt_size, const u64 *raw_args,
 				goto fmt_str;
 			}
 
-			if (fmt[i + 1] == 0 || isspace(fmt[i + 1]) ||
-			    ispunct(fmt[i + 1]) || fmt[i + 1] == 'K' ||
-			    fmt[i + 1] == 'x' || fmt[i + 1] == 's' ||
-			    fmt[i + 1] == 'S') {
+			if (fmt[i + 1] == 'K' || fmt[i + 1] == 'x' ||
+			    fmt[i + 1] == 's' || fmt[i + 1] == 'S') {
 				/* just kernel pointers */
 				if (tmp_buf)
 					cur_arg = raw_args[num_spec];
@@ -900,6 +898,13 @@ int bpf_bprintf_prepare(char *fmt, u32 fmt_size, const u64 *raw_args,
 				goto nocopy_fmt;
 			}
 
+			if (fmt[i + 1] == 0 || isspace(fmt[i + 1]) ||
+			    ispunct(fmt[i + 1])) {
+				if (tmp_buf)
+					cur_arg = raw_args[num_spec];
+				goto nocopy_fmt;
+			}
+
 			if (fmt[i + 1] == 'B') {
 				if (tmp_buf)  {
 					err = snprintf(tmp_buf,
-- 
2.43.0

[PATCH bpf v3 2/2] selftests/bpf: Add test cases for various pointer specifiers

[Ilya Shchipletsov]

Extend snprintf negative tests to cover pointer specifiers to prevent
possible invalid handling of %p% from happening again.

 ./test_progs -t snprintf
 #302/1   snprintf/snprintf_positive:OK
 #302/2   snprintf/snprintf_negative:OK
 #302     snprintf:OK
 #303     snprintf_btf:OK
 Summary: 2/2 PASSED, 0 SKIPPED, 0 FAILED

Co-developed-by: Nikita Marushkin <hfggklm@gmail.com>
Signed-off-by: Nikita Marushkin <hfggklm@gmail.com>
Signed-off-by: Ilya Shchipletsov <rabbelkin@mail.ru>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Florent Revest <revest@chromium.org>
---
 tools/testing/selftests/bpf/prog_tests/snprintf.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/snprintf.c b/tools/testing/selftests/bpf/prog_tests/snprintf.c
index 4be6fdb78c6a..b5b6371e09bb 100644
--- a/tools/testing/selftests/bpf/prog_tests/snprintf.c
+++ b/tools/testing/selftests/bpf/prog_tests/snprintf.c
@@ -116,6 +116,21 @@ static void test_snprintf_negative(void)
 	ASSERT_ERR(load_single_snprintf("%llc"), "invalid specifier 7");
 	ASSERT_ERR(load_single_snprintf("\x80"), "non ascii character");
 	ASSERT_ERR(load_single_snprintf("\x1"), "non printable character");
+
+	ASSERT_OK(load_single_snprintf("valid %p"), "valid usage");
+
+	ASSERT_ERR(load_single_snprintf("%p%"), "too many specifiers 1");
+	ASSERT_ERR(load_single_snprintf("%pK%"), "too many specifiers 2");
+	ASSERT_ERR(load_single_snprintf("%px%"), "too many specifiers 3");
+	ASSERT_ERR(load_single_snprintf("%ps%"), "too many specifiers 4");
+	ASSERT_ERR(load_single_snprintf("%pS%"), "too many specifiers 5");
+	ASSERT_ERR(load_single_snprintf("%pB%"), "too many specifiers 6");
+	ASSERT_ERR(load_single_snprintf("%pi4%"), "too many specifiers 7");
+	ASSERT_ERR(load_single_snprintf("%pI4%"), "too many specifiers 8");
+	ASSERT_ERR(load_single_snprintf("%pi6%"), "too many specifiers 9");
+	ASSERT_ERR(load_single_snprintf("%pI6%"), "too many specifiers 10");
+	ASSERT_ERR(load_single_snprintf("%pks%"), "too many specifiers 11");
+	ASSERT_ERR(load_single_snprintf("%pus%"), "too many specifiers 12");
 }
 
 void test_snprintf(void)
-- 
2.43.0