[BUG] WARNING: at lib/vsprintf.c:2659 format_decode+0x121a/0x1c00

[Yeqi Fu]

Hi there,
A warning is triggered in lib/vsprintf.c due to an unsupported '%' in a format string. This issue occurs in the function format_decode at line 2659 of kernel version 6.12.0-rc3-gb22db8b8befe. A proof-of-concept is available, and I have manually reproduced this bug.

Report:
```
Please remove unsupported % in format string
WARNING: CPU: 1 PID: 29307 at lib/vsprintf.c:2659 format_decode+0x121a/0x1c00 lib/vsprintf.c:2659
Modules linked in:
CPU: 1 UID: 0 PID: 29307 Comm: syz.5.9298 Not tainted 6.12.0-rc3-gb22db8b8befe #2
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
RIP: 0010:format_decode+0x121a/0x1c00 lib/vsprintf.c:2659
Code: 8b 9c 24 80 00 00 00 48 89 d8 48 c1 e8 03 42 8a 04 30 84 c0 0f 85 d5 09 00 00 0f b6 33 48 c7 c7 00 bd eb 92 e8 b7 59 67 fc 90 <0f> 0b 90 90 4d 89 f7 48 8b 5c 24 18 e9 d7 fc ff ff 89 d1 80 e1 07
RSP: 0018:ffff888041197600 EFLAGS: 00010246
RAX: ea46d93351edcc00 RBX: ffff88804119792c RCX: ffff888009a78000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffff8880411976f0 R08: ffffffff8ebc8e3b R09: 1ffff1100d9e515a
R10: dffffc0000000000 R11: ffffed100d9e515b R12: ffff0000ffffff00
R13: ffff888041197700 R14: dffffc0000000000 R15: dffffc0000000000
FS: 00007fbe06321640(0000) GS:ffff88806cf00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020a8c000 CR3: 00000000404b6005 CR4: 0000000000370ef0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
bstr_printf+0x136/0x1260 lib/vsprintf.c:3232
____bpf_trace_printk kernel/trace/bpf_trace.c:389 [inline]
bpf_trace_printk+0x1a1/0x220 kernel/trace/bpf_trace.c:374
bpf_prog_7ee8fe4dad0c4460+0x4e/0x50
bpf_dispatcher_nop_func include/linux/bpf.h:1257 [inline]
__bpf_prog_run include/linux/filter.h:692 [inline]
bpf_prog_run include/linux/filter.h:708 [inline]
bpf_test_run+0x7a9/0x910 net/bpf/test_run.c:433
bpf_prog_test_run_skb+0xc47/0x1750 net/bpf/test_run.c:1094
bpf_prog_test_run+0x2df/0x350 kernel/bpf/syscall.c:4247
__sys_bpf+0x484/0x850 kernel/bpf/syscall.c:5652
__do_sys_bpf kernel/bpf/syscall.c:5741 [inline]
__se_sys_bpf kernel/bpf/syscall.c:5739 [inline]
__x64_sys_bpf+0x7c/0x90 kernel/bpf/syscall.c:5739
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xd8/0x1c0 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x67/0x6f
RIP: 0033:0x7fbe07ccd72d
Code: 02 b8 ff ff ff ff c3 66 0f 1f 44 00 00 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fbe06320f98 EFLAGS: 00000246 ORIG_RAX: 0000000000000141
RAX: ffffffffffffffda RBX: 00007fbe07ea5f80 RCX: 00007fbe07ccd72d
RDX: 0000000000000050 RSI: 0000000020000700 RDI: 000000000000000a
RBP: 00007fbe07d57584 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000000 R14: 00007fbe07ea5f80 R15: 00007fbe06301000
</TASK>
irq event stamp: 39314
hardirqs last enabled at (39324): [<ffffffff8ed766cb>] __up_console_sem kernel/printk/printk.c:344 [inline]
hardirqs last enabled at (39324): [<ffffffff8ed766cb>] __console_unlock+0xfb/0x130 kernel/printk/printk.c:2844
hardirqs last disabled at (39335): [<ffffffff8ed766b0>] __up_console_sem kernel/printk/printk.c:342 [inline]
hardirqs last disabled at (39335): [<ffffffff8ed766b0>] __console_unlock+0xe0/0x130 kernel/printk/printk.c:2844
softirqs last enabled at (38482): [<ffffffff9195aaea>] bpf_test_run+0x31a/0x910
softirqs last disabled at (38484): [<ffffffff9195aaea>] bpf_test_run+0x31a/0x910
---[ end trace 0000000000000000 ]---
```

