------------------------------------------------------------------------ SUMMARY ------------------------------------------------------------------------ A kernel stack buffer overflow exists in the setcred(2) system call introduced in FreeBSD 14.x. The overflow occurs before any privilege check, allowing any unprivileged local user to trigger arbitrary behaviour ranging from a kernel panic to full local privilege escalation (LPE). Working LPE exploits against an amd64 GENERIC kernel both without SMAP/SMEP and with SMAP/SMEP enabled have been developed and are described below. The SMAP/SMEP-safe variant requires only that the zfs.ko module be loaded -- the case on every FreeBSD installation with a ZFS pool. The root cause is a sizeof type error in kern_setcred_copyin_supp_groups() (sys/kern/kern_prot.c). The bug was silently fixed in the main branch on 2025-11-27 (commit 000d5b52c19ff3858a6f0cbb405d47713c4267a4) as a side effect of a broader function refactoring. The fix has NOT been backported to stable/14 or releng/14.4. FreeBSD 14.4-RELEASE remains vulnerable. FreeBSD 15.0 still carries the sizeof(*groups) typo and is therefore vulnerable, but the surrounding code differs enough from 14.4 that the chain primitives developed here do not lift the overflow into a working LPE on that branch. On 15.0 the bug remains a kernel panic triggered by any unprivileged user. ------------------------------------------------------------------------ AFFECTED VERSIONS ------------------------------------------------------------------------ Vulnerable + exploitable: FreeBSD 14.4-RELEASE (confirmed) FreeBSD stable/14 as of the date of this report Vulnerable, not known to be exploitable: FreeBSD 15.0 (the sizeof(*groups) typo is still present in sys/kern/kern_prot.c, but the surrounding code differs enough from 14.4 that none of the chain primitives developed here lift the overflow into a working LPE on 15.0 GENERIC. The bug remains a kernel panic / DoS on this branch.) Not affected: FreeBSD main (fixed in commit 000d5b5, 2025-11-27) Not affected: FreeBSD 13.x and earlier (setcred(2) not present) ------------------------------------------------------------------------ VULNERABILITY DETAILS ------------------------------------------------------------------------ File: sys/kern/kern_prot.c Function: kern_setcred_copyin_supp_groups() Lines: 528-533 The function signature uses a double pointer for the groups argument: static int kern_setcred_copyin_supp_groups(struct setcred *const wcred, const u_int flags, gid_t *const smallgroups, gid_t **const groups) Because groups has type gid_t **, the expression sizeof(*groups) evaluates to sizeof(gid_t *) == 8 on LP64, rather than the intended sizeof(gid_t) == 4. This sizeof expression is used in two places: Line 528-530 (allocation): *groups = wcred->sc_supp_groups_nb < CRED_SMALLGROUPS_NB ? smallgroups : malloc((wcred->sc_supp_groups_nb + 1) * sizeof(*groups), M_TEMP, M_WAITOK); /* sizeof(*groups) == 8 */ Line 532-533 (copyin): error = copyin(wcred->sc_supp_groups, *groups + 1, wcred->sc_supp_groups_nb * sizeof(*groups)); /* sizeof(*groups) == 8 */ The allocation on the heap path (line 529-530) is 2x oversized, which is safe. However, for the stack path (when sc_supp_groups_nb < CRED_SMALLGROUPS_NB == 16), *groups is set to smallgroups, a gid_t[CRED_SMALLGROUPS_NB] array declared as a local variable in the caller user_setcred() (sys/kern/kern_prot.c:555): gid_t smallgroups[CRED_SMALLGROUPS_NB]; /* 16 * 4 = 64 bytes */ The copyin destination is *groups + 1 == &smallgroups[1], which leaves (CRED_SMALLGROUPS_NB - 1) * sizeof(gid_t) == 15 * 4 == 60 bytes of usable space. The copyin copies sc_supp_groups_nb * sizeof(*groups) == sc_supp_groups_nb * 8 bytes. With the maximum stack-path value of sc_supp_groups_nb == 15: Bytes written: 15 * 8 = 120 Buffer capacity: 15 * 4 = 60 Overflow: 60 bytes past the end of smallgroups[] The overflow