APPLIED(D,E): [PATCH v2] UBUNTU: SAUCE: shiftfs: prevent lower dentries from going negative during unlink

[Khaled Elmously]

On 2020-01-20 15:47:42 , 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>
> 
> If you send a V2, then its no longer acked and you should really send a
> self-nack on the v1.
> 
> > 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>
> > ---
> > /* 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)
> > ---
> >  fs/shiftfs.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/fs/shiftfs.c b/fs/shiftfs.c
> > index 04fba4689eb6..3623d02b061e 100644
> > --- a/fs/shiftfs.c
> > +++ b/fs/shiftfs.c
> > @@ -583,6 +583,7 @@ static int shiftfs_rm(struct inode *dir, struct dentry *dentry, bool rmdir)
> >  	int err;
> >  	const struct cred *oldcred;
> >  
> > +	dget(lowerd);
> >  	oldcred = shiftfs_override_creds(dentry->d_sb);
> >  	inode_lock_nested(loweri, I_MUTEX_PARENT);
> >  	if (rmdir)
> > @@ -602,6 +603,7 @@ static int shiftfs_rm(struct inode *dir, struct dentry *dentry, bool rmdir)
> >  	inode_unlock(loweri);
> >  
> >  	shiftfs_copyattr(loweri, dir);
> > +	dput(lowerd);
> >  
> >  	return err;
> >  }
> > 
> > base-commit: 5c6810aad94a79c86bee07cd19dba90b92f461b4
> > 
> 
> 




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