[PATCHv4 1/3] fs: add O_BENEATH flag to openat(2)

From: David Drysdale
Date: Thu Aug 13 2015 - 05:34:04 EST

Add a new O_BENEATH flag for openat(2) which restricts the
provided path, rejecting (with -EPERM) paths that are not beneath
the provided dfd. In particular, reject:
- paths that contain .. components
- paths that begin with /
- symlinks that have paths as above.

Also disallow use of nd_jump_link() for following symlinks without
path expansion, when O_BENEATH is set.

Signed-off-by: David Drysdale <drysdale@xxxxxxxxxx>
---
arch/alpha/include/uapi/asm/fcntl.h | 1 +
arch/parisc/include/uapi/asm/fcntl.h | 1 +
arch/sparc/include/uapi/asm/fcntl.h | 1 +
fs/fcntl.c | 4 ++--
fs/namei.c | 12 +++++++++++-
fs/open.c | 4 +++-
fs/proc/base.c | 4 +++-
fs/proc/namespaces.c | 8 ++++++--
include/linux/namei.h | 3 ++-
include/uapi/asm-generic/fcntl.h | 4 ++++
10 files changed, 34 insertions(+), 8 deletions(-)

diff --git a/arch/alpha/include/uapi/asm/fcntl.h b/arch/alpha/include/uapi/asm/fcntl.h
index 09f49a6b87d1..76a87038d2c1 100644
--- a/arch/alpha/include/uapi/asm/fcntl.h
+++ b/arch/alpha/include/uapi/asm/fcntl.h
@@ -33,6 +33,7 @@

#define O_PATH 040000000
#define __O_TMPFILE 0100000000
+#define O_BENEATH 0200000000 /* no / or .. in openat path */

#define F_GETLK 7
#define F_SETLK 8
diff --git a/arch/parisc/include/uapi/asm/fcntl.h b/arch/parisc/include/uapi/asm/fcntl.h
index 34a46cbc76ed..3adadf72f929 100644
--- a/arch/parisc/include/uapi/asm/fcntl.h
+++ b/arch/parisc/include/uapi/asm/fcntl.h
@@ -21,6 +21,7 @@

#define O_PATH 020000000
#define __O_TMPFILE 040000000
+#define O_BENEATH 080000000 /* no / or .. in openat path */

#define F_GETLK64 8
#define F_SETLK64 9
diff --git a/arch/sparc/include/uapi/asm/fcntl.h b/arch/sparc/include/uapi/asm/fcntl.h
index 7e8ace5bf760..ea38f0bd6cec 100644
--- a/arch/sparc/include/uapi/asm/fcntl.h
+++ b/arch/sparc/include/uapi/asm/fcntl.h
@@ -36,6 +36,7 @@

#define O_PATH 0x1000000
#define __O_TMPFILE 0x2000000
+#define O_BENEATH 0x4000000 /* no / or .. in openat path */

#define F_GETOWN 5 /* for sockets. */
#define F_SETOWN 6 /* for sockets. */
diff --git a/fs/fcntl.c b/fs/fcntl.c
index ee85cd4e136a..3169693e9390 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -740,7 +740,7 @@ static int __init fcntl_init(void)
* Exceptions: O_NONBLOCK is a two bit define on parisc; O_NDELAY
* is defined as O_NONBLOCK on some platforms and not on others.
*/
- BUILD_BUG_ON(21 - 1 /* for O_RDONLY being 0 */ != HWEIGHT32(
+ BUILD_BUG_ON(22 - 1 /* for O_RDONLY being 0 */ != HWEIGHT32(
O_RDONLY | O_WRONLY | O_RDWR |
O_CREAT | O_EXCL | O_NOCTTY |
O_TRUNC | O_APPEND | /* O_NONBLOCK | */
@@ -748,7 +748,7 @@ static int __init fcntl_init(void)
O_DIRECT | O_LARGEFILE | O_DIRECTORY |
O_NOFOLLOW | O_NOATIME | O_CLOEXEC |
__FMODE_EXEC | O_PATH | __O_TMPFILE |
- __FMODE_NONOTIFY
+ __FMODE_NONOTIFY| O_BENEATH
));

fasync_cache = kmem_cache_create("fasync_cache",
diff --git a/fs/namei.c b/fs/namei.c
index fbbcf0993312..978f07d91a11 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -827,14 +827,18 @@ static inline void path_to_nameidata(const struct path *path,
* Helper to directly jump to a known parsed path from ->follow_link,
* caller must have taken a reference to path beforehand.
*/
-void nd_jump_link(struct path *path)
+int nd_jump_link(struct path *path)
{
struct nameidata *nd = current->nameidata;
+
+ if (nd->flags & LOOKUP_BENEATH)
+ return -EPERM;
path_put(&nd->path);

nd->path = *path;
nd->inode = nd->path.dentry->d_inode;
nd->flags |= LOOKUP_JUMPED;
+ return 0;
}

