[PATCH net v2] ppp: ensure file->private_data can't be overridden

[Guillaume Nault]

Locking ppp_mutex must be done before dereferencing file->private_data,
otherwise it could be modified before ppp_unattached_ioctl() takes the
lock. This could lead ppp_unattached_ioctl() to override ->private_data,
thus leaking reference to the ppp_file previously pointed to.

v2: lock all ppp_ioctl() instead of just checking private_data in
    ppp_unattached_ioctl(), to avoid ambiguous behaviour.

Fixes: f3ff8a4d80e8 ("ppp: push BKL down into the driver")
Signed-off-by: Guillaume Nault <g.na...@alphalink.fr>
---
 drivers/net/ppp/ppp_generic.c | 31 +++++++++++++++++--------------
 1 file changed, 17 insertions(+), 14 deletions(-)

diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index d61da9ec..8c8eedb 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -575,7 +575,7 @@ static int get_filter(void __user *arg, struct sock_filter 
**p)
 
 static long ppp_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 {
-       struct ppp_file *pf = file->private_data;
+       struct ppp_file *pf;
        struct ppp *ppp;
        int err = -EFAULT, val, val2, i;
        struct ppp_idle idle;
@@ -585,9 +585,14 @@ static long ppp_ioctl(struct file *file, unsigned int cmd, 
unsigned long arg)
        void __user *argp = (void __user *)arg;
        int __user *p = argp;
 
-       if (!pf)
-               return ppp_unattached_ioctl(current->nsproxy->net_ns,
-                                       pf, file, cmd, arg);
+       mutex_lock(&ppp_mutex);
+
+       pf = file->private_data;
+       if (!pf) {
+               err = ppp_unattached_ioctl(current->nsproxy->net_ns,
+                                          pf, file, cmd, arg);
+               goto out;
+       }
 
        if (cmd == PPPIOCDETACH) {
                /*
@@ -602,7 +607,6 @@ static long ppp_ioctl(struct file *file, unsigned int cmd, 
unsigned long arg)
                 * this fd and reopening /dev/ppp.
                 */
                err = -EINVAL;
-               mutex_lock(&ppp_mutex);
                if (pf->kind == INTERFACE) {
                        ppp = PF_TO_PPP(pf);
                        rtnl_lock();
@@ -616,15 +620,13 @@ static long ppp_ioctl(struct file *file, unsigned int 
cmd, unsigned long arg)
                } else
                        pr_warn("PPPIOCDETACH file->f_count=%ld\n",
                                atomic_long_read(&file->f_count));
-               mutex_unlock(&ppp_mutex);
-               return err;
+               goto out;
        }
 
        if (pf->kind == CHANNEL) {
                struct channel *pch;
                struct ppp_channel *chan;
 
-               mutex_lock(&ppp_mutex);
                pch = PF_TO_CHANNEL(pf);
 
                switch (cmd) {
@@ -646,17 +648,16 @@ static long ppp_ioctl(struct file *file, unsigned int 
cmd, unsigned long arg)
                                err = chan->ops->ioctl(chan, cmd, arg);
                        up_read(&pch->chan_sem);
                }
-               mutex_unlock(&ppp_mutex);
-               return err;
+               goto out;
        }
 
        if (pf->kind != INTERFACE) {
                /* can't happen */
                pr_err("PPP: not interface or channel??\n");
-               return -EINVAL;
+               err = -EINVAL;
+               goto out;
        }
 
-       mutex_lock(&ppp_mutex);
        ppp = PF_TO_PPP(pf);
        switch (cmd) {
        case PPPIOCSMRU:
@@ -831,7 +832,10 @@ static long ppp_ioctl(struct file *file, unsigned int cmd, 
unsigned long arg)
        default:
                err = -ENOTTY;
        }
+
+out:
        mutex_unlock(&ppp_mutex);
+
        return err;
 }
 
@@ -844,7 +848,6 @@ static int ppp_unattached_ioctl(struct net *net, struct 
ppp_file *pf,
        struct ppp_net *pn;
        int __user *p = (int __user *)arg;
 
-       mutex_lock(&ppp_mutex);
        switch (cmd) {
        case PPPIOCNEWUNIT:
                /* Create a new ppp unit */
@@ -894,7 +897,7 @@ static int ppp_unattached_ioctl(struct net *net, struct 
ppp_file *pf,
        default:
                err = -ENOTTY;
        }
-       mutex_unlock(&ppp_mutex);
+
        return err;
 }
 
-- 
2.7.0