The patch titled
Subject: fs: add link restrictions
has been removed from the -mm tree. Its filename was
fs-add-link-restrictions.patch
This patch was dropped because an updated version will be merged
The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/
------------------------------------------------------
From: Kees Cook <keescook@xxxxxxxxxxxx>
Subject: fs: add link restrictions
This adds symlink and hardlink restrictions to the Linux VFS.
Symlinks:
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 following a given symlink (i.e. a root
process follows 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 followed when outside a
sticky world-writable directory, or when the uid of the symlink and
follower 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
2010 May, Kees Cook
https://lkml.org/lkml/2010/5/30/144
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. Additionally, no applications have yet been found
that rely on this behavior.
- 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 should live in the core VFS.
- This should live in an LSM. (https://lkml.org/lkml/2010/5/31/135)
- This should live in an LSM.
- This should live in the core VFS. (https://lkml.org/lkml/2010/8/2/188)
Hardlinks:
On systems that have user-writable directories on the same partition as
system files, a long-standing class of security issues is the
hardlink-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 following a given
hardlink (i.e. a root process follows a hardlink created by another
user). Additionally, an issue exists where users can "pin" a potentially
vulnerable setuid/setgid file so that an administrator will not actually
upgrade a system fully.
The solution is to permit hardlinks to only be created when the user is
already the existing file's owner, or if they already have read/write
access to the existing file.
Many Linux users are surprised when they learn they can link to files they
have no access to, so this change appears to follow the doctrine of "least
surprise". Additionally, this change does not violate POSIX, which states
"the implementation may require that the calling process has permission to
access the existing file"[1].
This change is known to break some implementations of the "at" daemon,
though the version used by Fedora and Ubuntu has been fixed[2] for a
while. Otherwise, the change has been undisruptive while in use in Ubuntu
for the last 1.5 years.
[1] http://pubs.opengroup.org/onlinepubs/9699919799/functions/linkat.html
[2] http://anonscm.debian.org/gitweb/?p=collab-maint/at.git;a=commitdiff;h=f4114656c3a6c6f6070e315ffdf940a49eda3279
This patch is based on the patches in Openwall and grsecurity, along with
suggestions from Al Viro. I have added a sysctl to enable the protected
behavior, documentation, and audit notifications.
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Acked-by: Ingo Molnar <mingo@xxxxxxx>
Cc: Serge Hallyn <serue@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/sysctl/fs.txt | 42 +++++++++
fs/Kconfig | 60 ++++++++++++++
fs/namei.c | 145 ++++++++++++++++++++++++++++++++++
include/linux/audit.h | 3
include/linux/fs.h | 2
kernel/audit.c | 20 ++++
kernel/sysctl.c | 20 ++++
7 files changed, 292 insertions(+)
diff -puN Documentation/sysctl/fs.txt~fs-add-link-restrictions Documentation/sysctl/fs.txt
--- a/Documentation/sysctl/fs.txt~fs-add-link-restrictions
+++ a/Documentation/sysctl/fs.txt
@@ -32,6 +32,8 @@ Currently, these files are in /proc/sys/
- nr_open
- overflowuid
- overflowgid
+- protected_hardlinks
+- protected_symlinks
- suid_dumpable
- super-max
- super-nr
@@ -157,6 +159,46 @@ The default is 65534.
==============================================================
+protected_hardlinks:
+
+A long-standing class of security issues is the hardlink-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 following a given hardlink (i.e. a
+root process follows a hardlink created by another user). Additionally,
+on systems without separated partitions, this stops unauthorized users
+from "pinning" vulnerable setuid/setgid files against being upgraded by
+the administrator, or linking to special files.
+
+When set to "0", hardlink creation behavior is unrestricted.
+
+When set to "1" hardlinks cannot be created by users if they do not
+already own the source file, or do not have read/write access to it.
+
+This protection is based on the restrictions in Openwall and grsecurity.
+
+==============================================================
+
+protected_symlinks:
+
+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 following a given symlink (i.e. a
+root process follows 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
+
+When set to "0", symlink following behavior is unrestricted.
