Mobile/Android
Android Kernel Exploit - Basic
pyozzi
2020. 4. 30. 00:18
이번 글에서는 Android Kernel Exploit의 기초를 다루려고 한다.
Android Kernel을 대상으로 익스플로잇을 진행할 때, '주 타겟은 어떤 부분이고 어떤 방식으로 root권한을 획득하는지',
'고려해야 하는 점은 무엇인지'와 같은 Kernel Exploit의 전반적인 부분들을 간단하게 다룰 것이다.
Linux Privilege Escalation
Linux Privilege Escalation(LPE)은 Android 커널 취약점으로 권한 상승, 즉 root권한을 획득하는 것을 말한다.
Linux에서 root는 uid=0(root), gid=0(root) 인 SuperUser를 말하며, 모든 액세스 권한을 갖는다.
AOSP에서는 LPE를 EOP(Elevation of privilege)라고 부른다.
Light Weight Process
Linux는 멀티 스레딩을 보다 더 효율적으로 하기 위해서 Light Weight Process(LWP), 경량 프로세스를 사용한다.
간단하게 Thread라고 생각하면 되는데, 하나의 프로세스에서 여러 개의 스레드가 동작하는 것을 멀티 스레딩이라고 한다.
이 멀티 스레딩을 보다 효율적으로 동작시키기 위해서 스레드들끼리 서로 리소스들을 공유하면서 동작한다고 생각하면 된다.
각 경량 프로세스에는 task_struct라는 프로세스 디스크립터가 지정되며, include/linux/sched.h에 정의되어 있다.
아래는 task_struct구조체 모습이다.
struct task_struct {
#ifdef CONFIG_THREAD_INFO_IN_TASK
/*
* For reasons of header soup (see current_thread_info()), this
* must be the first element of task_struct.
*/
struct thread_info thread_info;
#endif
/* -1 unrunnable, 0 runnable, >0 stopped: */
volatile long state;
/*
* This begins the randomizable portion of task_struct. Only
* scheduling-critical items should be added above here.
*/
randomized_struct_fields_start
void *stack;
atomic_t usage;
/* Per task flags (PF_*), defined further below: */
unsigned int flags;
unsigned int ptrace;
#ifdef CONFIG_SMP
struct llist_node wake_entry;
int on_cpu;
#ifdef CONFIG_THREAD_INFO_IN_TASK
/* Current CPU: */
unsigned int cpu;
#endif
unsigned int wakee_flips;
unsigned long wakee_flip_decay_ts;
struct task_struct *last_wakee;
int wake_cpu;
#endif
int on_rq;
int prio;
int static_prio;
int normal_prio;
unsigned int rt_priority;
const struct sched_class *sched_class;
struct sched_entity se;
struct sched_rt_entity rt;
#ifdef CONFIG_SCHED_WALT
struct ravg ravg;
/*
* 'init_load_pct' represents the initial task load assigned to children
* of this task
*/
u32 init_load_pct;
u64 last_sleep_ts;
#endif
#ifdef CONFIG_CGROUP_SCHED
struct task_group *sched_task_group;
#endif
struct sched_dl_entity dl;
#ifdef CONFIG_PREEMPT_NOTIFIERS
/* List of struct preempt_notifier: */
struct hlist_head preempt_notifiers;
#endif
#ifdef CONFIG_BLK_DEV_IO_TRACE
unsigned int btrace_seq;
#endif
unsigned int policy;
int nr_cpus_allowed;
cpumask_t cpus_allowed;
#ifdef CONFIG_PREEMPT_RCU
int rcu_read_lock_nesting;
union rcu_special rcu_read_unlock_special;
struct list_head rcu_node_entry;
struct rcu_node *rcu_blocked_node;
#endif /* #ifdef CONFIG_PREEMPT_RCU */
#ifdef CONFIG_TASKS_RCU
unsigned long rcu_tasks_nvcsw;
u8 rcu_tasks_holdout;
u8 rcu_tasks_idx;
int rcu_tasks_idle_cpu;
struct list_head rcu_tasks_holdout_list;
#endif /* #ifdef CONFIG_TASKS_RCU */
struct sched_info sched_info;
struct list_head tasks;
#ifdef CONFIG_SMP
struct plist_node pushable_tasks;
struct rb_node pushable_dl_tasks;
#endif
struct mm_struct *mm;
struct mm_struct *active_mm;
/* Per-thread vma caching: */
struct vmacache vmacache;
#ifdef SPLIT_RSS_COUNTING
struct task_rss_stat rss_stat;
#endif
int exit_state;
int exit_code;
int exit_signal;
/* The signal sent when the parent dies: */
int pdeath_signal;
/* JOBCTL_*, siglock protected: */
unsigned long jobctl;
