linux-kernel - RE: [PATCH] Fix the race between the fget() and close()

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Date:	Sat, 31 Aug 2013 07:01:33 +0000
From:	"Liu, Chuansheng" <chuansheng.liu@...el.com>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] Fix the race between the fget() and close()



> -----Original Message-----
> From: Al Viro [mailto:viro@....linux.org.uk] On Behalf Of Al Viro
> Sent: Saturday, August 31, 2013 2:48 PM
> To: Liu, Chuansheng
> Cc: Eric Dumazet; linux-fsdevel@...r.kernel.org; linux-kernel@...r.kernel.org
> Subject: Re: [PATCH] Fix the race between the fget() and close()
> 
> On Sat, Aug 31, 2013 at 05:53:11AM +0000, Liu, Chuansheng wrote:
> 
> > I think I found one of possible race here(two processes P1 and P2):
> > P1 has the the files_struct pointer FILES1, P2 has the files_struct pointer
> FILES2,
> >
> > When P1 open file, the new struct file pointer SHARE_FILE will be installed
> into FILES1,
> > and file refcount is 1;
> >
> > And in P1, we can get P2's files_struct FILES2, and thru _fd_install(), we can
> add SHARE_FILE
> > into P2's FILES2.
> >
> > Then the same file pointer SHARE_FILE stayed in both P1 and P2's files_struct,
> and the panic case
> > will happen:
> > P1
> P2
> > Open the SHARE_FILE
> > Installed SHARE_FILE into P2's file_struct FILES2
> 
> ... without bumping refcount on SHARE_FILE?  Then you really have a big
> problem.  task_fd_install() call is preceded by grabbing a reference
> to the file we are installing, though...  BTW, /* TODO: fput? */ after
> that call is really bogus - the code doesn't call fput() there and it's
> quite correct as is, since at that point the reference had gone into
> descriptor table we'd been installing into and doesn't need to be dropped.
> 
> > Ioctl(SHARE_FILE)
> When P2 exiting,
> >  fget_light()
> >    due to FILES1->refcount is 1,
> put_files_struct will be called,
> >    there will be no RCU and SHARE_FILE refcount increasing
> will close all files including SHARE_FILE
> >
> > But at this time, P1 is still operate SHARE_FILE without the refcount safety.
> >
> > Then the panic will happen at vfs_ioctl() due to the SHARE_FILE has been
> freed.
> >
> > Is it allowable that installing one file pointer into another FILES_STRUCT?
> Seems binder is doing the similar things.
> > In fact, if in ioctl function, we can call fget() instead of fget_light(), this panic
> can be avoided.
> >
> > Is it making sense?
> 
> No, it doesn't.  For one thing, any reference in any files_struct should
> contribute 1 to refcount of struct file.  For another, you can modify
> files_struct *ONLY* if you hold a reference to it.  binder, a misdesigned
My scenario is:
P1 files_struct refcount is 1, P2's is 1 also.
P1 get_files_struct(P2)
P1 install one file into P2's files_struct
P1 put_files_struct(P2)

Then P1 and P2's files_struct refcount are 1, then when P1 is doing ioctl() and P2 is exiting
with put_files_struct(P2), the race will occur, my understanding is wrong?

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