From 672c40a112898c4a6467892e22d5979c14aa0820 Mon Sep 17 00:00:00 2001 From: Tom Marshall Date: Wed, 25 Jan 2017 18:01:03 +0100 Subject: [PATCH] kernel: Only expose su when daemon is running It has been claimed that the PG implementation of 'su' has security vulnerabilities even when disabled. Unfortunately, the people that find these vulnerabilities often like to keep them private so they can profit from exploits while leaving users exposed to malicious hackers. In order to reduce the attack surface for vulnerabilites, it is therefore necessary to make 'su' completely inaccessible when it is not in use (except by the root and system users). Change-Id: I79716c72f74d0b7af34ec3a8054896c6559a181d --- fs/exec.c | 5 +++++ fs/namei.c | 8 ++++++++ fs/readdir.c | 15 +++++++++++++++ include/linux/dcache.h | 7 +++++++ include/linux/fs.h | 1 + include/linux/sched.h | 8 ++++++++ include/linux/uidgid.h | 3 +++ kernel/exit.c | 5 +++++ kernel/fork.c | 3 +++ kernel/sched/core.c | 32 ++++++++++++++++++++++++++++++++ 10 files changed, 87 insertions(+) diff --git a/fs/exec.c b/fs/exec.c index fd2778918e8..62de118065e 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -1587,6 +1587,11 @@ static int do_execve_common(const char *filename, if (retval < 0) goto out; + if (d_is_su(file->f_dentry) && capable(CAP_SYS_ADMIN)) { + current->flags |= PF_SU; + su_exec(); + } + /* execve succeeded */ current->fs->in_exec = 0; current->in_execve = 0; diff --git a/fs/namei.c b/fs/namei.c index c87e15ee925..36d4b29459e 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -2009,6 +2009,14 @@ static int path_lookupat(int dfd, const char *name, } } + if (!err) { + struct super_block *sb = nd->inode->i_sb; + if (sb->s_flags & MS_RDONLY) { + if (d_is_su(nd->path.dentry) && !su_visible()) + err = -ENOENT; + } + } + if (base) fput(base); diff --git a/fs/readdir.c b/fs/readdir.c index 5d6578affbb..516fc904513 100644 --- a/fs/readdir.c +++ b/fs/readdir.c @@ -39,6 +39,7 @@ int iterate_dir(struct file *file, struct dir_context *ctx) if (!IS_DEADDIR(inode)) { if (file->f_op->iterate) { ctx->pos = file->f_pos; + ctx->romnt = (inode->i_sb->s_flags & MS_RDONLY); res = file->f_op->iterate(file, ctx); file->f_pos = ctx->pos; } else { @@ -53,6 +54,14 @@ out: } EXPORT_SYMBOL(iterate_dir); +static bool hide_name(const char *name, int namlen) +{ + if (namlen == 2 && !memcmp(name, "su", 2)) + if (!su_visible()) + return true; + return false; +} + /* * Traditional linux readdir() handling.. * @@ -91,6 +100,8 @@ static int fillonedir(void * __buf, const char * name, int namlen, loff_t offset buf->result = -EOVERFLOW; return -EOVERFLOW; } + if (hide_name(name, namlen) && buf->ctx.romnt) + return 0; buf->result++; dirent = buf->dirent; if (!access_ok(VERIFY_WRITE, dirent, @@ -169,6 +180,8 @@ static int filldir(void * __buf, const char * name, int namlen, loff_t offset, buf->error = -EOVERFLOW; return -EOVERFLOW; } + if (hide_name(name, namlen) && buf->ctx.romnt) + return 0; dirent = buf->previous; if (dirent) { if (__put_user(offset, &dirent->d_off)) @@ -249,6 +262,8 @@ static int filldir64(void * __buf, const char * name, int namlen, loff_t offset, buf->error = -EINVAL; /* only used if we fail.. */ if (reclen > buf->count) return -EINVAL; + if (hide_name(name, namlen) && buf->ctx.romnt) + return 0; dirent = buf->previous; if (dirent) { if (__put_user(offset, &dirent->d_off)) diff --git a/include/linux/dcache.h b/include/linux/dcache.h index