Introduce O_CLOEXEC

The problem is as follows: in multi-threaded code (or more correctly: all
code using clone() with CLONE_FILES) we have a race when exec'ing.

   thread #1                       thread #2

   fd=open()

                                   fork + exec

  fcntl(fd,F_SETFD,FD_CLOEXEC)

In some applications this can happen frequently.  Take a web browser.  One
thread opens a file and another thread starts, say, an external PDF viewer.
 The result can even be a security issue if that open file descriptor
refers to a sensitive file and the external program can somehow be tricked
into using that descriptor.

Just adding O_CLOEXEC support to open() doesn't solve the whole set of
problems.  There are other ways to create file descriptors (socket,
epoll_create, Unix domain socket transfer, etc).  These can and should be
addressed separately though.  open() is such an easy case that it makes not
much sense putting the fix off.

The test program:

#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>

#ifndef O_CLOEXEC
# define O_CLOEXEC 02000000
#endif

int
main (int argc, char *argv[])
{
  int fd;
  if (argc > 1)
    {
      fd = atol (argv[1]);
      printf ("child: fd = %d\n", fd);
      if (fcntl (fd, F_GETFD) == 0 || errno != EBADF)
        {
          puts ("file descriptor valid in child");
          return 1;
        }
      return 0;
    }

  fd = open ("/proc/self/exe", O_RDONLY | O_CLOEXEC);
  printf ("in parent: new fd = %d\n", fd);
  char buf[20];
  snprintf (buf, sizeof (buf), "%d", fd);
  execl ("/proc/self/exe", argv[0], buf, NULL);
  puts ("execl failed");
  return 1;
}

[kyle@parisc-linux.org: parisc fix]
Signed-off-by: Ulrich Drepper <drepper@redhat.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Cc: Davide Libenzi <davidel@xmailserver.org>
Cc: Michael Kerrisk <mtk-manpages@gmx.net>
Cc: Chris Zankel <chris@zankel.net>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 29 insertions, 17 deletions

diff --git a/fs/open.c b/fs/open.c
index 0d515d161974..e6991c1b5874 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -855,7 +855,7 @@ EXPORT_SYMBOL(dentry_open);
 /*
  * Find an empty file descriptor entry, and mark it busy.
  */
-int get_unused_fd(void)
+static int get_unused_fd_flags(int flags)
 {
         struct files_struct * files = current->files;
         int fd, error;
@@ -891,7 +891,10 @@ repeat:
         }
 
         FD_SET(fd, fdt->open_fds);
-        FD_CLR(fd, fdt->close_on_exec);
+        if (flags & O_CLOEXEC)
+                FD_SET(fd, fdt->close_on_exec);
+        else
+                FD_CLR(fd, fdt->close_on_exec);
         files->next_fd = fd + 1;
 #if 1
         /* Sanity check */
@@ -907,6 +910,11 @@ out:
         return error;
 }
 
+int get_unused_fd(void)
+{
+        return get_unused_fd_flags(0);
+}
+
 EXPORT_SYMBOL(get_unused_fd);
 
 static void __put_unused_fd(struct files_struct *files, unsigned int fd)
@@ -959,7 +967,7 @@ long do_sys_open(int dfd, const char __user *filename, int flags, int mode)
         int fd = PTR_ERR(tmp);
 
         if (!IS_ERR(tmp)) {
-                fd = get_unused_fd();
+                fd = get_unused_fd_flags(flags);
                 if (fd >= 0) {
                         struct file *f = do_filp_open(dfd, tmp, flags, mode);
                         if (IS_ERR(f)) {
diff --git a/include/asm-generic/fcntl.h b/include/asm-generic/fcntl.h
index c154b9d6e7e5..b8477414c5c8 100644
--- a/include/asm-generic/fcntl.h
+++ b/include/asm-generic/fcntl.h
@@ -48,6 +48,9 @@
 #ifndef O_NOATIME
 #define O_NOATIME       01000000
 #endif
+#ifndef O_CLOEXEC
+#define O_CLOEXEC       02000000        /* set close_on_exec */
+#endif
 #ifndef O_NDELAY
 #define O_NDELAY        O_NONBLOCK
 #endif
diff --git a/include/asm-parisc/fcntl.h b/include/asm-parisc/fcntl.h
index 317851fa78f3..1e1c824764ee 100644
--- a/include/asm-parisc/fcntl.h
+++ b/include/asm-parisc/fcntl.h
@@ -3,21 +3,22 @@
 
 /* open/fcntl - O_SYNC is only implemented on blocks devices and on files
    located on an ext2 file system */
-#define O_APPEND        00000010
-#define O_BLKSEEK       00000100 /* HPUX only */
-#define O_CREAT         00000400 /* not fcntl */
-#define O_EXCL          00002000 /* not fcntl */
-#define O_LARGEFILE     00004000
-#define O_SYNC          00100000
-#define O_NONBLOCK      00200004 /* HPUX has separate NDELAY & NONBLOCK */
-#define O_NOCTTY        00400000 /* not fcntl */
-#define O_DSYNC         01000000 /* HPUX only */
-#define O_RSYNC         02000000 /* HPUX only */
-#define O_NOATIME       04000000
+#define O_APPEND        000000010
+#define O_BLKSEEK       000000100 /* HPUX only */
+#define O_CREAT         000000400 /* not fcntl */
+#define O_EXCL          000002000 /* not fcntl */
+#define O_LARGEFILE     000004000
+#define O_SYNC          000100000
+#define O_NONBLOCK      000200004 /* HPUX has separate NDELAY & NONBLOCK */
+#define O_NOCTTY        000400000 /* not fcntl */
+#define O_DSYNC         001000000 /* HPUX only */
+#define O_RSYNC         002000000 /* HPUX only */
+#define O_NOATIME       004000000
+#define O_CLOEXEC       010000000 /* set close_on_exec */
 
-#define O_DIRECTORY     00010000 /* must be a directory */
-#define O_NOFOLLOW      00000200 /* don't follow links */
-#define O_INVISIBLE     04000000 /* invisible I/O, for DMAPI/XDSM */
+#define O_DIRECTORY     000010000 /* must be a directory */
+#define O_NOFOLLOW      000000200 /* don't follow links */
+#define O_INVISIBLE     004000000 /* invisible I/O, for DMAPI/XDSM */
 
 #define F_GETLK64       8
 #define F_SETLK64       9