-
由 Eric Biggers 提交于
'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: d59729f4 ("ext4: fix races in ext4_sync_parent()") Cc: stable@vger.kernel.org Signed-off-by: NEric Biggers <ebiggers@google.com> Link: https://lore.kernel.org/r/20200506183140.541194-1-ebiggers@kernel.orgSigned-off-by: NTheodore Ts'o <tytso@mit.edu>
08adf452