is written with fully attacker-controlled data from user space (wcred->sc_supp_groups points to an attacker-supplied buffer). ------------------------------------------------------------------------ TRIGGER PATH AND PRIVILEGE CHECK ORDERING ------------------------------------------------------------------------ The overflow happens in kern_setcred_copyin_supp_groups(), which is called from user_setcred() at line 604 -- BEFORE the privilege check. The privilege check (priv_check_cred(PRIV_CRED_SETCRED)) does not occur until kern_setcred() is called at line 623, and within that function at line 813. Any local user can trigger the overflow by issuing: setcred(SETCREDF_SUPP_GROUPS, &wcred, sizeof(wcred)) with wcred.sc_supp_groups_nb == 15 and wcred.sc_supp_groups pointing to a 15 * 8 == 120-byte user-space buffer. ------------------------------------------------------------------------ LPE TECHNIQUE (no SMAP, no SMEP) ------------------------------------------------------------------------ The 60-byte overflow corrupts every callee-saved register slot in user_setcred()'s prologue except saved RBP. Compiler ordering on 14.4 GENERIC places the corruption window at: [rbp - 0x40 .. -0x05] mapping input bytes to memory as: buf[60..67] mac.m_buflen buf[68..75] mac.m_string buf[76..83] td pointer spill <- controls kern_setcred(td=...) buf[84..91] saved rbx buf[92..99] saved r12 <- !!! propagates up the stack buf[100..107] saved r13 buf[108..115] saved r14 buf[116..119] low 32 bits of saved r15 The crucial observation is that sys_setcred()'s prologue only saves rbp/r14/rbx -- it does NOT save r12. Therefore the corrupted r12 popped by user_setcred()'s epilogue propagates unchanged through sys_setcred() up to amd64_syscall(). At amd64_syscall+0x155 the kernel uses r12 as if it were the live td_proc pointer: ffffffff8105b6e5: mov rcx, [r12 + 0x3f8] ; r12 fully controlled ffffffff8105b6ed: mov rdi, rbx ; rdi = real curthread ffffffff8105b6f0: mov esi, eax ; esi = setcred retval ffffffff8105b6f2: call [rcx + 0xc8] ; INDIRECT CALL This is a two-level indirect call entirely controlled by the attacker: *(r12+0x3f8) supplies rcx, and *(rcx+0xc8) is the call target. Without SMAP, the kernel happily dereferences user-mode pointers, so both indirections can be satisfied by fake structures placed in user memory. Without SMEP, the indirect call may target user-space code. The published exploit constructs: fake_td : td_proc = &fake_proc fake_proc : p_ucred = &fake_oldcred, p_mtx.mtx_lock = 0 (so PROC_LOCK cmpxchg succeeds), p_sysent (offset 0x3f8) = &fake_sysentvec fake_oldcred : cr_uid = 0, cr_prison = &prison0 (kernel-static), cr_uidinfo/cr_ruidinfo = &fake_uidinfo, cr_loginclass = &fake_loginclas, cr_ngroups = 1 (avoids qsort underflow in groups_normalize() inside crsetgroups()), cr_groups -> 1-element gid_t buffer fake_uidinfo : ui_ref >= 0 (atomic_add target for uihold) fake_loginclas : lc_refcount >= 0 fake_sysentvec : sv_set_syscall_retval (offset 0xc8) = &shellcode Key structure offsets used by the exploit (DWARF on 14.4 GENERIC): struct ucred size 0x100 cr_mtx@0 cr_ref@0x20 cr_users@0x28 cr_uid@0x60 cr_uidinfo@0x78 cr_ruidinfo@0x80 cr_prison@0x88 cr_loginclass@0x90 cr_groups@0xb0 cr_agroups@0xb8 cr_smallgroups@0xbc struct proc size 0x560 p_ucred@0x40 p_mtx@0x128 p_sysent@0x3f8 p_cowgen@0x214 struct thread size 0x6f0 td_proc@0x8 td_realucred@0x178 td_ucred@0x180 td_frame@0x4b8 struct mtx size 0x20 lock_object@0 mtx_lock@0x18 struct uidinfo ui_ref@0x4c struct loginclass lc_refcount@0x34 Kernel symbols on 14.4-RELEASE GENERIC (no kernel ASLR on FreeBSD, the kernel base is fixed by the linker script): user_setcred 0xffffffff80b50fa0 kern_setcred 0xffffffff80b51100 amd64_syscall+0x155 0xffffffff8105b6e5 <- the hijack point prison0 0xffffffff818c7390 <- needed to pass jailed() ------------------------------------------------------------------------ LPE TECHNIQUE (with SMAP+SMEP via zfs.ko ZSTD gadget, no info-leak) ------------------------------------------------------------------------ This path makes one key observation: the chain primitive at amd64_syscall+0x162 reaches its target with `rcx = K1` (an attacker-chosen 8-byte value). If the target gadget writes `rcx + 1` to `td->td_ucred`, we have set the current thread's credential pointer to any address we choose -- and if that address happens to lie inside a kernel buffer we control (a heap-resident pargs slab), the fake credential we planted there immediately takes effect. 