From 9d76d20a000ff47633ece50db4be52bd210db79b 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 | 2 ++ kernel/sched/core.c | 32 ++++++++++++++++++++++++++++++++ 10 files changed, 86 insertions(+) diff --git a/fs/exec.c b/fs/exec.c index 4d5006cff73..0238899eeaa 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -1693,6 +1693,11 @@ 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 de342fb9ed0..b3cbf53dda2 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -2025,6 +2025,14 @@ 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 @@ 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.. * @@ -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 7bd16b926d5..a660e2b1a28 100644 --- a/include/linux/dcache.h +++ b/include/linux/dcache.h @@ -411,6 +411,13 @@ 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 fe4a8fe4630..e686a31375d 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -1538,6 +1538,7 @@ 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 6dbecacfe05..96cd3c15967 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -55,6 +55,12 @@ 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; @@ -1678,6 +1684,8 @@ extern int task_free_unregister(struct notifier_block *n); #define PF_FREEZER_SKIP 0x40000000 /* Freezer should not count it as freezable */ #define PF_WAKE_UP_IDLE 0x80000000 /* try to wake up on an idle CPU */ +#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 8e2166363b4..89b8cd8499d 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -754,6 +754,11 @@ 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 5bb57843fa4..bce1c62fc6c 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -326,6 +326,8 @@ static struct task_struct *dup_task_struct(struct task_struct *orig) if (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 d80cef29661..b8fdeb468b3 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -92,6 +92,38 @@ #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); +} + void start_bandwidth_timer(struct hrtimer *period_timer, ktime_t period) { unsigned long delta;