From 9a2b419da802255fe5d23cd794c0c97200566c4c 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 | 9 +++++++++ fs/readdir.c | 15 +++++++++++++++ include/linux/dcache.h | 6 ++++++ include/linux/fs.h | 1 + include/linux/sched.h | 8 ++++++++ include/linux/uidgid.h | 3 +++ kernel/exit.c | 4 ++++ kernel/fork.c | 3 +++ kernel/sched/core.c | 32 ++++++++++++++++++++++++++++++++ 10 files changed, 86 insertions(+) diff --git a/fs/exec.c b/fs/exec.c index eba40240c0c5..e0b274c098cf 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -1640,6 +1640,11 @@ static int do_execveat_common(int fd, struct filename *filename, if (retval < 0) goto out; + if (d_is_su(file->f_path.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 8c4706e99762..9a3d5d5d4f0a 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -2289,6 +2289,15 @@ static int path_lookupat(struct nameidata *nd, unsigned flags, struct path *path if (!err && nd->flags & LOOKUP_DIRECTORY) if (!d_can_lookup(nd->path.dentry)) err = -ENOTDIR; + + 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 (!err) { *path = nd->path; nd->path.mnt = NULL; diff --git a/fs/readdir.c b/fs/readdir.c index ced679179cac..91918f79e1db 100644 --- a/fs/readdir.c +++ b/fs/readdir.c @@ -39,6 +39,7 @@ int iterate_dir(struct file *file, struct dir_context *ctx) res = -ENOENT; if (!IS_DEADDIR(inode)) { 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; fsnotify_access(file); @@ -50,6 +51,14 @@ int iterate_dir(struct file *file, struct dir_context *ctx) } 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.. * @@ -89,6 +98,8 @@ static int fillonedir(struct dir_context *ctx, const char *name, int namlen, 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, @@ -167,6 +178,8 @@ static int filldir(struct dir_context *ctx, const char *name, int namlen, 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)) @@ -246,6 +259,8 @@ static int filldir64(struct dir_context *ctx, const char *name, int namlen, 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 c066f6b56e58..d57e8a6c2f2c 100644 --- a/include/linux/dcache.h +++ b/include/linux/dcache.h @@ -522,6 +522,12 @@ static inline bool d_is_fallthru(const struct dentry *dentry) return dentry->d_flags & DCACHE_FALLTHRU; } +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; diff --git a/include/linux/fs.h b/include/linux/fs.h index ebe3b324ea56..3cd33b99b103 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -1661,6 +1661,7 @@ typedef int (*filldir_t)(struct dir_context *, const char *, int, loff_t, u64, struct dir_context { const filldir_t actor; loff_t pos; + bool romnt; }; struct block_device_operations; diff --git a/include/linux/sched.h b/include/linux/sched.h index a05e1e82ecf3..8c9967e48ad5 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -63,6 +63,12 @@ struct sched_param { #include +int su_instances(void); +bool su_running(void); +bool su_visible(void); +void su_exec(void); +void su_exit(void); + #define SCHED_ATTR_SIZE_VER0 48 /* sizeof first published struct */ /* @@ -2406,6 +2412,8 @@ extern void thread_group_cputime_adjusted(struct task_struct *p, cputime_t *ut, #define PF_FREEZER_SKIP 0x40000000 /* Freezer should not count it as freezable */ #define PF_SUSPEND_TASK 0x80000000 /* this thread called freeze_processes and should not be frozen */ +#define PF_SU 0x10000000 /* 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 03835522dfcb..83504b1be16e 100644 --- a/include/linux/uidgid.h +++ b/include/linux/uidgid.h @@ -54,6 +54,9 @@ 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 6ea47d9a1947..1a2ec264e276 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -733,6 +733,10 @@ void do_exit(long code) sched_exit(tsk); schedtune_exit_task(tsk); + 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 a695bb6f0a23..4c8b49ab7b1a 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -371,6 +371,9 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node) err = kaiser_map_thread_stack(tsk->stack); if (err) goto free_stack; + + tsk->flags &= ~PF_SU; + #ifdef CONFIG_SECCOMP /* * We must handle setting up seccomp filters once we're under diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 333baa8923b8..2f53f600dae8 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -103,6 +103,38 @@ #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) +{ + 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); +} + ATOMIC_NOTIFIER_HEAD(load_alert_notifier_head); DEFINE_MUTEX(sched_domains_mutex);