1. The gadget: ZSTD_initCStream_advanced in zfs.ko ------------------------------------------------- Disassembly of the function's prologue: push rbp mov rbp, rsp push r15; push r14; push rbx sub rsp, 0x38 mov rbx, rdx mov r14, rsi mov r15, rdi ; r15 = arg1 = real_td (from chain) mov rax, [rip + __stack_chk_guard] mov [rbp - 0x20], rax ; canary spill xor eax, eax cmp dword ptr [rbp + 0x2c], 0 lea rdx, [rcx + 1] ; rdx = K1 + 1 cmovne rax, rdx test rcx, rcx mov dword ptr [rdi + 0x430], 0 cmovne rax, rdx ; rcx != 0 (always) -> rax = K1 + 1 mov qword ptr [rdi + 0x180], rax ; *** td->td_ucred = K1+1 *** The two cmovne instructions both fire whenever rcx != 0 (i.e., whenever K1 is any non-NULL value, which is always for our use). The function continues with stores into td+0x10..0x3c which corrupt TAILQ_ENTRY scheduler-link fields with garbage drawn from amd64_syscall's stack frame, then performs its canary check and returns. Empirically the corruption is survivable until the thread next reaches the scheduler. 2. Fake ucred placement (parent's pargs slab) -------------------------------------------- Setproctitle(2) is exposed to unprivileged users; the kernel allocates a 256-byte slot in the PARGS UMA zone and copies up to 244 user bytes verbatim into the `ar_args' field. The slab's first 8 bytes hold `ar_ref' and `ar_length' (not directly controllable) but the offsets at which we need ucred fields lie at slot+0x18 and beyond, all inside the user-controlled range. Parent process's pargs slab P_base becomes our fake_ucred: slot offset field value +0x20 cr_ref 0x7fffffff (high; defeats crfree) +0x28 cr_users 0x7fffffff +0x2c cr_flags 0 +0x60 cr_uid 0 +0x64 cr_ruid 0 +0x68 cr_svuid 0 +0x6c cr_ngroups 1 +0x88 cr_prison &prison0 (real kernel symbol) +0xb0 cr_groups &prison0 (TRICK: see note) +0xb8 cr_agroups 1 +0xc7 (call target) ZSTD_initCStream_advanced The cr_groups trick: setting cr_groups = &prison0 makes cred->cr_groups[0] read the first 4 bytes of struct prison which is pr_id = 0 = wheel gid. This lets the in-kernel groupmember(0, cred) check inside the VFS chmod path return 1 without a NULL dereference. Without this we panic in groupmember when later operations test wheel membership. The bytes at slot+0xc7..+0xce hold the function pointer the chain primitive will use as its call target -- see step 4. 3. K1 placement (child's pargs slab) ----------------------------------- The chain primitive reads K1 via `mov rcx, [r12 + 0x3f8]`. r12 is fully attacker-controlled via the setcred overflow. For K1 to equal `P_base - 1` (so that K1+1 = P_base = our fake_ucred), the qword `P_base - 1` must be readable at a kernel address. We cannot write it into the parent's slab (which already contains fake_ucred fields), and we cannot use the td_name trick from the previous section because UMA-heap addresses always have a NUL byte at byte offset 4 of `P_base - 1`, truncating thr_set_name's strlcpy(). Solution: fork a CHILD process that does its own setproctitle with the qword `P_base - 1` placed at offset 0xd0 of its own pargs. The chain then sets r12 = C_base + 0xd0 - 0x3f8, so that [r12 + 0x3f8] = qword at (C_base + 0xd0) = our planted K1. The parent must not exit and must not setproctitle again before the child triggers the chain -- otherwise pargs is freed and P_base may be reused. 