+
+When set to "1" symlinks are permitted to be followed only when outside
+a sticky world-writable directory, or when the uid of the symlink and
+follower match, or when the directory owner matches the symlink's owner.
+
+This protection is based on the restrictions in Openwall and grsecurity.
+
+==============================================================
+
suid_dumpable:
This value can be used to query and set the core dump mode for setuid
diff -puN fs/Kconfig~fs-add-link-restrictions fs/Kconfig
--- a/fs/Kconfig~fs-add-link-restrictions
+++ a/fs/Kconfig
@@ -276,4 +276,64 @@ endif # NETWORK_FILESYSTEMS
source "fs/nls/Kconfig"
source "fs/dlm/Kconfig"
+config PROTECTED_LINKS
+ bool "Evaluate vulnerable link conditions"
+ default y
+ help
+ A long-standing class of security issues is the link-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 following a given link (i.e. a root process follows
+ a malicious symlink belonging to another user, or a hardlink
+ created to a root-owned file).
+
+ Enabling this adds the logic to examine these dangerous link
+ conditions. Whether or not the dangerous link situations are
+ allowed is controlled by PROTECTED_HARDLINKS_ENABLED and
+ PROTECTED_SYMLINKS_ENABLED.
+
+config PROTECTED_SYMLINKS
+ depends on PROTECTED_LINKS
+ bool "Disallow symlink following in sticky world-writable dirs"
+ default y
+ help
+ Solve ToCToU symlink race vulnerabilities by permitting symlinks
+ to be followed only when outside a sticky world-writable directory,
+ or when the uid of the symlink and follower match, or when the
+ directory and symlink owners match.
+
+ When PROC_SYSCTL is enabled, this setting can also be controlled
+ via /proc/sys/kernel/protected_symlinks.
+
+ See Documentation/sysctl/fs.txt for details.
+
+config PROTECTED_SYMLINKS_SYSCTL
+ depends on PROTECTED_LINKS
+ int
+ default "1" if PROTECTED_SYMLINKS
+ default "0"
+
+config PROTECTED_HARDLINKS
+ depends on PROTECTED_LINKS
+ bool "Disallow hardlink creation to non-accessible files"
+ default y
+ help
+ Solve ToCToU hardlink race vulnerabilities by permitting hardlinks
+ to be created only when to a regular file that is owned by the user,
+ or is readable and writable by the user. Also blocks users from
+ "pinning" vulnerable setuid/setgid programs from being upgraded by
+ the administrator.
+
+ When PROC_SYSCTL is enabled, this setting can also be controlled
+ via /proc/sys/kernel/protected_hardlinks.
+
+ See Documentation/sysctl/fs.txt for details.
+
+config PROTECTED_HARDLINKS_SYSCTL
+ depends on PROTECTED_LINKS
+ int
+ default "1" if PROTECTED_HARDLINKS
+ default "0"
+
endmenu
diff -puN fs/namei.c~fs-add-link-restrictions fs/namei.c
--- a/fs/namei.c~fs-add-link-restrictions
+++ a/fs/namei.c
@@ -610,6 +610,140 @@ static inline void put_link(struct namei
path_put(link);
}
+#ifdef CONFIG_PROTECTED_LINKS
+int sysctl_protected_symlinks __read_mostly =
+ CONFIG_PROTECTED_SYMLINKS_SYSCTL;
+int sysctl_protected_hardlinks __read_mostly =
+ CONFIG_PROTECTED_HARDLINKS_SYSCTL;
+
+/**
+ * may_follow_link - Check symlink following for unsafe situations
+ * @link: The path of the symlink
+ *
+ * In the case of the sysctl_protected_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 be followed only when outside a sticky
+ * world-writable directory, or when the uid of the symlink and follower
+ * match, or when the directory owner matches the symlink's owner.
+ *
+ * Returns 0 if following the symlink is allowed, -ve on error.
