Re: [PATCH] ext4: fix race between ext4_sync_parent() and rename()

[Eric Biggers <ebiggers@kernel.org>]

On Wed, May 06, 2020 at 11:31:40AM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> 'igrab(d_inode(dentry->d_parent))' without holding dentry->d_lock is
> broken because without d_lock, d_parent can be concurrently changed due
> to a rename().  Then if the old directory is immediately deleted, old
> d_parent->inode can be NULL.  That causes a NULL dereference in igrab().
> 
> To fix this, use dget_parent() to safely grab a reference to the parent
> dentry, which pins the inode.  This also eliminates the need to use
> d_find_any_alias() other than for the initial inode, as we no longer
> throw away the dentry at each step.
> 
> This is an extremely hard race to hit, but it is possible.  Adding a
> udelay() in between the reads of ->d_parent and its ->d_inode makes it
> reproducible on a no-journal filesystem using the following program:
> 
>     #include <fcntl.h>
>     #include <unistd.h>
> 
>     int main()
>     {
>         if (fork()) {
>             for (;;) {
>                 mkdir("dir1", 0700);
>                 int fd = open("dir1/file", O_RDWR|O_CREAT|O_SYNC);
>                 write(fd, "X", 1);
>                 close(fd);
>             }
>         } else {
>             mkdir("dir2", 0700);
>             for (;;) {
>                 rename("dir1/file", "dir2/file");
>                 rmdir("dir1");
>             }
>         }
>     }
> 
> Fixes: d59729f4e794 ("ext4: fix races in ext4_sync_parent()")
> Cc: stable@vger.kernel.org
> Signed-off-by: Eric Biggers <ebiggers@google.com>


Any comments on this patch?

- Eric