linux-kernel - Re: fs/coda oops bisected to (925b9cd1b8) "locking/rwsem: Make owner store task pointer of last owni

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Date:   Tue, 2 Apr 2019 15:17:03 -0400
From:   Jan Harkes <jaharkes@...cmu.edu>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Pedro Cuadra Chamorro <pcuadrac@...cmu.edu>,
        linux-kernel@...r.kernel.org
Subject: Re: fs/coda oops bisected to (925b9cd1b8) "locking/rwsem: Make owner
 store task pointer of last owni

On Sun, Mar 31, 2019 at 03:13:47PM -0400, Jan Harkes wrote:
> On Sun, Mar 31, 2019 at 02:14:13PM -0400, Waiman Long wrote:
> > One possibility is that there is a previous reference to the memory
> > currently occupied by the spinlock. If the memory location is previously
> > part of a rwsem structure and someone is still using it, you may get
> > memory corruption.
> 
> Ah, I hadn't even thought of that possibility. Good, it will open up

First of all, I have to thank you for your original patch because
otherwise I probably would never have discovered that something was
seriously wrong. Your patch made the problem visible.

I ended up changing 'owner' to '_RET_IP_' and dumping the value of the
clobbered coda inode spinlock and surrounding memory and found that the
'culprit' is in ext4_filemap_fault and despite it being in ext4, it is
still a Coda specific problem.

Effectively Coda overlays other filesystems' inodes for mmap, but
the vma->vm_file still points at Coda's file. So when we use
file_inode() in ext4_filemap_fault we end up with the Coda inode instead
of the ext4 inode and when trying to grab ext4's mmap_sem we really just
scribble over the memory region that happens to contain the Coda inode
spinlock. A fix is to use vm_file->f_mapping->host instead of
file_inode(vm_file).

Of course everyone looks at ext4 as a canonical example so this problem
has spread pretty much everywhere and I'm wondering how to best resolve
this.

- change file_inode() to follow file->f_mapping->host

  would fix most places, but maybe f_mapping is not always guaranteed to
  point at a usable place?

- change Coda's mmap to replace vma->vm_file with the host file

  we'd probably no longer get notified when the last reference to the
  host file goes away, so we'd call coda_release and notify userspace on
  close() even when there are still active mmap regions.

- fix every in-tree file system to use vma->vm_file->f_mapping->host.

Jan

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 69d65d49837b..122d691d3eda 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -284,7 +284,7 @@ static vm_fault_t ext4_dax_huge_fault(struct vm_fault *vmf,
 	vm_fault_t result;
 	int retries = 0;
 	handle_t *handle = NULL;
-	struct inode *inode = file_inode(vmf->vma->vm_file);
+	struct inode *inode = vmf->vma->vm_file->f_mapping->host;
 	struct super_block *sb = inode->i_sb;

 	/*
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index b54b261ded36..62a0025ce7f8 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -6211,7 +6211,7 @@ vm_fault_t ext4_page_mkwrite(struct vm_fault *vmf)
 	int err;
 	vm_fault_t ret;
 	struct file *file = vma->vm_file;
-	struct inode *inode = file_inode(file);
+	struct inode *inode = file->f_mapping->host;
 	struct address_space *mapping = inode->i_mapping;
 	handle_t *handle;
 	get_block_t *get_block;
@@ -6302,7 +6302,7 @@ vm_fault_t ext4_page_mkwrite(struct vm_fault *vmf)

 vm_fault_t ext4_filemap_fault(struct vm_fault *vmf)
 {
-	struct inode *inode = file_inode(vmf->vma->vm_file);
+	struct inode *inode = vmf->vma->vm_file->f_mapping->host;
 	vm_fault_t ret;

 	down_read(&EXT4_I(inode)->i_mmap_sem);