On Wed, May 06, 2020 at 11:31:40AM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@xxxxxxxxxx>
>
> '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@xxxxxxxxxxxxxxx
> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
Any comments on this patch?
- Eric