static inline void put_link(struct nameidata *nd)
@@ -1000,6 +1004,8 @@ const char *get_link(struct nameidata *nd)
}
}
if (*res == '/') {
+ if (nd->flags & LOOKUP_BENEATH)
+ return ERR_PTR(-EPERM);
if (nd->flags & LOOKUP_RCU) {
struct dentry *d;
if (!nd->root.mnt)
@@ -1888,6 +1894,8 @@ static int link_path_walk(const char *name, struct nameidata *nd)
if (name[0] == '.') switch (hashlen_len(hash_len)) {
case 2:
if (name[1] == '.') {
+ if (nd->flags & LOOKUP_BENEATH)
+ return -EPERM;
type = LAST_DOTDOT;
nd->flags |= LOOKUP_JUMPED;
}
@@ -2000,6 +2008,8 @@ static const char *path_init(struct nameidata *nd, unsigned flags)

nd->m_seq = read_seqbegin(&mount_lock);
if (*s == '/') {
+ if (flags & LOOKUP_BENEATH)
+ return ERR_PTR(-EPERM);
if (flags & LOOKUP_RCU) {
rcu_read_lock();
set_root_rcu(nd);
diff --git a/fs/open.c b/fs/open.c
index e33dab287fa0..29208cd307f7 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -917,7 +917,7 @@ static inline int build_open_flags(int flags, umode_t mode, struct open_flags *o
* If we have O_PATH in the open flag. Then we
* cannot have anything other than the below set of flags
*/
- flags &= O_DIRECTORY | O_NOFOLLOW | O_PATH;
+ flags &= O_DIRECTORY | O_NOFOLLOW | O_PATH | O_BENEATH;
acc_mode = 0;
} else {
acc_mode = MAY_OPEN | ACC_MODE(flags);
@@ -948,6 +948,8 @@ static inline int build_open_flags(int flags, umode_t mode, struct open_flags *o
lookup_flags |= LOOKUP_DIRECTORY;
if (!(flags & O_NOFOLLOW))
lookup_flags |= LOOKUP_FOLLOW;
+ if (flags & O_BENEATH)
+ lookup_flags |= LOOKUP_BENEATH;
op->lookup_flags = lookup_flags;
return 0;
}
diff --git a/fs/proc/base.c b/fs/proc/base.c
index aa50d1ac28fc..281f7a8d8060 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1589,7 +1589,9 @@ static const char *proc_pid_follow_link(struct dentry *dentry, void **cookie)
if (error)
goto out;

- nd_jump_link(&path);
+ error = nd_jump_link(&path);
+ if (error)
+ goto out;
return NULL;
out:
return ERR_PTR(error);
diff --git a/fs/proc/namespaces.c b/fs/proc/namespaces.c
index f6e8354b8cea..beb5aa3ad38c 100644
--- a/fs/proc/namespaces.c
+++ b/fs/proc/namespaces.c
@@ -36,6 +36,7 @@ static const char *proc_ns_follow_link(struct dentry *dentry, void **cookie)
const struct proc_ns_operations *ns_ops = PROC_I(inode)->ns_ops;
struct task_struct *task;
struct path ns_path;
+ int err;
void *error = ERR_PTR(-EACCES);

task = get_proc_task(inode);
@@ -44,8 +45,11 @@ static const char *proc_ns_follow_link(struct dentry *dentry, void **cookie)

if (ptrace_may_access(task, PTRACE_MODE_READ)) {
error = ns_get_path(&ns_path, task, ns_ops);
- if (!error)
- nd_jump_link(&ns_path);
+ if (!error) {
+ err = nd_jump_link(&ns_path);
+ if (err)
+ error = ERR_PTR(err);
+ }
}
put_task_struct(task);
return error;
diff --git a/include/linux/namei.h b/include/linux/namei.h
index d8c6334cd150..a5a262c85e49 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -27,6 +27,7 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
#define LOOKUP_FOLLOW 0x0001
#define LOOKUP_DIRECTORY 0x0002
#define LOOKUP_AUTOMOUNT 0x0004
+#define LOOKUP_BENEATH 0x0008

#define LOOKUP_PARENT 0x0010
#define LOOKUP_REVAL 0x0020
@@ -85,7 +86,7 @@ extern int follow_up(struct path *);
extern struct dentry *lock_rename(struct dentry *, struct dentry *);
extern void unlock_rename(struct dentry *, struct dentry *);

-extern void nd_jump_link(struct path *path);
+extern int nd_jump_link(struct path *path);

static inline void nd_terminate_link(void *name, size_t len, size_t maxlen)
{
diff --git a/include/uapi/asm-generic/fcntl.h b/include/uapi/asm-generic/fcntl.h
index e063effe0cc1..4542bc6a2950 100644
--- a/include/uapi/asm-generic/fcntl.h
+++ b/include/uapi/asm-generic/fcntl.h
@@ -92,6 +92,10 @@
#define O_TMPFILE (__O_TMPFILE | O_DIRECTORY)
#define O_TMPFILE_MASK (__O_TMPFILE | O_DIRECTORY | O_CREAT)

+#ifndef O_BENEATH
+#define O_BENEATH 040000000 /* no / or .. in openat path */
+#endif
+
#ifndef O_NDELAY
#define O_NDELAY O_NONBLOCK
#endif
--
1.9.1

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