[PATCH 2.6.28] Fix FASYNC race

Jonathan Corbet
2008-12-05T23:13:01Z

Hi, Linus,

The following patch closes a race which was introduced by the fasync()
BKL pushdown patch. As it turns out, f_flags handling was already
racy, but that change made it worse. Here, we fix it (and a
longer-standing race) minimally by reintroducing the BKL; a better fix
will be prepared for 2.6.29.

jon

---
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>
---
fs/fcntl.c | 7 +++++++
fs/ioctl.c | 12 ++++++++----
2 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/fs/fcntl.c b/fs/fcntl.c
index ac4f7db..549daf8 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 d152856..fc8fcaa 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:
--
1.6.0.4