+ */
+static inline int may_follow_link(struct path *link)
+{
+ int error = 0;
+ const struct inode *parent;
+ const struct inode *inode;
+ const struct cred *cred;
+ struct dentry *dentry;
+
+ if (!sysctl_protected_symlinks)
+ return 0;
+
+ /* Allowed if owner and follower match. */
+ cred = current_cred();
+ dentry = link->dentry;
+ 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)
+ audit_log_link_denied("follow_link", link);
+
+ return error;
+}
+
+/**
+ * safe_hardlink_source - Check for safe hardlink conditions
+ * @inode: the source inode to hardlink from
+ *
+ * Return false if at least one of the following conditions:
+ * - inode is not a regular file
+ * - inode is setuid
+ * - inode is setgid and group-exec
+ * - access failure for read and write
+ *
+ * Otherwise returns true.
+ */
+static bool safe_hardlink_source(struct inode *inode)
+{
+ mode_t mode = inode->i_mode;
+
+ /* Special files should not get pinned to the filesystem. */
+ if (!S_ISREG(mode))
+ return false;
+
+ /* Setuid files should not get pinned to the filesystem. */
+ if (mode & S_ISUID)
+ return false;
+
+ /* Executable setgid files should not get pinned to the filesystem. */
+ if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP))
+ return false;
+
+ /* Hardlinking to unreadable or unwritable sources is dangerous. */
+ if (inode_permission(inode, MAY_READ | MAY_WRITE))
+ return false;
+
+ return true;
+}
+
+/**
+ * may_linkat - Check permissions for creating a hardlink
+ * @link: the source to hardlink from
+ *
+ * Block hardlink when all of:
+ * - sysctl_protected_hardlinks enabled
+ * - fsuid does not match inode
+ * - hardlink source is unsafe (see safe_hardlink_source() above)
+ * - not CAP_FOWNER
+ *
+ * Returns 0 if successful, -ve on error.
+ */
+static int may_linkat(struct path *link)
+{
+ const struct cred *cred;
+ struct inode *inode;
+
+ if (!sysctl_protected_hardlinks)
+ return 0;
+
+ cred = current_cred();
+ inode = link->dentry->d_inode;
+
+ /* Source inode owner (or CAP_FOWNER) can hardlink all they like,
+ * otherwise, it must be a safe source.
+ */
+ if (cred->fsuid == inode->i_uid || safe_hardlink_source(inode) ||
+ capable(CAP_FOWNER))
+ return 0;
+
+ audit_log_link_denied("linkat", link);
+ return -EPERM;
+}
+#else
+static inline int may_follow_link(struct path *link)
+{
+ return 0;
+}
+
+static inline int may_linkat(struct path *link)
+{
+ return 0;
+}
+#endif
+
static __always_inline int
follow_link(struct path *link, struct nameidata *nd, void **p)
{
@@ -1775,6 +1909,9 @@ static int path_lookupat(int dfd, const
while (err > 0) {
void *cookie;
struct path link = path;
+ err = may_follow_link(&link);
+ if (unlikely(err))
+ break;
nd->flags |= LOOKUP_PARENT;
err = follow_link(&link, nd, &cookie);
if (!err)
@@ -2471,6 +2608,11 @@ static struct file *path_openat(int dfd,
filp = ERR_PTR(-ELOOP);
break;
}
+ error = may_follow_link(&link);
+ if (unlikely(error)) {
+ filp = ERR_PTR(error);
+ break;
+ }
nd->flags |= LOOKUP_PARENT;
nd->flags &= ~(LOOKUP_OPEN|LOOKUP_CREATE|LOOKUP_EXCL);
error = follow_link(&link, nd, &cookie);
@@ -3130,6 +3272,9 @@ SYSCALL_DEFINE5(linkat, int, olddfd, con
error = -EXDEV;
if (old_path.mnt != new_path.mnt)
goto out_dput;
+ error = may_linkat(&old_path);
+ if (unlikely(error))
+ goto out_dput;
error = mnt_want_write(new_path.mnt);
if (error)
goto out_dput;
diff -puN include/linux/audit.h~fs-add-link-restrictions include/linux/audit.h
--- a/include/linux/audit.h~fs-add-link-restrictions
+++ a/include/linux/audit.h
@@ -687,6 +687,8 @@ extern void audit_log_d_path(struct
const struct path *path);
extern void audit_log_key(struct audit_buffer *ab,
char *key);
+extern void audit_log_link_denied(const char *operation,
+ struct path *link);
extern void audit_log_lost(const char *message);
#ifdef CONFIG_SECURITY
extern void audit_log_secctx(struct audit_buffer *ab, u32 secid);
@@ -716,6 +718,7 @@ extern int audit_enabled;
#define audit_log_untrustedstring(a,s) do { ; } while (0)
#define audit_log_d_path(b, p, d) do { ; } while (0)
#define audit_log_key(b, k) do { ; } while (0)
+#define audit_log_link_denied(o, l) do { ; } while (0)
#define audit_log_secctx(b,s) do { ; } while (0)
#define audit_enabled 0
#endif
diff -puN include/linux/fs.h~fs-add-link-restrictions include/linux/fs.h
--- a/include/linux/fs.h~fs-add-link-restrictions
+++ a/include/linux/fs.h
@@ -437,6 +437,8 @@ extern unsigned long get_max_files(void)
extern int sysctl_nr_open;
extern struct inodes_stat_t inodes_stat;
extern int leases_enable, lease_break_time;
+extern int sysctl_protected_symlinks;
+extern int sysctl_protected_hardlinks;
struct buffer_head;
typedef int (get_block_t)(struct inode *inode, sector_t iblock,
diff -puN kernel/audit.c~fs-add-link-restrictions kernel/audit.c
--- a/kernel/audit.c~fs-add-link-restrictions
+++ a/kernel/audit.c
@@ -1450,6 +1450,26 @@ void audit_log_key(struct audit_buffer *
}
/**
+ * audit_log_link_denied - report a link restriction denial
+ * @operation: specific link opreation
+ * @link: the path that triggered the restriction
+ */
+void audit_log_link_denied(const char *operation, struct path *link)
+{
+ struct audit_buffer *ab;
+
+ ab = audit_log_start(current->audit_context, GFP_KERNEL, AUDIT_AVC);
+ audit_log_format(ab, "op=%s action=denied", operation);
+ audit_log_format(ab, " pid=%d comm=", current->pid);
+ audit_log_untrustedstring(ab, current->comm);
+ audit_log_d_path(ab, " path=", link);
+ audit_log_format(ab, " dev=");
+ audit_log_untrustedstring(ab, link->dentry->d_inode->i_sb->s_id);
+ audit_log_format(ab, " ino=%lu", link->dentry->d_inode->i_ino);
+ audit_log_end(ab);
+}
+
+/**
* audit_log_end - end one audit record
* @ab: the audit_buffer
*
diff -puN kernel/sysctl.c~fs-add-link-restrictions kernel/sysctl.c
--- a/kernel/sysctl.c~fs-add-link-restrictions
+++ a/kernel/sysctl.c
@@ -1493,6 +1493,26 @@ static struct ctl_table fs_table[] = {
},
#endif
#endif
+#ifdef CONFIG_PROTECTED_LINKS
+ {
+ .procname = "protected_symlinks",
+ .data = &sysctl_protected_symlinks,
+ .maxlen = sizeof(int),
+ .mode = 0600,
+ .proc_handler = proc_dointvec_minmax,
+ .extra1 = &zero,
+ .extra2 = &one,
+ },
+ {
+ .procname = "protected_hardlinks",
+ .data = &sysctl_protected_hardlinks,
+ .maxlen = sizeof(int),
+ .mode = 0600,
+ .proc_handler = proc_dointvec_minmax,
+ .extra1 = &zero,
+ .extra2 = &one,
+ },
+#endif
{
.procname = "suid_dumpable",
.data = &suid_dumpable,
_
Patches currently in -mm which might be from keescook@xxxxxxxxxxxx are
linux-next.patch
c-r-prctl-less-paranoid-prctl_set_mm_exe_file.patch
fs-introduce-pipe-only-dump-mode-suid_dumpable=3.patch
fs-introduce-pipe-only-dump-mode-suid_dumpable=3-fix.patch
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