c1999d1fe6f..65a8fb6c769 100644 --- a/include/linux/dcache.h +++ b/include/linux/dcache.h @@ -410,6 +410,13 @@ static inline bool d_mountpoint(struct dentry *dentry) return dentry->d_flags & DCACHE_MOUNTED; } +static inline bool d_is_su(const struct dentry *dentry) +{ + return dentry && + dentry->d_name.len == 2 && + !memcmp(dentry->d_name.name, "su", 2); +} + extern int sysctl_vfs_cache_pressure; #endif /* __LINUX_DCACHE_H */ diff --git a/include/linux/fs.h b/include/linux/fs.h index 3fe45fa4b78..0d4d337e723 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -1509,6 +1509,7 @@ typedef int (*filldir_t)(void *, const char *, int, loff_t, u64, unsigned); struct dir_context { filldir_t actor; loff_t pos; + bool romnt; }; static inline bool dir_emit(struct dir_context *ctx, diff --git a/include/linux/sched.h b/include/linux/sched.h index 7d5ffda7111..b290b0c0301 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -56,6 +56,12 @@ struct sched_param { #include #include +int su_instances(void); +bool su_running(void); +bool su_visible(void); +void su_exec(void); +void su_exit(void); + struct exec_domain; struct futex_pi_state; struct robust_list_head; @@ -1774,6 +1780,8 @@ extern int task_free_unregister(struct notifier_block *n); #define task_in_mtkpasr(task) unlikely(task->flags & PF_MTKPASR) +#define PF_SU 0x00000002 /* task is su */ + /* * Only the _current_ task can read/write to tsk->flags, but other * tasks can access tsk->flags in readonly mode for example diff --git a/include/linux/uidgid.h b/include/linux/uidgid.h index 8e522cbcef2..cb4c867a523 100644 --- a/include/linux/uidgid.h +++ b/include/linux/uidgid.h @@ -64,6 +64,9 @@ static inline gid_t __kgid_val(kgid_t gid) #define GLOBAL_ROOT_UID KUIDT_INIT(0) #define GLOBAL_ROOT_GID KGIDT_INIT(0) +#define GLOBAL_SYSTEM_UID KUIDT_INIT(1000) +#define GLOBAL_SYSTEM_GID KGIDT_INIT(1000) + #define INVALID_UID KUIDT_INIT(-1) #define INVALID_GID KGIDT_INIT(-1) diff --git a/kernel/exit.c b/kernel/exit.c index b75ebb365ce..72bde1db3bd 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -778,6 +778,11 @@ void do_exit(long code) } exit_signals(tsk); /* sets PF_EXITING */ + + if (tsk->flags & PF_SU) { + su_exit(); + } + /* * tsk->flags are checked in the futex code to protect against * an exiting task cleaning up the robust pi futexes. diff --git a/kernel/fork.c b/kernel/fork.c index 8b32b9205cf..99408165ded 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -334,6 +334,9 @@ static struct task_struct *dup_task_struct(struct task_struct *orig) printk("[%d:%s] fork fail at arch_dup_task_struct, err:%d \n", current->pid, current->comm, err); goto free_ti; } + + tsk->flags &= ~PF_SU; + tsk->stack = ti; #ifdef CONFIG_SECCOMP /* diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 3c359e0c838..b817cf43ff4 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -102,6 +102,38 @@ # include #endif +static atomic_t __su_instances; + +int su_instances(void) +{ + return atomic_read(&__su_instances); +} + +bool su_running(void) +{ + return su_instances() > 0; +} + +bool su_visible(void) +{ + kuid_t uid = current_uid(); + if (su_running()) + return true; + if (uid_eq(uid, GLOBAL_ROOT_UID) || uid_eq(uid, GLOBAL_SYSTEM_UID)) + return true; + return false; +} + +void su_exec(void) +{ + atomic_inc(&__su_instances); +} + +void su_exit(void) +{ + atomic_dec(&__su_instances); +} + void start_bandwidth_timer(struct hrtimer *period_timer, ktime_t period) { unsigned long delta;