Fix a race condition in FASYNC handling

Changeset a238b790d5f99c7832f9b73ac8847025815b85f7 (Call fasync()
functions without the BKL) introduced a race which could leave
file->f_flags in a state inconsistent with what the underlying
driver/filesystem believes.  Revert that change, and also fix the same
races in ioctl_fioasync() and ioctl_fionbio().

This is a minimal, short-term fix; the real fix will not involve the
BKL.

Reported-by: Oleg Nesterov <oleg@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: stable@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 15 insertions, 4 deletions

diff --git a/fs/fcntl.c b/fs/fcntl.c
index ac4f7db9f134..549daf8005fb 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -19,6 +19,7 @@
 #include <linux/signal.h>
 #include <linux/rcupdate.h>
 #include <linux/pid_namespace.h>
+#include <linux/smp_lock.h>
 
 #include <asm/poll.h>
 #include <asm/siginfo.h>
@@ -175,6 +176,11 @@ static int setfl(int fd, struct file * filp, unsigned long arg)
         if (error)
                 return error;
 
+        /*
+         * We still need a lock here for now to keep multiple FASYNC calls
+         * from racing with each other.
+         */
+        lock_kernel();
         if ((arg ^ filp->f_flags) & FASYNC) {
                 if (filp->f_op && filp->f_op->fasync) {
                         error = filp->f_op->fasync(fd, filp, (arg & FASYNC) != 0);
@@ -185,6 +191,7 @@ static int setfl(int fd, struct file * filp, unsigned long arg)
 
         filp->f_flags = (arg & SETFL_MASK) | (filp->f_flags & ~SETFL_MASK);
  out:
+        unlock_kernel();
         return error;
 }
 
diff --git a/fs/ioctl.c b/fs/ioctl.c
index d152856c371b..43e8b2c0664b 100644
--- a/fs/ioctl.c
+++ b/fs/ioctl.c
@@ -400,11 +400,9 @@ static int ioctl_fioasync(unsigned int fd, struct file *filp,
 
         /* Did FASYNC state change ? */
         if ((flag ^ filp->f_flags) & FASYNC) {
-                if (filp->f_op && filp->f_op->fasync) {
-                        lock_kernel();
+                if (filp->f_op && filp->f_op->fasync)
                         error = filp->f_op->fasync(fd, filp, on);
-                        unlock_kernel();
-                } else
+                else
                         error = -ENOTTY;
         }
         if (error)
@@ -440,11 +438,17 @@ int do_vfs_ioctl(struct file *filp, unsigned int fd, unsigned int cmd,
                 break;
 
         case FIONBIO:
+                /* BKL needed to avoid races tweaking f_flags */
+                lock_kernel();
                 error = ioctl_fionbio(filp, argp);
+                unlock_kernel();
                 break;
 
         case FIOASYNC:
+                /* BKL needed to avoid races tweaking f_flags */
+                lock_kernel();
                 error = ioctl_fioasync(fd, filp, argp);
+                unlock_kernel();
                 break;
 
         case FIOQSIZE: