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Subject[PATCH 2/4] Expose O_PATHSTATIC to userspace
Date
From: "Demi M. Obenour" <demiobenour@gmail.com>

This adds the file open flag O_PATHSTATIC, which ensures that symbolic
links are *never* followed, even in path components other than the last.
This is distinct from O_NOFOLLOW, which only prevents symlinks in the
*last* component from being followed.

This is useful for avoiding race conditions in userspace code that
should expose only a subset of the filesystem to clients. This includes
FTP and SFTP servers, QEMU, and others.

Currently, O_NOFOLLOW must be set if O_PATHSTATIC is set. Otherwise,
open() fails with -EINVAL.
---
fs/fcntl.c | 2 +-
fs/namei.c | 6 ++++++
fs/open.c | 24 ++++++++++++++++++++++--
include/linux/fcntl.h | 2 +-
include/uapi/asm-generic/fcntl.h | 4 ++++
5 files changed, 34 insertions(+), 4 deletions(-)

diff --git a/fs/fcntl.c b/fs/fcntl.c
index 083185174c6d..6c85c4d0c006 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -1031,7 +1031,7 @@ 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 */ !=
+ BUILD_BUG_ON(22 - 1 /* for O_RDONLY being 0 */ !=
HWEIGHT32(
(VALID_OPEN_FLAGS & ~(O_NONBLOCK | O_NDELAY)) |
__FMODE_EXEC | __FMODE_NONOTIFY));
diff --git a/fs/namei.c b/fs/namei.c
index 54fbd2c7ba82..4c90f265c103 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3282,6 +3282,12 @@ static int do_last(struct nameidata *nd,
if (!(open_flag & O_CREAT)) {
if (nd->last.name[nd->last.len])
nd->flags |= LOOKUP_FOLLOW | LOOKUP_DIRECTORY;
+
+ if (open_flag & O_PATHSTATIC) {
+ nd->flags |= LOOKUP_NEVER_FOLLOW;
+ nd->flags &= ~LOOKUP_FOLLOW;
+ }
+
/* we _can_ be in RCU mode here */
error = lookup_fast(nd, &path, &inode, &seq);
if (likely(error > 0))
diff --git a/fs/open.c b/fs/open.c
index 0285ce7dbd51..717afa8179c0 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -940,6 +940,24 @@ static inline int build_open_flags(int flags, umode_t mode, struct open_flags *o
/* Must never be set by userspace */
flags &= ~FMODE_NONOTIFY & ~O_CLOEXEC;

+ /*
+ * If nonzero, setting O_PATHSTATIC but not O_NOFOLLOW fails with
+ * -EINVAL. Otherwise, setting O_PATHSTATIC automatically sets
+ * O_NOFOLLOW.
+ */
+#define REQUIRE_NOFOLLOW_FOR_PATHSTATIC 1
+
+#if REQUIRE_NOFOLLOW_FOR_PATHSTATIC
+ /* O_PATHSTATIC doesn't make sense without O_NOFOLLOW */
+ if (unlikely((flags & O_PATHSTATIC) && !(flags & O_NOFOLLOW)))
+ return -EINVAL;
+#elif defined REQUIRE_NOFOLLOW_FOR_PATHSTATIC
+ if (flags & O_PATHSTATIC)
+ flags &= O_NOFOLLOW;
+#else
+#error REQUIRE_NOFOLLOW_FOR_PATHSTATIC must be defined
+#endif
+
/*
* O_SYNC is implemented as __O_SYNC|O_DSYNC. As many places only
* check for O_DSYNC if the need any syncing at all we enforce it's
@@ -959,7 +977,7 @@ 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_PATHSTATIC;
acc_mode = 0;
}

@@ -986,7 +1004,9 @@ static inline int build_open_flags(int flags, umode_t mode, struct open_flags *o

if (flags & O_DIRECTORY)
lookup_flags |= LOOKUP_DIRECTORY;
- if (!(flags & O_NOFOLLOW))
+ if (flags & O_PATHSTATIC)
+ lookup_flags |= LOOKUP_NEVER_FOLLOW;
+ else if (!(flags & O_NOFOLLOW))
lookup_flags |= LOOKUP_FOLLOW;
op->lookup_flags = lookup_flags;
return 0;
diff --git a/include/linux/fcntl.h b/include/linux/fcntl.h
index 27dc7a60693e..6f91e1490592 100644
--- a/include/linux/fcntl.h
+++ b/include/linux/fcntl.h
@@ -9,7 +9,7 @@
(O_RDONLY | O_WRONLY | O_RDWR | O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC | \
O_APPEND | O_NDELAY | O_NONBLOCK | O_NDELAY | __O_SYNC | O_DSYNC | \
FASYNC | O_DIRECT | O_LARGEFILE | O_DIRECTORY | O_NOFOLLOW | \
- O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE)
+ O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE | O_PATHSTATIC)

#ifndef force_o_largefile
#define force_o_largefile() (BITS_PER_LONG != 32)
diff --git a/include/uapi/asm-generic/fcntl.h b/include/uapi/asm-generic/fcntl.h
index 9dc0bf0c5a6e..314ea1cecf44 100644
--- a/include/uapi/asm-generic/fcntl.h
+++ b/include/uapi/asm-generic/fcntl.h
@@ -89,6 +89,10 @@
#define __O_TMPFILE 020000000
#endif

+#ifndef O_PATHSTATIC
+#define O_PATHSTATIC 040000000
+#endif
+
/* a horrid kludge trying to make sure that this will fail on old kernels */
#define O_TMPFILE (__O_TMPFILE | O_DIRECTORY)
#define O_TMPFILE_MASK (__O_TMPFILE | O_DIRECTORY | O_CREAT)
--
2.20.1
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