Poc.c:
```
#define _GNU_SOURCE

#include <dirent.h>
#include <endian.h>
#include <errno.h>
#include <fcntl.h>
#include <signal.h>
#include <stdarg.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/prctl.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <time.h>
#include <unistd.h>

#ifndef __NR_bpf
#define __NR_bpf 321
#endif

static void sleep_ms(uint64_t ms)
{
usleep(ms * 1000);
}

static uint64_t current_time_ms(void)
{
struct timespec ts;
if (clock_gettime(CLOCK_MONOTONIC, &ts))
exit(1);
return (uint64_t)ts.tv_sec * 1000 + (uint64_t)ts.tv_nsec / 1000000;
}

static bool write_file(const char* file, const char* what, ...)
{
char buf[1024];
va_list args;
va_start(args, what);
vsnprintf(buf, sizeof(buf), what, args);
va_end(args);
buf[sizeof(buf) - 1] = 0;
int len = strlen(buf);
int fd = open(file, O_WRONLY | O_CLOEXEC);
if (fd == -1)
return false;
if (write(fd, buf, len) != len) {
int err = errno;
close(fd);
errno = err;
return false;
}
close(fd);
return true;
}

static void kill_and_wait(int pid, int* status)
{
kill(-pid, SIGKILL);
kill(pid, SIGKILL);
for (int i = 0; i < 100; i++) {
if (waitpid(-1, status, WNOHANG | __WALL) == pid)
return;
usleep(1000);
}
DIR* dir = opendir("/sys/fs/fuse/connections");
if (dir) {
for (;;) {
struct dirent* ent = readdir(dir);
if (!ent)
break;
if (strcmp(ent->d_name, ".") == 0 || strcmp(ent->d_name, "..") == 0)
continue;
char abort[300];
snprintf(abort, sizeof(abort), "/sys/fs/fuse/connections/%s/abort",
ent->d_name);
int fd = open(abort, O_WRONLY);
if (fd == -1) {
continue;
}
if (write(fd, abort, 1) < 0) {
}
close(fd);
}
closedir(dir);
} else {
}
while (waitpid(-1, status, __WALL) != pid) {
}
}

static void setup_test()
{
prctl(PR_SET_PDEATHSIG, SIGKILL, 0, 0, 0);
setpgrp();
write_file("/proc/self/oom_score_adj", "1000");
}

static void setup_sysctl()
{
int cad_pid = fork();
if (cad_pid < 0)
exit(1);
if (cad_pid == 0) {
for (;;)
sleep(100);
}
char tmppid[32];
snprintf(tmppid, sizeof(tmppid), "%d", cad_pid);
struct {
const char* name;
const char* data;
} files[] = {
{"/sys/kernel/debug/x86/nmi_longest_ns", "10000000000"},
{"/proc/sys/kernel/hung_task_check_interval_secs", "20"},
{"/proc/sys/net/core/bpf_jit_kallsyms", "1"},
{"/proc/sys/net/core/bpf_jit_harden", "0"},
{"/proc/sys/kernel/kptr_restrict", "0"},
{"/proc/sys/kernel/softlockup_all_cpu_backtrace", "1"},
{"/proc/sys/fs/mount-max", "100"},
{"/proc/sys/vm/oom_dump_tasks", "0"},
{"/proc/sys/debug/exception-trace", "0"},
{"/proc/sys/kernel/printk", "7 4 1 3"},
{"/proc/sys/kernel/keys/gc_delay", "1"},
{"/proc/sys/vm/oom_kill_allocating_task", "1"},
{"/proc/sys/kernel/ctrl-alt-del", "0"},
{"/proc/sys/kernel/cad_pid", tmppid},
};
for (size_t i = 0; i < sizeof(files) / sizeof(files[0]); i++) {
if (!write_file(files[i].name, files[i].data)) {
}
}
kill(cad_pid, SIGKILL);
while (waitpid(cad_pid, NULL, 0) != cad_pid)
;
}

static void execute_one(void);

#define WAIT_FLAGS __WALL

static void loop(void)
{
int iter = 0;
for (;; iter++) {
int pid = fork();
if (pid < 0)
exit(1);
if (pid == 0) {
setup_test();
execute_one();
exit(0);
}
int status = 0;
uint64_t start = current_time_ms();
for (;;) {
sleep_ms(10);
if (waitpid(-1, &status, WNOHANG | WAIT_FLAGS) == pid)
break;
if (current_time_ms() - start < 5000)
continue;
kill_and_wait(pid, &status);
