From: Al Viro viro@zeniv.linux.org.uk
commit 6404674acd596de41fd3ad5f267b4525494a891a upstream.
Brown paperbag time: fetching ->i_uid/->i_mode really should've been done from nd->inode. I even suggested that, but the reason for that has slipped through the cracks and I went for dir->d_inode instead - made for more "obvious" patch.
Analysis:
- at the entry into do_last() and all the way to step_into(): dir (aka nd->path.dentry) is known not to have been freed; so's nd->inode and it's equal to dir->d_inode unless we are already doomed to -ECHILD. inode of the file to get opened is not known.
- after step_into(): inode of the file to get opened is known; dir might be pointing to freed memory/be negative/etc.
- at the call of may_create_in_sticky(): guaranteed to be out of RCU mode; inode of the file to get opened is known and pinned; dir might be garbage.
The last was the reason for the original patch. Except that at the do_last() entry we can be in RCU mode and it is possible that nd->path.dentry->d_inode has already changed under us.
In that case we are going to fail with -ECHILD, but we need to be careful; nd->inode is pointing to valid struct inode and it's the same as nd->path.dentry->d_inode in "won't fail with -ECHILD" case, so we should use that.
Reported-by: "Rantala, Tommi T. (Nokia - FI/Espoo)" tommi.t.rantala@nokia.com Reported-by: syzbot+190005201ced78a74ad6@syzkaller.appspotmail.com Wearing-brown-paperbag: Al Viro viro@zeniv.linux.org.uk Cc: stable@kernel.org Fixes: d0cb50185ae9 ("do_last(): fetch directory ->i_mode and ->i_uid before it's too late") Signed-off-by: Al Viro viro@zeniv.linux.org.uk Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/namei.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
--- a/fs/namei.c +++ b/fs/namei.c @@ -3202,8 +3202,8 @@ static int do_last(struct nameidata *nd, struct file *file, const struct open_flags *op) { struct dentry *dir = nd->path.dentry; - kuid_t dir_uid = dir->d_inode->i_uid; - umode_t dir_mode = dir->d_inode->i_mode; + kuid_t dir_uid = nd->inode->i_uid; + umode_t dir_mode = nd->inode->i_mode; int open_flag = op->open_flag; bool will_truncate = (open_flag & O_TRUNC) != 0; bool got_write = false;