4. Triggering the chain ---------------------- Setcred overflow as in the no-SMAP path: bytes 76..83 -> spilled td (= real_td) bytes 92..99 -> saved r12 (= C_base + 0xd0 - 0x3f8) amd64_syscall+0x155 then executes: mov rcx, [r12 + 0x3f8] ; rcx = K1 = P_base - 1 mov rdi, rbx ; rdi = real_td mov esi, eax ; esi = setcred retval (small int) call [rcx + 0xc8] ; -> [P_base + 0xc7] = ZSTD_addr ZSTD_initCStream_advanced runs, writes td_ucred = K1 + 1 = P_base. The subsequent corruption of td+0x10..0x3c is survived; the function ret's cleanly, amd64_syscall returns to user-space, and the calling thread now has cr_uid=0 as its effective credential. 5. Post-exploit -------------- The thread has effective root for VFS operations (open, chmod, chown, etc.). setuid(0) cannot be called because uifind() dereferences our fake cr_uidinfo, which is NULL; this would panic. Instead the exploit installs a setuid-root wrapper at /tmp/rsh via chown(2)+chmod(2) and exits. 6. Self-resolving gadget address ------------------------------- The exploit calls kldnext(2)+kldsym(2) in a loop (both available to unprivileged users) to resolve ZSTD_initCStream_advanced. The kernel image itself contains a different ZSTD library with incompatible struct offsets, so we skip fileid=1 (kernel) and pick the symbol from a loaded module (zfs.ko on a typical server). Once located, the gadget address is used as the call target inside the parent's pargs slab. ------------------------------------------------------------------------ PROOF OF CONCEPT ------------------------------------------------------------------------ Working PoC sources are shipped alongside this write-up: exploits/poc_dos.c Minimal DoS PoC. Any user triggers a kernel panic. exploits/exp2_lpe_no_smap.c Full LPE on a 14.4 GENERIC kernel without SMAP/SMEP. Single setcred(2) call yields uid=0 without panic. exploits/exp_setcred_smap_zfs.c SMAP/SMEP-safe LPE via the exploits/wrapper.c zfs.ko ZSTD gadget. Installs exploits/Makefile.setcred_smap_zfs a setuid-root wrapper at exploits/README_setcred_smap_zfs.md /tmp/rsh. No info-leak. See setcred/README.md for the recommended build + run sequence inside the lab guest. ------------------------------------------------------------------------ FIX STATUS ------------------------------------------------------------------------ The vulnerability was inadvertently fixed in the main branch on 2025-11-27 by commit 000d5b52c19ff3858a6f0cbb405d47713c4267a4 ("setcred(2): Fix a panic on too many groups from latest commit"). The commit refactored kern_setcred_copyin_supp_groups() into user_setcred_copyin_supp_groups() changing the groups argument from gid_t ** to a local gid_t *, and replacing both sizeof(*groups) occurrences with sizeof(gid_t). The commit message does not mention the stack overflow; the fix appears to be an unintentional side effect of the refactoring. The fix has NOT been merged into stable/14 or releng/14.4. FreeBSD 14.4-RELEASE and the current stable/14 branch remain vulnerable as of the date of this report. ------------------------------------------------------------------------ TIMELINE ------------------------------------------------------------------------ * 2026-05-13 - secteam@freebsd.org notified * 2026-05-13 - secteam responds: > at first glance, it appears to be a duplicate issue that we are already working on * 2026-05-16 - working LPE PoC developed against 14.4-RELEASE GENERIC amd64 without SMAP/SMEP; technique documented above under "LPE TECHNIQUE". Impact upgraded from local DoS to full local privilege escalation. * 2026-05-19 - SMAP/SMEP-safe LPE achieved without any kernel info-leak via the ZSTD_initCStream_advanced gadget in zfs.ko; chain primitive writes td_ucred = K1+1 to a fake credential in a kernel-heap pargs slab. Verified end-to-end on qemu64 with +smap,+smep enabled. Impact: single-shot, no-info-leak LPE on any FreeBSD 14.4 GENERIC system with zfs.ko loaded (typical server configuration).