On Jul 7, 2022, at 12:59 PM, Jeff Layton [off-list ref] wrote:

On Thu, 2022-07-07 at 12:55 -0400, Jeff Layton wrote:

quoted

On Thu, 2022-07-07 at 11:58 -0400, Chuck Lever wrote:

quoted

The documenting comment for struct nf_file states:

/*
* A representation of a file that has been opened by knfsd. These are hashed
* in the hashtable by inode pointer value. Note that this object doesn't
* hold a reference to the inode by itself, so the nf_inode pointer should
* never be dereferenced, only used for comparison.
*/

However, nfsd_file_mark_find_or_create() does dereference the pointer stored
in this field.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
fs/nfsd/filecache.c | 3 +++
fs/nfsd/filecache.h | 4 +---
2 files changed, 4 insertions(+), 3 deletions(-)

Hi Jeff-

I'm still testing this one, but I'm wondering what you think of it.
I did hit a KASAN splat that might be related, but it's not 100%
reproducible.

My first thought is "what the hell was I thinking, tracking an inode
field without holding a reference to it?"

But now that I look, the nf_inode value only gets dereferenced in one
place -- nfs4_show_superblock, and I think that's a bug. The comments
over struct nfsd_file say:

"Note that this object doesn't hold a reference to the inode by itself,
so the nf_inode pointer should never be dereferenced, only used for
comparison."

We should probably annotate nf_inode better. __attribute__((noderef))
maybe? It would also be good to make nfs4_show_superblock use a
different way to get the sb.

In any case, this is unlikely to fix anything unless the crash happened
in nfs4_show_superblock.

One other spot. We also dereference it in nfsd_file_mark_find_or_create,
but I think that specific instance is OK. We know that we still hold a
reference to the inode at that point since it comes from fhp->fh_dentry,
so we shouldn't need to worry about it disappearing out from under us.

Needs some annotation. I would prefer not to get that pointer from
nf_inode, then. As your comment says: compare only, never deref.

What did the crash look like?

Jul 06 11:19:32 klimt.1015granger.net kernel: BUG: KASAN: use-after-free in nfsd_file_obj_cmpfn+0x26b/0x49a [nfsd]
Jul 06 11:19:32 klimt.1015granger.net kernel: Read of size 4 at addr ffff888180623e1c by task nfsd/1003
Jul 06 11:19:32 klimt.1015granger.net kernel: 
Jul 06 11:19:32 klimt.1015granger.net kernel: CPU: 3 PID: 1003 Comm: nfsd Not tainted 5.19.0-rc5-00037-g17ba024b204f #3522
Jul 06 11:19:32 klimt.1015granger.net kernel: Hardware name: Supermicro Super Server/X10SRL-F, BIOS 3.3 10/28/2020
Jul 06 11:19:32 klimt.1015granger.net kernel: Call Trace:
Jul 06 11:19:32 klimt.1015granger.net kernel:  <TASK>
Jul 06 11:19:32 klimt.1015granger.net kernel:  dump_stack_lvl+0x56/0x7c
Jul 06 11:19:32 klimt.1015granger.net kernel:  print_report+0x101/0x4bb
Jul 06 11:19:32 klimt.1015granger.net kernel:  kasan_report+0x9f/0xbf
Jul 06 11:19:32 klimt.1015granger.net kernel:  nfsd_file_obj_cmpfn+0x26b/0x49a [nfsd]
Jul 06 11:19:32 klimt.1015granger.net kernel:  rhashtable_lookup.constprop.0+0x143/0x1ca [nfsd]
Jul 06 11:19:32 klimt.1015granger.net kernel:  nfsd_file_do_acquire+0x20b/0x1189 [nfsd]
Jul 06 11:19:32 klimt.1015granger.net kernel:  nfsd_write+0x138/0x255 [nfsd]
Jul 06 11:19:32 klimt.1015granger.net kernel:  nfsd3_proc_write+0x37e/0x3fc [nfsd]
Jul 06 11:19:32 klimt.1015granger.net kernel:  nfsd_dispatch+0x5ed/0x7d0 [nfsd]
Jul 06 11:19:32 klimt.1015granger.net kernel:  svc_process_common+0x8e9/0xefe [sunrpc]
Jul 06 11:19:32 klimt.1015granger.net kernel:  svc_process+0x34d/0x378 [sunrpc]
Jul 06 11:19:32 klimt.1015granger.net kernel:  nfsd+0x26b/0x34c [nfsd]
Jul 06 11:19:32 klimt.1015granger.net kernel:  kthread+0x249/0x258
Jul 06 11:19:32 klimt.1015granger.net kernel:  ret_from_fork+0x22/0x30
Jul 06 11:19:32 klimt.1015granger.net kernel:  </TASK>

The freed object was actually not an inode, so it's a red herring.
Still chasing it.

quoted

quoted

diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
index 9cb2d590c036..7b43bb427a53 100644
--- a/fs/nfsd/filecache.c
+++ b/fs/nfsd/filecache.c

@@ -180,6 +180,7 @@ nfsd_file_alloc(struct inode *inode, unsigned int may, unsigned int hashval,

		nf->nf_cred = get_current_cred();
		nf->nf_net = net;
		nf->nf_flags = 0;
+		ihold(inode);
		nf->nf_inode = inode;
		nf->nf_hashval = hashval;
		refcount_set(&nf->nf_ref, 1);

@@ -210,6 +211,7 @@ nfsd_file_free(struct nfsd_file *nf)

		fput(nf->nf_file);
		flush = true;
	}
+	iput(nf->nf_inode);
	call_rcu(&nf->nf_rcu, nfsd_file_slab_free);
	return flush;
}

@@ -940,6 +942,7 @@ nfsd_do_file_acquire(struct svc_rqst *rqstp, struct svc_fh *fhp,

	if (nf == NULL)
		goto open_file;
	spin_unlock(&nfsd_file_hashtbl[hashval].nfb_lock);
+	iput(new->nf_inode);
	nfsd_file_slab_free(&new->nf_rcu);

wait_for_construction:

diff --git a/fs/nfsd/filecache.h b/fs/nfsd/filecache.h
index 1da0c79a5580..01fbf6e88cce 100644
--- a/fs/nfsd/filecache.h
+++ b/fs/nfsd/filecache.h

@@ -24,9 +24,7 @@ struct nfsd_file_mark {

/*
* A representation of a file that has been opened by knfsd. These are hashed
- * in the hashtable by inode pointer value. Note that this object doesn't
- * hold a reference to the inode by itself, so the nf_inode pointer should
- * never be dereferenced, only used for comparison.
+ * in the hashtable by inode pointer value.
*/
struct nfsd_file {
	struct hlist_node	nf_node;

-- 
Jeff Layton [off-list ref]

--
Chuck Lever