/* Used for emulating ABI behavior of previous Linux versions: */
unsigned int personality;
/* Scheduler bits, serialized by scheduler locks: */
unsigned sched_reset_on_fork:1;
unsigned sched_contributes_to_load:1;
unsigned sched_migrated:1;
unsigned sched_remote_wakeup:1;
#ifdef CONFIG_PSI
unsigned sched_psi_wake_requeue:1;
#endif
/* Force alignment to the next boundary: */
unsigned :0;
/* Unserialized, strictly 'current' */
/* Bit to tell LSMs we're in execve(): */
unsigned in_execve:1;
unsigned in_iowait:1;
#ifndef TIF_RESTORE_SIGMASK
unsigned restore_sigmask:1;
#endif
#ifdef CONFIG_MEMCG
unsigned memcg_may_oom:1;
#ifndef CONFIG_SLOB
unsigned memcg_kmem_skip_account:1;
#endif
#endif
#ifdef CONFIG_COMPAT_BRK
unsigned brk_randomized:1;
#endif
#ifdef CONFIG_CGROUPS
/* disallow userland-initiated cgroup migration */
unsigned no_cgroup_migration:1;
#endif
unsigned long atomic_flags; /* Flags requiring atomic access. */
struct restart_block restart_block;
pid_t pid;
pid_t tgid;
#ifdef CONFIG_CC_STACKPROTECTOR
/* Canary value for the -fstack-protector GCC feature: */
unsigned long stack_canary;
#endif
/*
* Pointers to the (original) parent process, youngest child, younger sibling,
* older sibling, respectively. (p->father can be replaced with
* p->real_parent->pid)
*/
/* Real parent process: */
struct task_struct __rcu *real_parent;
/* Recipient of SIGCHLD, wait4() reports: */
struct task_struct __rcu *parent;
/*
* Children/sibling form the list of natural children:
*/
struct list_head children;
struct list_head sibling;
struct task_struct *group_leader;
/*
* 'ptraced' is the list of tasks this task is using ptrace() on.
*
* This includes both natural children and PTRACE_ATTACH targets.
* 'ptrace_entry' is this task's link on the p->parent->ptraced list.
*/
struct list_head ptraced;
struct list_head ptrace_entry;
/* PID/PID hash table linkage. */
struct pid_link pids[PIDTYPE_MAX];
struct list_head thread_group;
struct list_head thread_node;
struct completion *vfork_done;
/* CLONE_CHILD_SETTID: */
int __user *set_child_tid;
/* CLONE_CHILD_CLEARTID: */
int __user *clear_child_tid;
u64 utime;
u64 stime;
#ifdef CONFIG_ARCH_HAS_SCALED_CPUTIME
u64 utimescaled;
u64 stimescaled;
#endif
u64 gtime;
#ifdef CONFIG_CPU_FREQ_TIMES
u64 *time_in_state;
unsigned int max_state;
#endif
struct prev_cputime prev_cputime;
#ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
struct vtime vtime;
#endif
#ifdef CONFIG_NO_HZ_FULL
atomic_t tick_dep_mask;
#endif
/* Context switch counts: */
unsigned long nvcsw;
unsigned long nivcsw;
/* Monotonic time in nsecs: */
u64 start_time;
/* Boot based time in nsecs: */
u64 real_start_time;
/* MM fault and swap info: this can arguably be seen as either mm-specific or thread-specific: */
unsigned long min_flt;
unsigned long maj_flt;
#ifdef CONFIG_POSIX_TIMERS
struct task_cputime cputime_expires;
struct list_head cpu_timers[3];
#endif
/* Process credentials: */
/* Tracer's credentials at attach: */
const struct cred __rcu *ptracer_cred;
/* Objective and real subjective task credentials (COW): */
const struct cred __rcu *real_cred;
/* Effective (overridable) subjective task credentials (COW): */
const struct cred __rcu *cred;
/*
* executable name, excluding path.
