A long-standing class of security issues is the symlink-based time-of-check-time-of-use race, most commonly seen in world-writable directories like /tmp. The common method of exploitation of this flaw is to cross privilege boundaries when opening a file through a given symlink (i.e. a root process opens a symlink belonging to another user). For a likely incomplete list of hundreds of examples across the years, please see: http://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=/tmp The solution is to permit symlinks to only be opened when outside a sticky world-writable directory, or when the uid of the symlink and opener match, or when the directory owner matches the symlink's owner. Some pointers to the history of earlier discussion that I could find: 1996 Aug, Zygo Blaxell http://marc.info/?l=bugtraq&m=87602167419830&w=2 1996 Oct, Andrew Tridgell http://lkml.indiana.edu/hypermail/linux/kernel/9610.2/0086.html 1997 Dec, Albert D Cahalan http://lkml.org/lkml/1997/12/16/4 2005 Feb, Lorenzo Hernández García-Hierro http://lkml.indiana.edu/hypermail/linux/kernel/0502.0/1896.html Past objections and rebuttals could be summarized as: - Violates POSIX. - POSIX didn't consider this situation and it's not useful to follow a broken specification at the cost of security. - Might break unknown applications that use this feature. - Applications that break because of the change are easy to spot and fix. Applications that are vulnerable to symlink ToCToU by not having the change aren't. - Applications should just use mkstemp() or O_CREATE|O_EXCL. - True, but applications are not perfect, and new software is written all the time that makes these mistakes; blocking this flaw at the kernel is a single solution to the entire class of vulnerability. This patch is based on the patch in Openwall and grsecurity, but with the scope changed to be only "opening" a symlink. I have added a sysctl to enable the protected behavior, documentation, and a ratelimited warning. v2: - dropped redundant S_ISLNK check. - moved sysctl extern into security.h. - asked to include CC to linux-fsdevel. v3: - move into VFS core. - add CONFIG entry for build-time default. - rename sysctl, invert logic. - use get_task_comm for task name. - lock dentry when checking parent. v4: - limit check to leaf symlink opening. v5: - Kconfig whitespace regressed (thanks to Randy Dunlap for pointing it out) Signed-off-by: Kees Cook <kees.cook@xxxxxxxxxxxxx> --- Documentation/sysctl/fs.txt | 15 ++++++++++ fs/Kconfig | 15 ++++++++++ fs/namei.c | 61 +++++++++++++++++++++++++++++++++++++++++++ kernel/sysctl.c | 10 +++++++ 4 files changed, 101 insertions(+), 0 deletions(-) diff --git a/Documentation/sysctl/fs.txt b/Documentation/sysctl/fs.txt index 6268250..9986bce 100644 --- a/Documentation/sysctl/fs.txt +++ b/Documentation/sysctl/fs.txt @@ -32,6 +32,7 @@ Currently, these files are in /proc/sys/fs: - nr_open - overflowuid - overflowgid +- protected-sticky-symlinks - suid_dumpable - super-max - super-nr @@ -158,6 +159,20 @@ The default is 65534. ============================================================== +protected-sticky-symlinks: + +Opening symlinks in sticky world-writable directories (like /tmp) can be +dangerous due to time-of-check-time-of-use races that frequently result +in security vulnerabilities. + +The default value is "0", leaving the behavior of symlink opening +unchanged from POSIX. A value of "1" will enable the protection, causing +symlinks to be openable only if outside a sticky world-writable directory, +or if the symlink and the opener's uid match, or if the symlink and its +directory are owned by the same uid. + +============================================================== + suid_dumpable: This value can be used to query and set the core dump mode for setuid diff --git a/fs/Kconfig b/fs/Kconfig index 5f85b59..b2cdff3 100644 --- a/fs/Kconfig +++ b/fs/Kconfig @@ -256,3 +256,18 @@ source "fs/nls/Kconfig" source "fs/dlm/Kconfig" endmenu + +config PROTECTED_STICKY_SYMLINKS + bool "Protect symlink opening in sticky world-writable directories" + help + A long-standing