/* * linux/kernel/seccomp.c * * Copyright 2004-2005 Andrea Arcangeli <andrea@cpushare.com> * * Copyright (C) 2012 Google, Inc. * Will Drewry <wad@chromium.org> * * This defines a simple but solid secure-computing facility. * * Mode 1 uses a fixed list of allowed system calls. * Mode 2 allows user-defined system call filters in the form * of Berkeley Packet Filters/Linux Socket Filters. */ #include <linux/atomic.h> #include <linux/audit.h> #include <linux/compat.h> #include <linux/sched.h> #include <linux/seccomp.h> /* #define SECCOMP_DEBUG 1 */ #ifdef CONFIG_SECCOMP_FILTER #include <asm/syscall.h> #include <linux/filter.h> #include <linux/ptrace.h> #include <linux/security.h> #include <linux/slab.h> #include <linux/tracehook.h> #include <linux/uaccess.h> /** * struct seccomp_filter - container for seccomp BPF programs * * @usage: reference count to manage the object lifetime. * get/put helpers should be used when accessing an instance * outside of a lifetime-guarded section. In general, this * is only needed for handling filters shared across tasks. * @prev: points to a previously installed, or inherited, filter * @len: the number of instructions in the program * @insns: the BPF program instructions to evaluate * * seccomp_filter objects are organized in a tree linked via the @prev * pointer. For any task, it appears to be a singly-linked list starting * with current->seccomp.filter, the most recently attached or inherited filter. * However, multiple filters may share a @prev node, by way of fork(), which * results in a unidirectional tree existing in memory. This is similar to * how namespaces work. * * seccomp_filter objects should never be modified after being attached * to a task_struct (other than @usage). */ struct seccomp_filter { atomic_t usage; struct seccomp_filter *prev; unsigned short len; /* Instruction count */ struct sock_filter insns[]; }; /* Limit any path through the tree to 256KB worth of instructions. */ #define MAX_INSNS_PER_PATH ((1 << 18) / sizeof(struct sock_filter)) /** * get_u32 - returns a u32 offset into data * @data: a unsigned 64 bit value * @index: 0 or 1 to return the first or second 32-bits * * This inline exists to hide the length of unsigned long. If a 32-bit * unsigned long is passed in, it will be extended and the top 32-bits will be * 0. If it is a 64-bit unsigned long, then whatever data is resident will be * properly returned. * * Endianness is explicitly ignored and left for BPF program authors to manage * as per the specific architecture. */ static inline u32 get_u32(u64 data, int index) { return ((u32 *)&data)[index]; } /* Helper for bpf_load below. */ #define BPF_DATA(_name) offsetof(struct seccomp_data, _name) /** * bpf_load: checks and returns a pointer to the requested offset * @off: offset into struct seccomp_data to load from * * Returns the requested 32-bits of data. * seccomp_check_filter() should assure that @off is 32-bit aligned * and not out of bounds. Failure to do so is a BUG. */ u32 seccomp_bpf_load(int off) { struct pt_regs *regs = task_pt_regs(current); if (off == BPF_DATA(nr)) return syscall_get_nr(current, regs); if (off == BPF_DATA(arch)) return syscall_get_arch(current, regs); if (off >= BPF_DATA(args[0]) && off < BPF_DATA(args[6])) { unsigned long value; int arg = (off - BPF_DATA(args[0])) / sizeof(u64); int index = !!