From e864a35d26a940bd763568c5f74162978eca2d26 Mon Sep 17 00:00:00 2001 From: Tom Marshall Date: Fri, 28 Apr 2017 22:46:37 +0000 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: Ia7d50ba46c3d932c2b0ca5fc8e9ec69ec9045f85 Signed-off-by: Kevin F. Haggerty --- fs/exec.c | 5 +++++ fs/namei.c | 8 ++++++++ fs/readdir.c | 20 ++++++++++++++++++++ include/linux/dcache.h | 7 +++++++ include/linux/sched.h | 8 ++++++++ kernel/exit.c | 5 +++++ kernel/fork.c | 2 ++ kernel/sched/core.c | 32 ++++++++++++++++++++++++++++++++ 8 files changed, 87 insertions(+) diff --git a/fs/exec.c b/fs/exec.c index bd452fdd1e2..90c5bf94c7d 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -1563,6 +1563,11 @@ 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 7b557793899..1bec45c8b0f 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -1785,6 +1785,14 @@ 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 cc0a8227cdd..536054080bc 100644 --- a/fs/readdir.c +++ b/fs/readdir.c @@ -47,6 +47,14 @@ out: EXPORT_SYMBOL(vfs_readdir); +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.. * @@ -68,6 +76,7 @@ struct old_linux_dirent { struct readdir_callback { struct old_linux_dirent __user * dirent; int result; + bool romnt; }; static int fillonedir(void * __buf, const char * name, int namlen, loff_t offset, @@ -84,6 +93,8 @@ static int fillonedir(void * __buf, const char * name, int namlen, loff_t offset buf->result = -EOVERFLOW; return -EOVERFLOW; } + if (hide_name(name, namlen) && buf->romnt) + return 0; buf->result++; dirent = buf->dirent; if (!access_ok(VERIFY_WRITE, dirent, @@ -116,6 +127,7 @@ SYSCALL_DEFINE3(old_readdir, unsigned int, fd, buf.result = 0; buf.dirent = dirent; + buf.romnt = (file->f_path.dentry->d_sb->s_flags & MS_RDONLY); error = vfs_readdir(file, fillonedir, &buf); if (buf.result) @@ -144,6 +156,7 @@ struct getdents_callback { struct linux_dirent __user * previous; int count; int error; + bool romnt; }; static int filldir(void * __buf, const char * name, int namlen, loff_t offset, @@ -163,6 +176,8 @@ static int filldir(void * __buf, const char * name, int namlen, loff_t offset, buf->error = -EOVERFLOW; return -EOVERFLOW; } + if (hide_name(name, namlen) && buf->romnt) + return 0; dirent = buf->previous; if (dirent) { if (__put_user(offset, &dirent->d_off)) @@ -210,6 +225,7 @@ SYSCALL_DEFINE3(getdents, unsigned int, fd, buf.previous = NULL; buf.count = count; buf.error = 0; + buf.romnt = (file->f_path.dentry->d_sb->s_flags & MS_RDONLY); error = vfs_readdir(file, filldir, &buf); if (error >= 0) @@ -231,6 +247,7 @@ struct getdents_callback64 { struct linux_dirent64 __user * previous; int count; int error; + bool romnt; }; static int filldir64(void * __buf, const char * name, int namlen, loff_t offset, @@ -244,6 +261,8 @@ 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->romnt) + return 0; dirent = buf->previous; if (dirent) { if (__put_user(offset, &dirent->d_off)) @@ -293,6 +312,7 @@ SYSCALL_DEFINE3(getdents64, unsigned int, fd, buf.previous = NULL; buf.count = count; buf.error = 0; + buf.romnt = (file->f_path.dentry->d_sb->s_flags & MS_RDONLY); error = vfs_readdir(file, filldir64, &buf); if (error >= 0) diff --git a/include/linux/dcache.h b/include/linux/dcache.h index d6afb7628e7..72f8fdf9bc9 100644 --- a/include/linux/dcache.h +++ b/include/linux/dcache.h @@ -398,6 +398,13 @@ static inline bool d_need_lookup(struct dentry *dentry) extern void d_clear_need_lookup(struct dentry *dentry); +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; struct name_snapshot { diff --git a/include/linux/sched.h b/include/linux/sched.h index 0f88be2c38b..0c83f9940e1 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -93,6 +93,12 @@ struct sched_param { #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; @@ -1850,6 +1856,8 @@ extern int task_free_unregister(struct notifier_block *n); #define PF_MUTEX_TESTER 0x20000000 /* Thread belongs to the rt mutex tester */ #define PF_FREEZER_SKIP 0x40000000 /* Freezer should not count it as freezable */ +#define PF_SU 0x80000000 /* 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/kernel/exit.c b/kernel/exit.c index 43e326e7418..908d96d46b8 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -956,6 +956,11 @@ 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 b706730e1e8..cbf3e3f20c4 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -293,6 +293,8 @@ static struct task_struct *dup_task_struct(struct task_struct *orig) if (err) goto out; + tsk->flags &= ~PF_SU; + tsk->stack = ti; setup_thread_stack(tsk, orig); diff --git a/kernel/sched/core.c b/kernel/sched/core.c index d8475092ac5..9f9768d6de8 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -88,6 +88,38 @@ #define CREATE_TRACE_POINTS #include +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) +{ + uid_t uid = current_uid(); + if (su_running()) + return true; + if (uid == 0 || uid == 1000) + return true; + return false; +} + +void su_exec(void) +{ + atomic_inc(&__su_instances); +} + +void su_exit(void) +{ + atomic_dec(&__su_instances); +} + ATOMIC_NOTIFIER_HEAD(migration_notifier_head); void start_bandwidth_timer(struct hrtimer *period_timer, ktime_t period)