ksmbd - Fuzzing Improvements and Vulnerability Discovery (2/3)
02 Sep 2025 - Posted by Norbert SzeteiIntroduction
This is a follow-up to the article originally published here.
Our initial research uncovered several unauthenticated bugs, but we had only touched the attack surface lightly. Even after patching the code to bypass authentication, most interesting operations required interacting with handlers and state we initially omitted. In this part, we explain how we increased coverage and applied different fuzzing strategies to identify more bugs.
Some functionalities require additional configuration options. We tried to enable many available features to maximize the exposed attack surface. This helped us trigger code paths that are disabled in the minimalistic configuration example. However, to simplify our setup, we did not consider features like Kerberos support or RDMA. These could be targets for further improvement.
Configuration-Dependent Attack Surface
The following functionalities helped expand the attack surface. Only oplocks are enabled by default.
G = Global scope only
S = Per-share, but can also be set globally as a default
- durable handles (G)
- oplocks (S)
- server multi channel support (G)
- smb2 leases (G)
- vfs objects (S)
From a code perspective, in addition to smb2pdu.c, these source files were involved:
ndr.c– NDR encoding/decoding used in SMB structuresoplock.c– Oplock request and break handlingsmbacl.c– Parsing and enforcement of SMB ACLsvfs.c– Interface to virtual file system operationsvfs_cache.c– Cache layer for file and directory lookups
The remaining files in the fs/smb/server directory were either part of standard communication or exercising them required a more complex setup, as in the case of various authentication schemes.
Fuzzer Improvements
SMB3 expects a valid session setup before most operations, and its authentication flow is multi-step, requiring correct ordering. Implementing valid Kerberos authentication was impractical for fuzzing.
As described in the first part, we patched the NTLMv2 authentication to be able to interact with resources. We also explicitly allowed guest accounts and specified map to guest = bad user to allow a fallback to “guest” when credentials were invalid. After reporting CVE-2024-50285: ksmbd: check outstanding simultaneous SMB operations, credit limitations became more strict, so we patched that out as well to avoid rate limiting.
When we restarted syzkaller with a larger corpus, a few minutes later, all remaining candidates were rejected. After some investigation, we realized it was due to the default max connections = 128, which we had to increase to the maximum value 65536. No other limits were changed.
State Management
SMB interactions are stateful, relying on sessions, TreeIDs, and FileIDs. Fuzzing required simulating valid transitions like smb2_create ⇢ smb2_ioctl ⇢ smb2_close. When we initiated operations such as smb2_tree_connect, smb2_sess_setup, or smb2_create, we manually parsed responses in the pseudo-syscall to extract resource identifiers and reused them in subsequent calls. Our harness was programmed to send multiple messages per pseudo-syscall.
Example code for resources parsing is displayed below:
// process response. does not contain +4B PDU length
void process_buffer(int msg_no, const char *buffer, size_t received) {
// .. snip ..
// Extract SMB2 command
uint16_t cmd_rsp = u16((const uint8_t *)(buffer + CMD_OFFSET));
debug("Response command: 0x%04x\n", cmd_rsp);
switch (cmd_rsp) {
case SMB2_TREE_CONNECT:
if (received >= TREE_ID_OFFSET + sizeof(uint32_t)) {
tree_id = u32((const uint8_t *)(buffer + TREE_ID_OFFSET));
debug("Obtained tree_id: 0x%x\n", tree_id);
}
break;
case SMB2_SESS_SETUP:
// First session setup response carries session_id
if (msg_no == 0x01 &&
received >= SESSION_ID_OFFSET + sizeof(uint64_t)) {
session_id = u64((const uint8_t *)(buffer + SESSION_ID_OFFSET));
debug("Obtained session_id: 0x%llx\n", session_id);
}
break;
case SMB2_CREATE:
if (received >= CREATE_VFID_OFFSET + sizeof(uint64_t)) {
persistent_file_id = u64((const uint8_t *)(buffer + CREATE_PFID_OFFSET));
volatile_file_id = u64((const uint8_t *)(buffer + CREATE_VFID_OFFSET));
debug("Obtained p_fid: 0x%llx, v_fid: 0x%llx\n",
persistent_file_id, volatile_file_id);
}
break;
default:
debug("Unknown command (0x%04x)\n", cmd_rsp);
break;
}
}
Another issue we had to solve was that ksmbd relies on global state-memory pools or session tables, which makes fuzzing less deterministic. We tried enabling the experimental reset_acc_state feature to reset accumulated state, but it slowed down fuzzing significantly. We decided to not care much about reproducibility, since each bug typically appeared in dozens or even hundreds of test cases. For the rest, we used focused fuzzing, as described below.