(off % sizeof(u64)); syscall_get_arguments(current, regs, arg, 1, &value); return get_u32(value, index); } if (off == BPF_DATA(instruction_pointer)) return get_u32(KSTK_EIP(current), 0); if (off == BPF_DATA(instruction_pointer) + sizeof(u32)) return get_u32(KSTK_EIP(current), 1); /* seccomp_check_filter should make this impossible. */ BUG(); } /** * seccomp_check_filter - verify seccomp filter code * @filter: filter to verify * @flen: length of filter * * Takes a previously checked filter (by sk_chk_filter) and * redirects all filter code that loads struct sk_buff data * and related data through seccomp_bpf_load. It also * enforces length and alignment checking of those loads. * * Returns 0 if the rule set is legal or -EINVAL if not. */ static int seccomp_check_filter(struct sock_filter *filter, unsigned int flen) { int pc; for (pc = 0; pc < flen; pc++) { struct sock_filter *ftest = &filter[pc]; u16 code = ftest->code; u32 k = ftest->k; switch (code) { case BPF_S_LD_W_ABS: ftest->code = BPF_S_ANC_SECCOMP_LD_W; /* 32-bit aligned and not out of bounds. */ if (k >= sizeof(struct seccomp_data) || k & 3) return -EINVAL; continue; case BPF_S_LD_W_LEN: ftest->code = BPF_S_LD_IMM; ftest->k = sizeof(struct seccomp_data); continue; case BPF_S_LDX_W_LEN: ftest->code = BPF_S_LDX_IMM; ftest->k = sizeof(struct seccomp_data); continue; /* Explicitly include allowed calls. */ case BPF_S_RET_K: case BPF_S_RET_A: case BPF_S_ALU_ADD_K: case BPF_S_ALU_ADD_X: case BPF_S_ALU_SUB_K: case BPF_S_ALU_SUB_X: case BPF_S_ALU_MUL_K: case BPF_S_ALU_MUL_X: case BPF_S_ALU_DIV_X: case BPF_S_ALU_AND_K: case BPF_S_ALU_AND_X: case BPF_S_ALU_OR_K: case BPF_S_ALU_OR_X: case BPF_S_ALU_XOR_K: case BPF_S_ALU_XOR_X: case BPF_S_ALU_LSH_K: case BPF_S_ALU_LSH_X: case BPF_S_ALU_RSH_K: case BPF_S_ALU_RSH_X: case BPF_S_ALU_NEG: case BPF_S_LD_IMM: case BPF_S_LDX_IMM: case BPF_S_MISC_TAX: case BPF_S_MISC_TXA: case BPF_S_ALU_DIV_K: case BPF_S_LD_MEM: case BPF_S_LDX_MEM: case BPF_S_ST: case BPF_S_STX: case BPF_S_JMP_JA: case BPF_S_JMP_JEQ_K: case BPF_S_JMP_JEQ_X: case BPF_S_JMP_JGE_K: case BPF_S_JMP_JGE_X: case BPF_S_JMP_JGT_K: case BPF_S_JMP_JGT_X: case BPF_S_JMP_JSET_K: case BPF_S_JMP_JSET_X: continue; default: return -EINVAL; } } return 0; } /** * seccomp_run_filters - evaluates all seccomp filters against @syscall * @syscall: number of the current system call * * Returns valid seccomp BPF response codes. */ static u32 seccomp_run_filters(int syscall) { struct seccomp_filter *f; u32 ret = SECCOMP_RET_ALLOW; /* Ensure unexpected behavior doesn't result in failing open. */ if (WARN_ON(current->seccomp.filter == NULL)) return SECCOMP_RET_KILL; /* * All filters in the list are evaluated and the lowest BPF return * value always takes priority (ignoring the DATA). */ for (f = current->seccomp.filter; f; f = f->prev) { u32 cur_ret = sk_run_filter(NULL, f->insns); if ((cur_ret & SECCOMP_RET_ACTION) < (ret & SECCOMP_RET_ACTION)) ret = cur_ret; } return ret; } /** * seccomp_attach_filter: Attaches a seccomp filter to current. * @fprog: BPF program to install * * Returns 0 on success or an errno on failure. */ static long seccomp_attach_filter(struct sock_fprog *fprog) { struct seccomp_filter *filter; unsigned long fp_size = fprog->len * sizeof(struct sock_filter); unsigned long total_insns = fprog->len; long ret; if (fprog->len == 0 || fprog->len > BPF_MAXINSNS) return -EINVAL; for (filter = current->seccomp.filter; filter; filter = filter->prev) total_insns += filter->len + 4; /* include a 4 instr penalty */ if (total_insns > MAX_INSNS_PER_PATH) return -ENOMEM; /* * Installing a seccomp filter requires that the task have * CAP_SYS_ADMIN in its namespace or be running with no_new_privs. * This avoids scenarios where unprivileged tasks can affect the * behavior of privileged children. */ if (!current->no_new_privs && security_capable_noaudit(current_cred(), current_user_ns(), CAP_SYS_ADMIN) != 0) return -EACCES; /* Allocate a new seccomp_filter */ filter = kzalloc(sizeof(struct seccomp_filter) + fp_size, GFP_KERNEL|__GFP_NOWARN); if (!filter) return -ENOMEM; atomic_set(&filter->usage, 1); filter->len = fprog->len; /* Copy the instructions from fprog. */ ret = -EFAULT; if (copy_from_user(filter->insns, fprog->filter, fp_size)) goto fail; /* Check and rewrite the fprog via the skb checker */ ret = sk_chk_filter(filter->insns, filter->len); if (ret) goto fail; /* Check and rewrite the fprog for seccomp use */ ret = seccomp_check_filter(filter->insns, filter->len); if (ret) goto fail; /* * If there is an existing filter, make it the prev and don't drop its * task reference. */ filter->prev = current->seccomp.filter; current->seccomp.filter = filter; return 0; fail: kfree(filter); return ret; } /** * seccomp_attach_user_filter - attaches a user-supplied sock_fprog * @user_filter: pointer to the user data containing a sock_fprog. * * Returns 0 on success and non-zero otherwise. */ long seccomp_attach_user_filter(char __user *user_filter) { struct sock_fprog fprog; long ret = -EFAULT; #ifdef CONFIG_COMPAT if (is_compat_task()) { struct compat_sock_fprog fprog32; if (copy_from_user(&fprog32, user_filter, sizeof(fprog32))) goto out; fprog.len = fprog32.len; fprog.filter = compat_ptr(fprog32.filter); } else /* falls through to the if below. */ #endif if (copy_from_user(&fprog, user_filter, sizeof(fprog))) goto out; ret = seccomp_attach_filter(&fprog); out: return ret; } /* get_seccomp_filter - increments the reference count of the filter on @tsk */ void get_seccomp_filter(struct task_struct *tsk) { struct seccomp_filter *orig = tsk->seccomp.filter; if (!orig) return; /* Reference count is bounded by the number of total processes. */ atomic_inc(&orig->usage); } /* put_seccomp_filter - decrements the ref count of tsk->seccomp.filter */ void put_seccomp_filter(struct task_struct *tsk) { struct seccomp_filter *orig = tsk->seccomp.filter; /* Clean up single-reference branches iteratively. */ while (orig && atomic_dec_and_test(&orig->usage)) { struct seccomp_filter *freeme = orig; orig = orig->prev; kfree(freeme); } } /** * seccomp_send_sigsys - signals the task to allow in-process syscall emulation * @syscall: syscall number to send to userland * @reason: filter-supplied reason code to send to userland (via si_errno) * * Forces a SIGSYS with a code of SYS_SECCOMP and related sigsys info. */ static void seccomp_send_sigsys(int syscall, int reason) { struct siginfo info; memset(&info, 0, sizeof(info)); info.si_signo = SIGSYS; info.si_code = SYS_SECCOMP; info.si_call_addr = (void __user *)KSTK_EIP(current); info.si_errno = reason; info.si_arch = syscall_get_arch(current, task_pt_regs(current)); info.si_syscall = syscall; force_sig_info(SIGSYS, &info, current); } #endif /* CONFIG_SECCOMP_FILTER */ /* * Secure computing mode 1 allows only read/write/exit/sigreturn. * To be fully