*
* - normally initialized setup_new_exec()
* - access it with [gs]et_task_comm()
* - lock it with task_lock()
*/
char comm[TASK_COMM_LEN];
struct nameidata *nameidata;
#ifdef CONFIG_SYSVIPC
struct sysv_sem sysvsem;
struct sysv_shm sysvshm;
#endif
#ifdef CONFIG_DETECT_HUNG_TASK
unsigned long last_switch_count;
#endif
/* Filesystem information: */
struct fs_struct *fs;
/* Open file information: */
struct files_struct *files;
/* Namespaces: */
struct nsproxy *nsproxy;
/* Signal handlers: */
struct signal_struct *signal;
struct sighand_struct *sighand;
sigset_t blocked;
sigset_t real_blocked;
/* Restored if set_restore_sigmask() was used: */
sigset_t saved_sigmask;
struct sigpending pending;
unsigned long sas_ss_sp;
size_t sas_ss_size;
unsigned int sas_ss_flags;
struct callback_head *task_works;
struct audit_context *audit_context;
#ifdef CONFIG_AUDITSYSCALL
kuid_t loginuid;
unsigned int sessionid;
#endif
struct seccomp seccomp;
/* Thread group tracking: */
u32 parent_exec_id;
u32 self_exec_id;
/* Protection against (de-)allocation: mm, files, fs, tty, keyrings, mems_allowed, mempolicy: */
spinlock_t alloc_lock;
/* Protection of the PI data structures: */
raw_spinlock_t pi_lock;
struct wake_q_node wake_q;
#ifdef CONFIG_RT_MUTEXES
/* PI waiters blocked on a rt_mutex held by this task: */
struct rb_root_cached pi_waiters;
/* Updated under owner's pi_lock and rq lock */
struct task_struct *pi_top_task;
/* Deadlock detection and priority inheritance handling: */
struct rt_mutex_waiter *pi_blocked_on;
#endif
#ifdef CONFIG_DEBUG_MUTEXES
/* Mutex deadlock detection: */
struct mutex_waiter *blocked_on;
#endif
#ifdef CONFIG_TRACE_IRQFLAGS
unsigned int irq_events;
unsigned long hardirq_enable_ip;
unsigned long hardirq_disable_ip;
unsigned int hardirq_enable_event;
unsigned int hardirq_disable_event;
int hardirqs_enabled;
int hardirq_context;
unsigned long softirq_disable_ip;
unsigned long softirq_enable_ip;
unsigned int softirq_disable_event;
unsigned int softirq_enable_event;
int softirqs_enabled;
int softirq_context;
#endif
#ifdef CONFIG_LOCKDEP
# define MAX_LOCK_DEPTH 48UL
u64 curr_chain_key;
int lockdep_depth;
unsigned int lockdep_recursion;
struct held_lock held_locks[MAX_LOCK_DEPTH];
#endif
#ifdef CONFIG_LOCKDEP_CROSSRELEASE
#define MAX_XHLOCKS_NR 64UL
struct hist_lock *xhlocks; /* Crossrelease history locks */
unsigned int xhlock_idx;
/* For restoring at history boundaries */
unsigned int xhlock_idx_hist[XHLOCK_CTX_NR];
unsigned int hist_id;
/* For overwrite check at each context exit */
unsigned int hist_id_save[XHLOCK_CTX_NR];
#endif
#ifdef CONFIG_UBSAN
unsigned int in_ubsan;
#endif
/* Journalling filesystem info: */
void *journal_info;
/* Stacked block device info: */
struct bio_list *bio_list;
#ifdef CONFIG_BLOCK
/* Stack plugging: */
struct blk_plug *plug;
#endif
/* VM state: */
struct reclaim_state *reclaim_state;
struct backing_dev_info *backing_dev_info;
struct io_context *io_context;
/* Ptrace state: */
unsigned long ptrace_message;
siginfo_t *last_siginfo;
struct task_io_accounting ioac;
#ifdef CONFIG_PSI
/* Pressure stall state */
unsigned int psi_flags;
#endif
#ifdef CONFIG_TASK_XACCT
/* Accumulated RSS usage: */
u64 acct_rss_mem1;
/* Accumulated virtual memory usage: */
u64 acct_vm_mem1;
/* stime + utime since last update: */
u64 acct_timexpd;
