[PATCH] devpts: If initialization failed, don't crash when opening /dev/ptmx

Josh Triplett
2015-05-07T00:35:58Z

If devpts failed to initialize, it would store an ERR_PTR in the global
devpts_mnt. A subsequent open of /dev/ptmx would call devpts_new_index,
which would dereference devpts_mnt and crash.

Avoid storing invalid values in devpts_mnt; leave it NULL instead.
Make both devpts_new_index and devpts_pty_new fail gracefully with
ENODEV in that case, which then becomes the return value to the
userspace open call on /dev/ptmx.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
---

This fixes a crash found by Fengguang Wu's 0-day service ("BUG: unable to
handle kernel paging request at ffffffee"). It doesn't yet fix the underlying
initialization failure in init_devpts_fs, but it stops that failure from
becoming a kernel crash. I'm working on the initialization failure now.

fs/devpts/inode.c | 30 +++++++++++++++++++++++-------
1 file changed, 23 insertions(+), 7 deletions(-)

diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
index cfe8466..03e9076 100644
--- a/fs/devpts/inode.c
+++ b/fs/devpts/inode.c
@@ -142,6 +142,8 @@ static inline struct super_block *pts_sb_from_inode(struct inode *inode)
if (inode->i_sb->s_magic == DEVPTS_SUPER_MAGIC)
return inode->i_sb;
#endif
+ if (!devpts_mnt)
+ return NULL;
return devpts_mnt->mnt_sb;
}

@@ -525,10 +527,14 @@ static struct file_system_type devpts_fs_type = {
int devpts_new_index(struct inode *ptmx_inode)
{
struct super_block *sb = pts_sb_from_inode(ptmx_inode);
- struct pts_fs_info *fsi = DEVPTS_SB(sb);
+ struct pts_fs_info *fsi;
int index;
int ida_ret;

+ if (!sb)
+ return -ENODEV;
+
+ fsi = DEVPTS_SB(sb);
retry:
if (!ida_pre_get(&fsi->allocated_ptys, GFP_KERNEL))
return -ENOMEM;
@@ -584,11 +590,18 @@ struct inode *devpts_pty_new(struct inode *ptmx_inode, dev_t device, int index,
struct dentry *dentry;
struct super_block *sb = pts_sb_from_inode(ptmx_inode);
struct inode *inode;
- struct dentry *root = sb->s_root;
- struct pts_fs_info *fsi = DEVPTS_SB(sb);
- struct pts_mount_opts *opts = &fsi->mount_opts;
+ struct dentry *root;
+ struct pts_fs_info *fsi;
+ struct pts_mount_opts *opts;
char s[12];

+ if (!sb)
+ return ERR_PTR(-ENODEV);
+
+ root = sb->s_root;
+ fsi = DEVPTS_SB(sb);
+ opts = &fsi->mount_opts;
+
inode = new_inode(sb);
if (!inode)
return ERR_PTR(-ENOMEM);
@@ -676,12 +689,15 @@ static int __init init_devpts_fs(void)
struct ctl_table_header *table;

if (!err) {
+ static struct vfsmount *mnt;
table = register_sysctl_table(pty_root_table);
- devpts_mnt = kern_mount(&devpts_fs_type);
- if (IS_ERR(devpts_mnt)) {
- err = PTR_ERR(devpts_mnt);
+ mnt = kern_mount(&devpts_fs_type);
+ if (IS_ERR(mnt)) {
+ err = PTR_ERR(mnt);
unregister_filesystem(&devpts_fs_type);
unregister_sysctl_table(table);
+ } else {
+ devpts_mnt = mnt;
}
}
return err;
--
2.1.4