Protocol Specification
We based our harness on the official SMB protocol specification by implementing a grammar for all supported SMB commands. Microsoft publishes detailed technical documents for SMB and other protocols as part of its Open Specifications program.
As an example, the wire format of the SMB2 IOCTL Request is shown below:
We then manually rewrote this specification into our grammar, which allowed our harness to automatically construct valid SMB2 IOCTL requests:
smb2_ioctl_req {
Header_Prefix SMB2Header_Prefix
Command const[0xb, int16]
Header_Suffix SMB2Header_Suffix
StructureSize const[57, int16]
Reserved const[0, int16]
CtlCode union_control_codes
PersistentFileId const[0x4, int64]
VolatileFileId const[0x0, int64]
InputOffset offsetof[Input, int32]
InputCount bytesize[Input, int32]
MaxInputResponse const[65536, int32]
OutputOffset offsetof[Output, int32]
OutputCount len[Output, int32]
MaxOutputResponse const[65536, int32]
Flags int32[0:1]
Reserved2 const[0, int32]
Input array[int8]
Output array[int8]
} [packed]
We did a final check against the source code to identify and verify possible mismatches during our translation.
Fuzzing Strategies
Since we were curious about the bugs that might be missed when using only the default syzkaller configuration with a corpus generated from scratch, we explored different fuzzing approaches, each of which is described in the following subsections.
FocusAreas
Occasionally, we triggered a bug that we were not able to reproduce, and it was not immediately clear from the crash log why it occurred. In other cases, we wanted to focus on a parsing function that had weak coverage. The experimental function focus_areas allows exactly that.
For instance, by targeting smb_check_perm_dacl with
"focus_areas": [
{"filter": {"functions": ["smb_check_perm_dacl"]}, "weight": 20.0},
{"filter": {"files": ["^fs/smb/server/"]}, "weight": 2.0},
{"weight": 1.0}
]
we identified multiple integer overflows and were able to quickly suggest and confirm the patch.
To reach the vulnerable code, syzkaller constructed an ACL that passed validation and led to an integer overflow. After rewriting it in Python, it looked like this:
def build_sd():
sd = bytearray(0x14)
sd[0x00] = 0x00
sd[0x01] = 0x00
struct.pack_into("<H", sd, 0x02, 0x0001)
struct.pack_into("<I", sd, 0x04, 0x78)
struct.pack_into("<I", sd, 0x08, 0x00)
struct.pack_into("<I", sd, 0x0C, 0x10000)
struct.pack_into("<I", sd, 0x10, 0xFFFFFFFF) # dacloffset
while len(sd) < 0x78:
sd += b"A"
sd += b"\x01\x01\x00\x00\x00\x00\x00\x00"
sd += b"\xCC" * 64
return bytes(sd)
sd = build_sd()
print(f"[+] Final SD length: {len(sd)}")
ANYBLOB
The anyTypes struct is used internally during fuzzing and it is less documented - probably because it’s not intended to be used directly. It is defined in prog/any.go and can represent multiple structures::
type anyTypes struct {
union *UnionType
array *ArrayType
blob *BufferType
// .. snip..
}
Implemented in commit 9fe8aa4, the use case is to squash complex structures into a flat byte array, and apply just generic mutations.
Reading the test case is more illustrative to see how it works, where:
foo$any_in(&(0x7f0000000000)={0x11, 0x11223344, 0x2233, 0x1122334455667788, {0x1, 0x7, 0x1, 0x1, 0x1bc, 0x4}, [{@res32=0x0, @i8=0x44, "aabb"}, {@res64=0x1, @i32=0x11223344, "1122334455667788"}, {@res8=0x2, @i8=0x55, "cc"}]})
translates to
foo$any_in(&(0x7f0000000000)=ANY=[@ANYBLOB="1100000044332211223300000000000088776655443322117d00bc11", @ANYRES32=0x0, @ANYBLOB="0000000044aabb00", @ANYRES64=0x1, @ANYBLOB="443322111122334455667788", @ANYRES8=0x2, @ANYBLOB="0000000000000055cc0000"])`
The translation happens automatically as part of the fuzzing process. After running the fuzzer for several weeks, it stopped producing new coverage. Instead of manually writing inputs that followed the grammar and reached new paths, we used ANYBLOB, which allowed us to generate them easily.
The ANYBLOB is represented as a BufferType data type and we used public pcaps obtained here and here to generate a new corpus.
import json
import os
# tshark -r smb2_dac_sample.pcap -Y "smb || smb2" -T json -e tcp.payload > packets.json
os.makedirs("corpus", exist_ok=True)
def load_packets(json_file):
with open(json_file, 'r') as file:
data = json.load(file)
packets = [entry["_source"]["layers"]["tcp.payload"] for entry in data]
return packets
if __name__ == "__main__":
json_file = "packets.json"
packets = load_packets(json_file)
for i, packet in enumerate(packets):
pdu_size = len(packet[0])
filename = f"corpus/packet_{i:03d}.txt"
with open(filename, "w") as f:
f.write(f"syz_ksmbd_send_req(&(0x7f0000000340)=ANY=[@ANYBLOB=\"{packet[0]}\"], {hex(pdu_size)}, 0x0, 0x0)")
After that, we used syz-db to pack all candidates into the corpus database and resumed fuzzing.
With that, we were able to immediately trigger ksmbd: fix use-after-free in ksmbd_sessions_deregister() and improve overall coverage by a few percent.
Sanitizer Coverage Beyond KASAN
In addition to KASAN, we tried other sanitizers such as KUBSAN and KCSAN. There was no significant improvement: KCSAN produced many false positives or reported bugs in unrelated components with seemingly no security impact. Interestingly, KUBSAN was able to identify one additional issue that KASAN did not detect:
id = le32_to_cpu(psid->sub_auth[psid->num_subauth - 1]);
In this case, the user was able to set psid->num_subauth to 0, which resulted in an incorrect read psid->sub_auth[-1]. Although this access still fell within the same struct allocation (smb_sid), UBSAN’s array index bounds check considered the declared bounds of the array
struct smb_sid {
__u8 revision; /* revision level */
__u8 num_subauth;
__u8 authority[NUM_AUTHS];
__le32 sub_auth[SID_MAX_SUB_AUTHORITIES]; /* sub_auth[num_subauth] */
} __attribute__((packed));
and was therefore able to catch the bug.
Coverage
One unresolved issue was fuzzing with multiple processes. Due to various locking mechanisms, and because we reused the same authentication state, we noticed that fuzzing was more stable and coverage increased faster when using only one process. We sent multiple requests within a single invocation, but initially worried that this would cause us to miss race conditions.
If we check the execution log, we see that syzkaller creates multiple threads inside one process, the same way it does when calling standard syscalls:
1.887619984s ago: executing program 0 (id=1628):
syz_ksmbd_send_req(&(0x7f0000000d40)={0xee, @smb2_read_req={{}, 0x8, {0x1, 0x0, 0x0, 0x0, 0x0, 0x1, 0x1, "fbac8eef056a860726ca964fb4f60999"}, 0x31, 0x6, 0x2, 0x7e, 0x70, 0x4, 0x0, 0xffffffff, 0x2, 0x7, 0xee, 0x0, "1cad48fb0cba2f253915fe074290eb3e10ed9ac895dde2a575e4caabc1f3a537e265fea8a440acfd66cf5e249b1ccaae941160f24282c81c9df0260d0403bb44b0461da80509bd756c155b191718caa5eabd4bd89aa9bed58bf87d42ef49bca4c9f08f22d495b601c9c025631b815bf6cbeb0aa4785aec4abf776d75e5be"}}, 0xf2, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0)
syz_ksmbd_send_req(&(0x7f0000000900)=ANY=[@ANYRES16=<r0=>0x0], 0xf0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0) (async, rerun: 32)
syz_ksmbd_send_req(&(0x7f0000001440)=ANY=[@ANYBLOB="000008c0fe534d4240000000000000000b0001000000000000000000030000000000000000000000010000000100000000000000684155244ffb955e3201e88679ed735a39000000040214000400000000000000000000000000000078000000480800000000010000000000000000000000010001"], 0x8c4, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0) (async, rerun: 32)
syz_ksmbd_send_req(&(0x7f0000000200)={0x58, @smb2_oplock_break_req={{}, 0x12, {0x1, 0x0, 0x0, 0x9, 0x0, 0x1, 0x1, "3c66dd1fe856ec397e7f8d7c8c293fd6"}, 0x24}}, 0x5c, &(0x7f0000000000)=ANY=[@ANYBLOB="00000080fe534d424000010000000000050001000800000000000000040000000000000000000000010000000100000000000000b31fae29f7ea148ad156304f457214a539000000020000000000000000000000000000000000000000000002"], 0x84, &(0x7f0000000100)=ANY=[@ANYBLOB="00000062fe534d4240000000000000000e00010000000000000000000700000000000000000000000100000001000000000000000002000000ffff0000000000000000002100030a08000000040000000000000000000000000000006000020009000000aedf"], 0x66, 0x0, 0x0) (async)
...
Observe the async keyword automatically added during the fuzzing process, which allows running commands in parallel without blocking, implemented in this commit fd8caa5. Hence, no UAF was missed due to the seemingly absent parallelism.
