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Date: Fri, 15 May 2020 12:14:15 -0700 From: Eric Biggers <ebiggers@...nel.org> To: linux-ext4@...r.kernel.org Cc: Jaegeuk Kim <jaegeuk@...nel.org>, linux-fsdevel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>, stable@...r.kernel.org Subject: Re: [PATCH] ext4: fix race between ext4_sync_parent() and rename() On Wed, May 06, 2020 at 11:31:40AM -0700, Eric Biggers wrote: > From: Eric Biggers <ebiggers@...gle.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@...r.kernel.org > Signed-off-by: Eric Biggers <ebiggers@...gle.com> Any comments on this patch? - Eric
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