class of security issues is the symlink-based + time-of-check-time-of-use race, most commonly seen in + world-writable directories like /tmp. The common method of + exploitation of this flaw is to cross privilege boundaries + when opening a given symlink (i.e. a root process opens a + malicious symlink belonging to another user). + + Enabling this solves the problem by permitting symlinks to only + be opened when outside a sticky world-writable directory, or + when the uid of the symlink and opener match, or when the + directory and symlink owners match. diff --git a/fs/namei.c b/fs/namei.c index 868d0cb..ee9d493 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -32,6 +32,7 @@ #include <linux/fcntl.h> #include <linux/device_cgroup.h> #include <linux/fs_struct.h> +#include <linux/ratelimit.h> #include <asm/uaccess.h> #include "internal.h" @@ -530,6 +531,60 @@ static inline void path_to_nameidata(struct path *path, struct nameidata *nd) nd->path.dentry = path->dentry; } +int protected_sticky_symlinks = CONFIG_PROTECTED_STICKY_SYMLINKS; + +/** + * may_open_sticky_symlink - Check symlink opening for unsafe situations + * @dentry: The inode/dentry of the symlink + * @nameidata: The path data of the symlink + * + * In the case of the protected_sticky_symlinks sysctl being enabled, + * CAP_DAC_OVERRIDE needs to be specifically ignored if the symlink is + * in a sticky world-writable directory. This is to protect privileged + * processes from failing races against path names that may change out + * from under them by way of other users creating malicious symlinks. + * It will permit symlinks to only be opened when outside a sticky + * world-writable directory, or when the uid of the symlink and opener + * match, or when the directory owner matches the symlink's owner. + * + * Returns 0 if opening the symlink is allowed, -ve on error. + */ +static __always_inline int +may_open_sticky_symlink(struct dentry *dentry, struct nameidata *nameidata) +{ + int error = 0; + const struct inode *parent; + const struct inode *inode; + const struct cred *cred; + + if (!protected_sticky_symlinks) + return 0; + + /* owner and opener match? */ + cred = current_cred(); + inode = dentry->d_inode; + if (cred->fsuid == inode->i_uid) + return 0; + + /* check parent directory mode and owner */ + spin_lock(&dentry->d_lock); + parent = dentry->d_parent->d_inode; + if ((parent->i_mode & (S_ISVTX|S_IWOTH)) == (S_ISVTX|S_IWOTH) && + parent->i_uid != inode->i_uid) { + error = -EACCES; + } + spin_unlock(&dentry->d_lock); + + if (error) { + char name[sizeof(current->comm)]; + printk_ratelimited(KERN_NOTICE "non-matching-uid symlink " + "opening attempted in sticky world-writable " + "directory by %s (fsuid %d)\n", + get_task_comm(name, current), cred->fsuid); + } + return error; +} + static __always_inline int __do_follow_link(struct path *path, struct nameidata *nd, void **p) { @@ -1844,6 +1899,12 @@ reval: goto exit_dput; if (count++ == 32) goto exit_dput; + + /* check if this symlink is in a sticky world-write dir */ + error = may_open_sticky_symlink(path.dentry, &nd); + if (error) + goto exit_dput; + /* * This is subtle. Instead of calling do_follow_link() we do * the thing by hands. The reason is that this way we have zero diff --git a/kernel/sysctl.c b/kernel/sysctl.c index 997080f..56affd6 100644 --- a/kernel/sysctl.c +++ b/kernel/sysctl.c @@ -87,6 +87,7 @@ extern int sysctl_oom_kill_allocating_task; extern int sysctl_oom_dump_tasks; extern int max_threads; extern int core_uses_pid; +extern int protected_sticky_symlinks; extern int suid_dumpable; extern char core_pattern[]; extern unsigned int core_pipe_limit; @@ -1455,6 +1456,15 @@ static struct ctl_table fs_table[] = { #endif #endif { + .procname = "protected-sticky-symlinks", + .data = &protected_sticky_symlinks, + .maxlen = sizeof(int), + .mode = 0644, + .proc_handler = proc_dointvec_minmax, + .extra1 = &zero, + .extra2 = &one, + }, + { .procname = "suid_dumpable", .data = &suid_dumpable, .maxlen = sizeof(int), -- 1.7.0.4 -- Kees Cook Ubuntu Security Team -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