#endif
#ifdef CONFIG_CPUSETS
/* Protected by ->alloc_lock: */
nodemask_t mems_allowed;
/* Seqence number to catch updates: */
seqcount_t mems_allowed_seq;
int cpuset_mem_spread_rotor;
int cpuset_slab_spread_rotor;
#endif
#ifdef CONFIG_CGROUPS
/* Control Group info protected by css_set_lock: */
struct css_set __rcu *cgroups;
/* cg_list protected by css_set_lock and tsk->alloc_lock: */
struct list_head cg_list;
#endif
#ifdef CONFIG_INTEL_RDT
u32 closid;
u32 rmid;
#endif
#ifdef CONFIG_FUTEX
struct robust_list_head __user *robust_list;
#ifdef CONFIG_COMPAT
struct compat_robust_list_head __user *compat_robust_list;
#endif
struct list_head pi_state_list;
struct futex_pi_state *pi_state_cache;
#endif
#ifdef CONFIG_PERF_EVENTS
struct perf_event_context *perf_event_ctxp[perf_nr_task_contexts];
struct mutex perf_event_mutex;
struct list_head perf_event_list;
#endif
#ifdef CONFIG_DEBUG_PREEMPT
unsigned long preempt_disable_ip;
#endif
#ifdef CONFIG_NUMA
/* Protected by alloc_lock: */
struct mempolicy *mempolicy;
short il_prev;
short pref_node_fork;
#endif
#ifdef CONFIG_NUMA_BALANCING
int numa_scan_seq;
unsigned int numa_scan_period;
unsigned int numa_scan_period_max;
int numa_preferred_nid;
unsigned long numa_migrate_retry;
/* Migration stamp: */
u64 node_stamp;
u64 last_task_numa_placement;
u64 last_sum_exec_runtime;
struct callback_head numa_work;
struct list_head numa_entry;
struct numa_group *numa_group;
/*
* numa_faults is an array split into four regions:
* faults_memory, faults_cpu, faults_memory_buffer, faults_cpu_buffer
* in this precise order.
*
* faults_memory: Exponential decaying average of faults on a per-node
* basis. Scheduling placement decisions are made based on these
* counts. The values remain static for the duration of a PTE scan.
* faults_cpu: Track the nodes the process was running on when a NUMA
* hinting fault was incurred.
* faults_memory_buffer and faults_cpu_buffer: Record faults per node
* during the current scan window. When the scan completes, the counts
* in faults_memory and faults_cpu decay and these values are copied.
*/
unsigned long *numa_faults;
unsigned long total_numa_faults;
/*
* numa_faults_locality tracks if faults recorded during the last
* scan window were remote/local or failed to migrate. The task scan
* period is adapted based on the locality of the faults with different
* weights depending on whether they were shared or private faults
*/
unsigned long numa_faults_locality[3];
unsigned long numa_pages_migrated;
#endif /* CONFIG_NUMA_BALANCING */
struct tlbflush_unmap_batch tlb_ubc;
struct rcu_head rcu;
/* Cache last used pipe for splice(): */
struct pipe_inode_info *splice_pipe;
struct page_frag task_frag;
#ifdef CONFIG_TASK_DELAY_ACCT
struct task_delay_info *delays;
#endif
#ifdef CONFIG_FAULT_INJECTION
int make_it_fail;
unsigned int fail_nth;
#endif
/*
* When (nr_dirtied >= nr_dirtied_pause), it's time to call
* balance_dirty_pages() for a dirty throttling pause:
*/
int nr_dirtied;
int nr_dirtied_pause;
/* Start of a write-and-pause period: */
unsigned long dirty_paused_when;
#ifdef CONFIG_LATENCYTOP
int latency_record_count;
struct latency_record latency_record[LT_SAVECOUNT];
#endif
/*
* Time slack values; these are used to round up poll() and
* select() etc timeout values. These are in nanoseconds.
