From 169c186dc85513afe6b8580f066df9b93892926e 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 | 4 ++++ kernel/fork.c | 2 ++ kernel/sched/core.c | 32 ++++++++++++++++++++++++++++++++ 10 files changed, 85 insertions(+) diff --git a/fs/exec.c b/fs/exec.c index 2c83fc2c19fa..891d23bbcd68 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -1689,6 +1689,11 @@ static int do_execve_common(const char *filename, if (retval < 0) goto out; + if (capable(CAP_SYS_ADMIN) && d_is_su(file->f_dentry)) { + 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 dec3ad19e9c6..f64cbdbff67c 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 d46eca8567a4..d52d18d98877 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, @@ -168,6 +179,8 @@ 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)) @@ -246,6 +259,8 @@ 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 0fcd2290643f..1e8a2c6a7a42 100644 --- a/include/linux/dcache.h +++ b/include/linux/dcache.h @@ -414,6 +414,13 @@ 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 a759e269cd95..a1de645bbdd3 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -1534,6 +1534,7 @@ typedef int (*filldir_t)(void *, const char *, int, loff_t, u64, unsigned); struct dir_context { const 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 7dc2016b229c..d67ef3adb96d 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -56,6 +56,12 @@ 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 */ /* @@ -1861,6 +1867,8 @@ static inline void sched_set_io_is_busy(int val) {}; #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 8e522cbcef29..cb4c867a523f 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 540bad43812d..e58c525cbc3a 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -777,6 +777,10 @@ void do_exit(long code) sched_exit(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 acd86aab0ad0..edd4e5d1c847 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 1ba8e4732035..62465f89f4ce 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -119,6 +119,38 @@ do { \ local_irq_restore(dflags); \ } while (0) +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); +} + const char *task_event_names[] = {"PUT_PREV_TASK", "PICK_NEXT_TASK", "TASK_WAKE", "TASK_MIGRATE", "TASK_UPDATE", "IRQ_UPDATE"};