Re: [PATCH -next 3/5] ceph: fix possible null-ptr-deref when parsing param

From: Xiubo Li
Date: Sun Oct 23 2022 - 22:17:27 EST


On 24/10/2022 10:04, Hawkins Jiawei wrote:
Hi Xiubo,
On Mon, 24 Oct 2022 at 08:55, Xiubo Li <xiubli@xxxxxxxxxx> wrote:

On 24/10/2022 00:39, Hawkins Jiawei wrote:
According to commit "vfs: parse: deal with zero length string value",
kernel will set the param->string to null pointer in vfs_parse_fs_string()
if fs string has zero length.

Yet the problem is that, ceph_parse_mount_param() will dereferences the
param->string, without checking whether it is a null pointer, which may
trigger a null-ptr-deref bug.

This patch solves it by adding sanity check on param->string
in ceph_parse_mount_param().

Signed-off-by: Hawkins Jiawei <yin31149@xxxxxxxxx>
---
fs/ceph/super.c | 3 +++
1 file changed, 3 insertions(+)

diff --git a/fs/ceph/super.c b/fs/ceph/super.c
index 3fc48b43cab0..341e23fe29eb 100644
--- a/fs/ceph/super.c
+++ b/fs/ceph/super.c
@@ -417,6 +417,9 @@ static int ceph_parse_mount_param(struct fs_context *fc,
param->string = NULL;
break;
case Opt_mds_namespace:
+ if (!param->string)
+ return invalfc(fc, "Bad value '%s' for mount option '%s'\n",
+ param->string, param->key);
if (!namespace_equals(fsopt, param->string, strlen(param->string)))
return invalfc(fc, "Mismatching mds_namespace");
kfree(fsopt->mds_namespace);
BTW, did you hit any crash issue when testing this ?

$ ./bin/mount.ceph :/ /mnt/kcephfs -o mds_namespace=

<5>[ 375.535442] ceph: module verification failed: signature and/or
required key missing - tainting kernel
<6>[ 375.698145] ceph: loaded (mds proto 32)
<3>[ 375.801621] ceph: Bad value for 'mds_namespace'

From my test, the 'fsparam_string()' has already make sure it won't
trigger the null-ptr-deref bug.
Did you test on linux-next tree?

No, I am using the ceph-client repo for ceph code developing.


I just write a reproducer based on syzkaller's template(So please
forgive me if it is too ugly to read)

===========================================================
// https://syzkaller.appspot.com/bug?id=76bbdfd28722f0160325e4350b57e33aa95b0bbe
// autogenerated by syzkaller (https://github.com/google/syzkaller)

#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>

unsigned long long procid;

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);
int i;
for (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 execute_one(void);

#define WAIT_FLAGS __WALL

static void loop(void)
{
int iter;
for (iter = 0;; 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 (;;) {
if (waitpid(-1, &status, WNOHANG | WAIT_FLAGS) == pid)
break;
sleep_ms(1);
if (current_time_ms() - start < 5 * 1000)
continue;
kill_and_wait(pid, &status);
break;
}
}
}

void execute_one(void)
{
char opt[] = "mds_namespace=,\x00";
memcpy((void*)0x20000080, "./file0\000", 8);
syscall(__NR_mknod, 0x20000080ul, 0ul, 0x700ul + procid * 2);
memcpy((void*)0x20000040, "[d::]:/8:", 9);
memcpy((void*)0x200000c0, "./file0\000", 8);
memcpy((void*)0x20000140, "ceph\000", 5);
memcpy((void*)0x20000150, opt, sizeof(opt));
syscall(__NR_mount, 0x20000040ul, 0x200000c0ul, 0x20000140ul, 0ul, 0x20000150);
}
int main(void)
{
syscall(__NR_mmap, 0x20000000ul, 0x1000000ul, 3ul, 0x32ul, -1, 0);
for (procid = 0; procid < 6; procid++) {
if (fork() == 0) {
loop();
}
}
sleep(1000000);
return 0;
}
===========================================================

And it triggers the null-ptr-deref bug described above,
its log is shown as below:
===========================================================
[ 90.779695][ T6513] KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
[ 90.782502][ T6513] RIP: 0010:strlen+0x1a/0x90
[ ... ]
[ 90.782502][ T6513] Call Trace:
[ 90.782502][ T6513] <TASK>
[ 90.782502][ T6513] ceph_parse_mount_param+0x89a/0x21e0
[ 90.782502][ T6513] ? __kasan_unpoison_range-0xf/0x10
[ 90.782502][ T6513] ? kasan_addr_to_slab-0xf/0x90
[ 90.782502][ T6513] ? __sanitizer_cov_trace_pc+0x1a/0x40
[ 90.782502][ T6513] ? ceph_parse_mount_param+0x0/0x21e0
[ 90.782502][ T6513] ? audit_kill_trees+0x2b0/0x300
[ 90.782502][ T6513] ? lock_release+0x0/0x760
[ 90.782502][ T6513] ? __sanitizer_cov_trace_pc+0x1a/0x40
[ 90.782502][ T6513] ? security_fs_context_parse_param+0x99/0xd0
[ 90.782502][ T6513] ? ceph_parse_mount_param+0x0/0x21e0
[ 90.782502][ T6513] vfs_parse_fs_param+0x20f/0x3d0
[ 90.782502][ T6513] vfs_parse_fs_string+0xe4/0x180
[ 90.782502][ T6513] ? vfs_parse_fs_string+0x0/0x180
[ 90.782502][ T6513] ? rcu_read_lock_sched_held+0x0/0xd0
[ 90.782502][ T6513] ? __sanitizer_cov_trace_pc+0x1a/0x40
[ 90.782502][ T6513] ? kfree+0x129/0x1a0
[ 90.782502][ T6513] ? __sanitizer_cov_trace_pc+0x1a/0x40
[ 90.782502][ T6513] ? __sanitizer_cov_trace_pc+0x1a/0x40
[ 90.782502][ T6513] generic_parse_monolithic+0x16f/0x1f0
[ 90.782502][ T6513] ? generic_parse_monolithic+0x0/0x1f0
[ 90.782502][ T6513] ? __sanitizer_cov_trace_pc+0x1a/0x40
[ 90.782502][ T6513] ? alloc_fs_context+0x5cb/0xa00
[ 90.782502][ T6513] path_mount+0x11d3/0x1cb0
[ 90.782502][ T6513] ? path_mount+0x0/0x1cb0
[ 90.782502][ T6513] ? putname+0xfe/0x140
[ 90.782502][ T6513] do_mount+0xf3/0x110
[ 90.782502][ T6513] ? do_mount+0x0/0x110
[ 90.782502][ T6513] ? _copy_from_user+0xf7/0x170
[ 90.782502][ T6513] ? __sanitizer_cov_trace_pc+0x1a/0x40
[ 90.782502][ T6513] __x64_sys_mount+0x18f/0x230
[ 90.782502][ T6513] do_syscall_64+0x35/0xb0
[ 90.782502][ T6513] entry_SYSCALL_64_after_hwframe+0x63/0xcd
[ ... ]
[ 90.782502][ T6513] </TASK>
===========================================================

By the way, commit "vfs: parse: deal with zero length string value"
is still in discussion as below, so maybe this patchset is not
needed.
https://lore.kernel.org/all/17a1fdc-14a0-cf3c-784f-baa939895aef@xxxxxxxxxx/

Okay, It's said that breaking commit will be reverted. Let's wait for a while to see what will it be.

Thanks!

- Xiubo

But it will always make sense to fix it in ceph code with your patch.

- Xiubo