*/
u64 timer_slack_ns;
u64 default_timer_slack_ns;
#ifdef CONFIG_KASAN
unsigned int kasan_depth;
#endif
#ifdef CONFIG_FUNCTION_GRAPH_TRACER
/* Index of current stored address in ret_stack: */
int curr_ret_stack;
/* Stack of return addresses for return function tracing: */
struct ftrace_ret_stack *ret_stack;
/* Timestamp for last schedule: */
unsigned long long ftrace_timestamp;
/*
* Number of functions that haven't been traced
* because of depth overrun:
*/
atomic_t trace_overrun;
/* Pause tracing: */
atomic_t tracing_graph_pause;
#endif
#ifdef CONFIG_TRACING
/* State flags for use by tracers: */
unsigned long trace;
/* Bitmask and counter of trace recursion: */
unsigned long trace_recursion;
#endif /* CONFIG_TRACING */
#ifdef CONFIG_KCOV
/* Coverage collection mode enabled for this task (0 if disabled): */
enum kcov_mode kcov_mode;
/* Size of the kcov_area: */
unsigned int kcov_size;
/* Buffer for coverage collection: */
void *kcov_area;
/* KCOV descriptor wired with this task or NULL: */
struct kcov *kcov;
#endif
#ifdef CONFIG_MEMCG
struct mem_cgroup *memcg_in_oom;
gfp_t memcg_oom_gfp_mask;
int memcg_oom_order;
/* Number of pages to reclaim on returning to userland: */
unsigned int memcg_nr_pages_over_high;
#endif
#ifdef CONFIG_UPROBES
struct uprobe_task *utask;
#endif
#if defined(CONFIG_BCACHE) || defined(CONFIG_BCACHE_MODULE)
unsigned int sequential_io;
unsigned int sequential_io_avg;
#endif
#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
unsigned long task_state_change;
#endif
int pagefault_disabled;
#ifdef CONFIG_MMU
struct task_struct *oom_reaper_list;
#endif
#ifdef CONFIG_VMAP_STACK
struct vm_struct *stack_vm_area;
#endif
#ifdef CONFIG_THREAD_INFO_IN_TASK
/* A live task holds one reference: */
atomic_t stack_refcount;
#endif
#ifdef CONFIG_LIVEPATCH
int patch_state;
#endif
#ifdef CONFIG_SECURITY
/* Used by LSM modules for access restriction: */
void *security;
#endif
/*
* New fields for task_struct should be added above here, so that
* they are included in the randomized portion of task_struct.
*/
randomized_struct_fields_end
/* CPU-specific state of this task: */
struct thread_struct thread;
/*
* WARNING: on x86, 'thread_struct' contains a variable-sized
* structure. It *MUST* be at the end of 'task_struct'.
*
* Do not put anything below here!
*/
};task_sturct에는 프로세스를 관리하기위한 모든 정보가 포함된다.
그 중 커널 익스플로잇 과정에서 흥미롭게 보는 멤버 중 하나가 프로세스의 크리데션을 관리하는 cred이다.
