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

[Stefan Bader]

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).


> ---
>  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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