break;
}
}
}

uint64_t r[1] = {0xffffffffffffffff};

void execute_one(void)
{
intptr_t res = 0;
if (write(1, "executing program\n", sizeof("executing program\n") - 1)) {
}
*(uint32_t*)0x200013c0 = 4;
*(uint32_t*)0x200013c4 = 0xc;
*(uint64_t*)0x200013c8 = 0x20000600;
memcpy(
(void*)0x20000600,
"\x18\x00\x00\x00\x09\x00\x00\x00\x00\x00\x00\x00\xff\xff\xff\xff\x85\x00"
"\x00\x00\x07\x00\x00\x00\x18\x01\x00\x00\x78\x6c\x6c\x25\x00\x00\x00\x00"
"\x00\x20\x20\x20\x7b\x0a\xf8\xff\x00\x00\x00\x00\xbf\xa1\x00\x00\x00\x00"
"\x00\x00\x07\x01\x00\x00\xf8\xff\xff\xff\xb7\x02\x00\x00\x08\x00\x00\x00"
"\xb7\x03\x00\x00\x05\x00\x00\x00\x85\x00\x00\x00\x06\x00\x00\x00\x95",
89);
*(uint64_t*)0x200013d0 = 0x20000200;
memcpy((void*)0x20000200, "GPL\000", 4);
*(uint32_t*)0x200013d8 = 9;
*(uint32_t*)0x200013dc = 0;
*(uint64_t*)0x200013e0 = 0;
*(uint32_t*)0x200013e8 = 0x41000;
*(uint32_t*)0x200013ec = 0;
memset((void*)0x200013f0, 0, 16);
*(uint32_t*)0x20001400 = 0;
*(uint32_t*)0x20001404 = 6;
*(uint32_t*)0x20001408 = -1;
*(uint32_t*)0x2000140c = 0;
*(uint64_t*)0x20001410 = 0;
*(uint32_t*)0x20001418 = 0;
*(uint32_t*)0x2000141c = 0;
*(uint64_t*)0x20001420 = 0;
*(uint32_t*)0x20001428 = 0;
*(uint32_t*)0x2000142c = 0;
*(uint32_t*)0x20001430 = 0;
*(uint32_t*)0x20001434 = 0;
*(uint64_t*)0x20001438 = 0;
*(uint64_t*)0x20001440 = 0;
*(uint32_t*)0x20001448 = 0;
*(uint32_t*)0x2000144c = 1;
*(uint32_t*)0x20001450 = 0;
res = syscall(__NR_bpf, /*cmd=*/5ul, /*arg=*/0x200013c0ul, /*size=*/0x94ul);
if (res != -1)
r[0] = res;
syscall(__NR_bpf, /*cmd=*/0ul, /*arg=*/0ul, /*size=*/0x48ul);
*(uint32_t*)0x20000700 = r[0];
*(uint32_t*)0x20000704 = 0;
*(uint32_t*)0x20000708 = 0xe;
*(uint32_t*)0x2000070c = 0;
*(uint64_t*)0x20000710 = 0x20000300;
memcpy((void*)0x20000300,
"\x8f\x41\x3f\x07\x00\x00\x00\x7e\xfb\xe3\xd9\xb2\x48\xb6", 14);
*(uint64_t*)0x20000718 = 0;
*(uint32_t*)0x20000720 = 0x2a12;
*(uint32_t*)0x20000724 = 0;
*(uint32_t*)0x20000728 = 0;
*(uint32_t*)0x2000072c = 0;
*(uint64_t*)0x20000730 = 0;
*(uint64_t*)0x20000738 = 0;
*(uint32_t*)0x20000740 = 0;
*(uint32_t*)0x20000744 = 0;
*(uint32_t*)0x20000748 = 0;
syscall(__NR_bpf, /*cmd=*/0xaul, /*arg=*/0x20000700ul, /*size=*/0x50ul);
}
int main(void)
{
syscall(__NR_mmap, /*addr=*/0x1ffff000ul, /*len=*/0x1000ul, /*prot=*/0ul,
/*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/ 0x32ul, /*fd=*/-1,
/*offset=*/0ul);
syscall(__NR_mmap, /*addr=*/0x20000000ul, /*len=*/0x1000000ul,
/*prot=PROT_WRITE|PROT_READ|PROT_EXEC*/ 7ul,
/*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/ 0x32ul, /*fd=*/-1,
/*offset=*/0ul);
syscall(__NR_mmap, /*addr=*/0x21000000ul, /*len=*/0x1000ul, /*prot=*/0ul,
/*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/ 0x32ul, /*fd=*/-1,
/*offset=*/0ul);
setup_sysctl();
const char* reason;
(void)reason;
loop();
return 0;
}
```

[Yonghong Song]

On 11/8/24 6:28 AM, Yeqi Fu wrote:
> Hi there,
> A warning is triggered in lib/vsprintf.c due to an unsupported '%' in a format string. This issue occurs in the function format_decode at line 2659 of kernel version 6.12.0-rc3-gb22db8b8befe. A proof-of-concept is available, and I have manually reproduced this bug.

I think the below patch set (not merged yet)
https://lore.kernel.org/bpf/20241028195343....@mail.ru/
should fix this issue.

[...]