Process Credentials
프로세스의 security context는 struct cred에 의해 정의되며 include/linux/cred.h에 정의되어 있다.
struct cred {
atomic_t usage;
#ifdef CONFIG_DEBUG_CREDENTIALS
atomic_t subscribers; /* number of processes subscribed */
void *put_addr;
unsigned magic;
#define CRED_MAGIC 0x43736564
#define CRED_MAGIC_DEAD 0x44656144
#endif
kuid_t uid; /* real UID of the task */
kgid_t gid; /* real GID of the task */
kuid_t suid; /* saved UID of the task */
kgid_t sgid; /* saved GID of the task */
kuid_t euid; /* effective UID of the task */
kgid_t egid; /* effective GID of the task */
kuid_t fsuid; /* UID for VFS ops */
kgid_t fsgid; /* GID for VFS ops */
unsigned securebits; /* SUID-less security management */
kernel_cap_t cap_inheritable; /* caps our children can inherit */
kernel_cap_t cap_permitted; /* caps we're permitted */
kernel_cap_t cap_effective; /* caps we can actually use */
kernel_cap_t cap_bset; /* capability bounding set */
kernel_cap_t cap_ambient; /* Ambient capability set */
#ifdef CONFIG_KEYS
unsigned char jit_keyring; /* default keyring to attach requested
* keys to */
struct key __rcu *session_keyring; /* keyring inherited over fork */
struct key *process_keyring; /* keyring private to this process */
struct key *thread_keyring; /* keyring private to this thread */
struct key *request_key_auth; /* assumed request_key authority */
#endif
#ifdef CONFIG_SECURITY
void *security; /* subjective LSM security */
#endif
struct user_struct *user; /* real user ID subscription */
struct user_namespace *user_ns; /* user_ns the caps and keyrings are relative to. */
struct group_info *group_info; /* supplementary groups for euid/fsgid */
/* RCU deletion */
union {
int non_rcu; /* Can we skip RCU deletion? */
struct rcu_head rcu; /* RCU deletion hook */
};
} __randomize_layout;대부분의 Linux 커널 익스플로잇에서 root 를 얻기 위해서 아래와 같은 방식을 사용한다.
commit_creds(prepare_kernel_cred(NULL));위에서 사용되는 commit_creds()와 prepare_kernel_cred()에 대해서 간단하게 살펴보도록 하자
먼저 prepare_kernel_cred()는 다음과 같이 정의되어 있다.
struct cred *prepare_kernel_cred(struct task_struct *daemon) // [1]
{
const struct cred *old;
struct cred *new;
new = kmem_cache_alloc(cred_jar, GFP_KERNEL);
if (!new)
return NULL;
kdebug("prepare_kernel_cred() alloc %p", new);
// [2]
if (daemon)
old = get_task_cred(daemon);
else
old = get_cred(&init_cred);
validate_creds(old);
*new = *old;
[...]
validate_creds(new);
return new;
error:
[...]
return NULL;
}[1]을 보면 task_struct 포인터를 사용하는 것을 볼 수 있는데, task_sturct는 커널 크리덴셜 정보를 정의하는 구조체이다.
if (daemon)
old = get_task_cred(daemon);
else
old = get_cred(&init_cred); // NULL인 경우이 함수에서 중요한 부분은 [2] 부분이다.
task_struct의 포인터가 NULL로 설정되어 있으면, Default로 설정된 크리덴셜인 init_cred을 얻게 된다.
init_cred는 kernel/cred.c에 정의 된 전역 구조체 struct cred이며, Linux에서 첫 번째 task인 root로 init_task의 크리덴셜을 초기화하는 데 사용된다.
/*
* The initial credentials for the initial task
*/
struct cred init_cred = {
.usage = ATOMIC_INIT(4),
#ifdef CONFIG_DEBUG_CREDENTIALS
.subscribers = ATOMIC_INIT(2),
.magic = CRED_MAGIC,
#endif
.uid = GLOBAL_ROOT_UID,
.gid = GLOBAL_ROOT_GID,
.suid = GLOBAL_ROOT_UID,
.sgid = GLOBAL_ROOT_GID,
.euid = GLOBAL_ROOT_UID,
.egid = GLOBAL_ROOT_GID,
.fsuid = GLOBAL_ROOT_UID,
.fsgid = GLOBAL_ROOT_GID,
.securebits = SECUREBITS_DEFAULT,
.cap_inheritable = CAP_EMPTY_SET,
.cap_permitted = CAP_FULL_SET,
.cap_effective = CAP_FULL_SET,
.cap_bset = CAP_FULL_SET,
.user = INIT_USER,
.user_ns = &init_user_ns,
.group_info = &init_groups,
};init_cred에서는 위와 같이 정의되어 있으며, uid, gid등의 값이 GLOBAL_ROOT_로 설정된다.GLOBAL_ROOT_는 다음과 같이 정의되어 있다.
