On Thu, 2022-07-07 at 17:01 +0000, Chuck Lever III wrote:
>
> > On Jul 7, 2022, at 12:59 PM, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
> >
> > On Thu, 2022-07-07 at 12:55 -0400, Jeff Layton wrote:
> > > On Thu, 2022-07-07 at 11:58 -0400, Chuck Lever wrote:
> > > > 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@xxxxxxxxxx>
> > > > ---
> > > > 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.
>
>
Yeah, maybe we should pass in the inode as a separate parameter to that
function?
> > 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.
>
ACK
>
> > > > 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 <jlayton@xxxxxxxxxx>
>
> --
> Chuck Lever
>
>
>
--
Jeff Layton <jlayton@xxxxxxxxxx>