secure this must be combined with rlimit * to limit the stack allocations too. */ static int mode1_syscalls[] = { __NR_seccomp_read, __NR_seccomp_write, __NR_seccomp_exit, __NR_seccomp_sigreturn, 0, /* null terminated */ }; #ifdef CONFIG_COMPAT static int mode1_syscalls_32[] = { __NR_seccomp_read_32, __NR_seccomp_write_32, __NR_seccomp_exit_32, __NR_seccomp_sigreturn_32, 0, /* null terminated */ }; #endif int __secure_computing(int this_syscall) { int mode = current->seccomp.mode; int exit_sig = 0; int *syscall; u32 ret; switch (mode) { case SECCOMP_MODE_STRICT: syscall = mode1_syscalls; #ifdef CONFIG_COMPAT if (is_compat_task()) syscall = mode1_syscalls_32; #endif do { if (*syscall == this_syscall) return 0; } while (*++syscall); exit_sig = SIGKILL; ret = SECCOMP_RET_KILL; break; #ifdef CONFIG_SECCOMP_FILTER case SECCOMP_MODE_FILTER: { int data; struct pt_regs *regs = task_pt_regs(current); ret = seccomp_run_filters(this_syscall); data = ret & SECCOMP_RET_DATA; ret &= SECCOMP_RET_ACTION; switch (ret) { case SECCOMP_RET_ERRNO: /* Set the low-order 16-bits as a errno. */ syscall_set_return_value(current, regs, -data, 0); goto skip; case SECCOMP_RET_TRAP: /* Show the handler the original registers. */ syscall_rollback(current, regs); /* Let the filter pass back 16 bits of data. */ seccomp_send_sigsys(this_syscall, data); goto skip; case SECCOMP_RET_TRACE: /* Skip these calls if there is no tracer. */ if (!ptrace_event_enabled(current, PTRACE_EVENT_SECCOMP)) { syscall_set_return_value(current, regs, -ENOSYS, 0); goto skip; } /* Allow the BPF to provide the event message */ ptrace_event(PTRACE_EVENT_SECCOMP, data); /* * The delivery of a fatal signal during event * notification may silently skip tracer notification. * Terminating the task now avoids executing a system * call that may not be intended. */ if (fatal_signal_pending(current)) break; if (syscall_get_nr(current, regs) < 0) goto skip; /* Explicit request to skip. */ return 0; case SECCOMP_RET_ALLOW: return 0; case SECCOMP_RET_KILL: default: break; } exit_sig = SIGSYS; break; } #endif default: BUG(); } #ifdef SECCOMP_DEBUG dump_stack(); #endif audit_seccomp(this_syscall, exit_sig, ret); do_exit(exit_sig); #ifdef CONFIG_SECCOMP_FILTER skip: audit_seccomp(this_syscall, exit_sig, ret); #endif return -1; } long prctl_get_seccomp(void) { return current->seccomp.mode; } /** * prctl_set_seccomp: configures current->seccomp.mode * @seccomp_mode: requested mode to use * @filter: optional struct sock_fprog for use with SECCOMP_MODE_FILTER * * This function may be called repeatedly with a @seccomp_mode of * SECCOMP_MODE_FILTER to install additional filters. Every filter * successfully installed will be evaluated (in reverse order) for each system * call the task makes. * * Once current->seccomp.mode is non-zero, it may not be changed. * * Returns 0 on success or -EINVAL on failure. */ long prctl_set_seccomp(unsigned long seccomp_mode, char __user *filter) { long ret = -EINVAL; if (current->seccomp.mode && current->seccomp.mode != seccomp_mode) goto out; switch (seccomp_mode) { case SECCOMP_MODE_STRICT: ret = 0; #ifdef TIF_NOTSC disable_TSC(); #endif break; #ifdef CONFIG_SECCOMP_FILTER case SECCOMP_MODE_FILTER: ret = seccomp_attach_user_filter(filter); if (ret) goto out; break; #endif default: goto out; } current->seccomp.mode = seccomp_mode; set_thread_flag(TIF_SECCOMP); out: return ret; }