#define GLOBAL_ROOT_UID (uint32_t)0
#define GLOBAL_ROOT_GID (uint32_t)0
#define SECUREBITS_DEFAULT (uint32_t)0x00000000
#define CAP_EMPTY_SET (uint64_t)0
#define CAP_FULL_SET (uint64_t)0x3FFFFFFFFF따라서, init_cred는 다음과 같이 root크리덴셜로 cred구조를 설정한다고 볼 수 있다.
cred->uid = 0;
cred->gid = 0;
cred->suid = 0;
cred->idid = 0;
cred->euid = 0;
cred->egid = 0;
cred->fsuid = 0;
cred->fsgid = 0;
cred->securebits = 0;
cred->cap_inheritable.cap[0] = 0;
cred->cap_inheritable.cap[1] = 0;
cred->cap_permitted.cap[0] = 0x3F;
cred->cap_permitted.cap[1] = 0xFFFFFFFF;
cred->cap_effective.cap[0] = 0x3F;
cred->cap_effective.cap[1] = 0xFFFFFFFF;
cred->cap_bset.cap[0] = 0x3F;
cred->cap_bset.cap[1] = 0xFFFFFFFF;
cred->cap_ambient.cap[0] = 0;
cred->cap_ambient.cap[1] = 0;
다음은 commit_creds이다.
int commit_creds(struct cred *new)
{
struct task_struct *task = current;
const struct cred *old = task->real_cred;
[...]
rcu_assign_pointer(task->real_cred, new);
rcu_assign_pointer(task->cred, new);
[...]
return 0;
}commit_creds는 인자로 전달 받은 cred구조에 대한 포인터로 task->real_cred 와 task->cred를 설정한다.
위에서 preparing_kernel_cred인자로 NULL을 전달 했으므로 init_cred로 설정되었을 것이고, 결국에는 root cred로 설정되는 것을 의미한다.
이 방법이 가장 기본적인 root권한 획득 방식이며, Privilege Escalation을 의미한다.
SELinux (Security Enhanced Linux)
SELinux는 NSA(National Security Agency)에서 LSM(Linux Security Modules)을 사용하여 개발되었다.
SELinux에는 다음 두 가지 모드로 동작한다.
permissive: permission denials로그가 기록은 되지만, 규칙은 적용되지 않습니다.
enforcing : permission denials로그가 기록 되고, 규칙이 적용됩니다.
Android에서 SELinux는 기본적으로 enforcing모드이며, root가 되더라도 SELinux 규칙이 적용된다.
generic_x86_64:/ $ getenforce
EnforcingSELinux는 getenforce 명령어로 확인할 수 있다.
따라서, 우리는 LPE를 성공하더라도 모든 자원에 접근하려면 SELinux를 permissive모드로 변경해줘야 한다.
selinux_enforcing
selinux_enforcing은 SELinux의 적용 여부를 나타내는 전역 변수다.
해당 변수의 값이 NULL로 설정되어 있으면 SELinux가 permissive로 동작하게 된다.
따라서, selinux_enforcing이 메모리에 어디에 있는지 알아 낸 뒤, NULL로 설정하여 SELinux를 비활성화 할 수 있다.
SecComp
SecComp는 Secure Computing 모드를 말하며, syscall을 필터링 할 수있는 Linux 커널 기능이다.
해당 기능이 활성화되면 프로세스는 read(), write(), exit(), sigreturn() 4개의 syscall만 사용할 수 있게 된다.
adb 쉘에서 익스플로잇을 진행할 때는 seccomp가 적용되지 않지만, Android 애플리케이션에 익스플로잇을 진행 할 때는 seccomp가 적용된다.
Ref. https://cloudfuzz.github.io/android-kernel-exploitation/chapters/linux-privilege-escalation.html