In the end, based on syzkaller’s benchmark, we executed 20-30 processes per second in 20 VMs, which still potentially meant running several hundred commands. For reference, we used a server with an average configuration - nothing particularly optimized for fuzzing performance.
We measured coverage using syzkaller’s built-in function-level metrics. While we’re aware that this does not capture state transitions, which are critical in a protocol like SMB, it still provides a useful approximation of code exercised. Overall, the fs/smb/server directory reached around 60%. For smb2pdu.c specifically, which handles most SMB command parsing and dispatch, we reached 70%.
The screenshot below shows coverage across key files.
Discovered Bugs
During our research period, we reported a grand total of 23 bugs. The majority of the bugs are use-after-frees or out-of-bounds read or write findings. Given this quantity, it is natural that the impact differs. For instance, fix the warning from __kernel_write_iter is a simple warning that could only be used for DoS in a specific setup (kernel.panic_on_warn), validate zero num_subauth before sub_auth is accessed is a simple out-of-bounds 1-byte read, and prevent rename with empty string will only cause a kernel oops.
There are additional issues where exploitability requires more thoughtful analysis (e.g., fix type confusion via race condition when using ipc_msg_send_request). Nevertheless, after evaluating potentially promising candidates, we were able to identify some powerful primitives, allowing an attacker to exploit the finding at least locally to gain remote code execution.
The list of the issues identified is reported hereby:
| Description | Commit | CVE |
|---|---|---|
| prevent out-of-bounds stream writes by validating *pos | 0ca6df4 | CVE-2025-37947 |
| prevent rename with empty string | 53e3e5b | CVE-2025-37956 |
| fix use-after-free in ksmbd_session_rpc_open | a1f46c9 | CVE-2025-37926 |
| fix the warning from __kernel_write_iter | b37f2f3 | CVE-2025-37775 |
| fix use-after-free in smb_break_all_levII_oplock() | 18b4fac | CVE-2025-37776 |
| fix use-after-free in __smb2_lease_break_noti() | 21a4e47 | CVE-2025-37777 |
| validate zero num_subauth before sub_auth is accessed | bf21e29 | CVE-2025-22038 |
| fix overflow in dacloffset bounds check | beff0bc | CVE-2025-22039 |
| fix use-after-free in ksmbd_sessions_deregister() | 15a9605 | CVE-2025-22041 |
| fix r_count dec/increment mismatch | ddb7ea3 | CVE-2025-22074 |
| add bounds check for create lease context | bab703e | CVE-2025-22042 |
| add bounds check for durable handle context | 542027e | CVE-2025-22043 |
| prevent connection release during oplock break notification | 3aa660c | CVE-2025-21955 |
| fix use-after-free in ksmbd_free_work_struct | bb39ed4 | CVE-2025-21967 |
| fix use-after-free in smb2_lock | 84d2d16 | CVE-2025-21945 |
| fix bug on trap in smb2_lock | e26e2d2 | CVE-2025-21944 |
| fix out-of-bounds in parse_sec_desc() | d6e13e1 | CVE-2025-21946 |
| fix type confusion via race condition when using ipc_msg_send_req.. | e2ff19f | CVE-2025-21947 |
| align aux_payload_buf to avoid OOB reads in cryptographic operati.. | 06a0254 | - |
| check outstanding simultaneous SMB operations | 0a77d94 | CVE-2024-50285 |
| fix slab-use-after-free in smb3_preauth_hash_rsp | b8fc56f | CVE-2024-50283 |
| fix slab-use-after-free in ksmbd_smb2_session_create | c119f4e | CVE-2024-50286 |
| fix slab-out-of-bounds in smb2_allocate_rsp_buf | 0a77715 | CVE-2024-26980 |
Note that we are aware of the controversy around CVE assignment since the Linux kernel became a CVE Numbering Authority (CNA) in February 2024. My personal take is that, while there were many disputable cases, the current approach is pragmatic: CVEs are now assigned for fixes with potential security impact, particularly memory corruptions and other classes of bugs that could potentially be exploitable.
For more information, the whole process is described in detail in this great presentation, or the relevant article. Lastly, the voting process for CVE approval is implemented in the vulns.git repository.
Conclusion
Our research yielded a few dozen bugs, although using pseudo-syscalls is generally discouraged and comes with several disadvantages. For instance, in all cases, we had to perform the triaging process manually by finding the relevant crash log entries, generating C programs, and minimizing them by hand.
Since syscalls can be tied using resources, this method could also be applied to ksmbd, which involves sending packets. It would be ideal for future research to explore this direction - SMB commands could yield resources that are then fed into different commands. Due to time restrictions, we followed the pseudo-syscall approach, relying on custom patches.
For the next and last part, we focus on exploiting CVE-2025-37947.