ACK/Cmnt: [PATCH v2] UBUNTU: SAUCE: shiftfs: prevent lower dentries from going negative during unlink

[Christian Brauner]

On Mon, Jan 20, 2020 at 04:08:10PM +0200, Stefan Bader wrote:
> On 17.01.20 17:17, Christian Brauner wrote:
> > BugLink: https://bugs.launchpad.net/bugs/1860041
> > 
> > All non-special files (For shiftfs this only includes fifos and - for
> > this case - unix sockets - since we don't allow character and block
> > devices to be created.) go through shiftfs_open() and have their dentry
> > pinned through this codepath preventing it from going negative. But
> > fifos don't use the shiftfs fops but rather use the pipefifo_fops which
> > means they do not go through shiftfs_open() and thus don't have their
> > dentry pinned that way. Thus, the lower dentries for such files can go
> > negative on unlink causing segfaults. The following C program can be
> > used to reproduce the crash:
> > 
> >  #include <stdio.h>
> >  #include <fcntl.h>
> >  #include <unistd.h>
> >  #include <sys/types.h>
> >  #include <sys/stat.h>
> >  #include <unistd.h>
> >  #include <stdlib.h>
> > 
> >  int main(int argc, char *argv[])
> >  {
> >         struct stat stat;
> > 
> >         unlink("./bbb");
> > 
> >         int ret = mknod("./bbb", S_IFIFO|0666, 0);
> >         if (ret < 0)
> >                 exit(1);
> > 
> >         int fd = open("./bbb", O_RDWR);
> >         if (fd < 0)
> >                 exit(2);
> > 
> >         if (unlink("./bbb"))
> >                 exit(4);
> > 
> >         fstat(fd, &stat);
> > 
> >         return 0;
> >  }
> > 
> > Similar to ecryptfs we need to dget() the lower dentry before calling
> > vfs_unlink() on it and dput() it afterwards.
> > 
> > Acked-by: Stefan Bader <stefan.bader at canonical.com>
> > Link: https://travis-ci.community/t/arm64-ppc64le-segfaults/6158/3
> > Signed-off-by: Seth Forshee <seth.forshee at canonical.com>
> > Signed-off-by: Christian Brauner <christian.brauner at ubuntu.com>
> Acked-by: Stefan Bader <stefan.bader at canonical.com>
> > ---
> > /* v1 */
> > Link: https://lists.ubuntu.com/archives/kernel-team/2020-January/106965.html
> > 
> > /* v2 */
> > - Christian Brauner <christian.brauner at ubuntu.com>:
> >   - add missing ; after dget(lowerd)
> 
> Agreed that I probably should have seen this but also test compiling before
> submitting (even more so for simple things) does help (and that is from
> self-experinece).

Yeah, I did but I had printk() statements in there and my intention was
to do a git fetch + git reset --hard but alas I forget the latter. So I
did test a correct change just not the final patch without the printks. :/

Christian