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From: David Howells <dhowells@redhat.com>
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To: syzbot <syzbot+d7c7a495a5e466c031b6@syzkaller.appspotmail.com>
Cc: dhowells@redhat.com, asmadeus@codewreck.org, brauner@kernel.org,
    hdanton@sina.com, linux-kernel@vger.kernel.org,
    syzkaller-bugs@googlegroups.com
Subject: Re: [syzbot] [v9fs?] KASAN: slab-use-after-free Read in p9_fid_destroy
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Date: Thu, 23 May 2024 15:37:38 +0100
Message-ID: <580959.1716475058@warthog.procyon.org.uk>

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.gi=
t master

netfs, 9p: Fix race between umount and async request completion [v2]

There's a problem in 9p's interaction with netfslib whereby a crash occurs
because the 9p_fid structs get forcibly destroyed during client teardown
(without paying attention to their refcounts) before netfslib has finished
with them.  However, it's not a simple case of deferring the clunking that
p9_fid_put() does as that requires the client.

The problem is that netfslib has to unlock pages and clear the IN_PROGRESS
flag before destroying the objects involved - including the pid - and, in
any case, nothing checks to see if writeback completed barring looking at
the page flags.

Fix this by keeping a count of outstanding I/O requests (of any type) and
waiting for it to quiesce during inode eviction.

Reported-by: syzbot+df038d463cca332e8414@syzkaller.appspotmail.com
Link: https://lore.kernel.org/all/0000000000005be0aa061846f8d6@google.com/
Reported-by: syzbot+d7c7a495a5e466c031b6@syzkaller.appspotmail.com
Link: https://lore.kernel.org/all/000000000000b86c5e06130da9c6@google.com/
Reported-by: syzbot+1527696d41a634cc1819@syzkaller.appspotmail.com
Link: https://lore.kernel.org/all/000000000000041f960618206d7e@google.com/
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Eric Van Hensbergen <ericvh@kernel.org>
cc: Latchesar Ionkov <lucho@ionkov.net>
cc: Dominique Martinet <asmadeus@codewreck.org>
cc: Christian Schoenebeck <linux_oss@crudebyte.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: Steve French <sfrench@samba.org>
cc: Hillf Danton <hdanton@sina.com>
cc: v9fs@lists.linux.dev
cc: linux-afs@lists.infradead.org
cc: linux-cifs@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org

Notes:
    Changes
    =3D=3D=3D=3D=3D=3D=3D
    ver #2)
     - Wait for outstanding I/O before clobbering the pagecache.

---
 fs/9p/vfs_inode.c      |    1 +
 fs/afs/inode.c         |    1 +
 fs/netfs/objects.c     |    5 +++++
 fs/smb/client/cifsfs.c |    1 +
 include/linux/netfs.h  |   18 ++++++++++++++++++
 5 files changed, 26 insertions(+)

diff --git a/fs/9p/vfs_inode.c b/fs/9p/vfs_inode.c
index 8c9a896d691e..effb3aa1f3ed 100644
--- a/fs/9p/vfs_inode.c
+++ b/fs/9p/vfs_inode.c
@@ -349,6 +349,7 @@ void v9fs_evict_inode(struct inode *inode)
 	__le32 __maybe_unused version;
 =

 	if (!is_bad_inode(inode)) {
+		netfs_wait_for_outstanding_io(inode);
 		truncate_inode_pages_final(&inode->i_data);
 =

 		version =3D cpu_to_le32(v9inode->qid.version);
diff --git a/fs/afs/inode.c b/fs/afs/inode.c
index 94fc049aff58..15bb7989c387 100644
--- a/fs/afs/inode.c
+++ b/fs/afs/inode.c
@@ -648,6 +648,7 @@ void afs_evict_inode(struct inode *inode)
 =

 	ASSERTCMP(inode->i_ino, =3D=3D, vnode->fid.vnode);
 =

+	netfs_wait_for_outstanding_io(inode);
 	truncate_inode_pages_final(&inode->i_data);
 =

 	afs_set_cache_aux(vnode, &aux);
diff --git a/fs/netfs/objects.c b/fs/netfs/objects.c
index c90d482b1650..f4a642727479 100644
--- a/fs/netfs/objects.c
+++ b/fs/netfs/objects.c
@@ -72,6 +72,7 @@ struct netfs_io_request *netfs_alloc_request(struct addr=
ess_space *mapping,
 		}
 	}
 =

+	atomic_inc(&ctx->io_count);
 	trace_netfs_rreq_ref(rreq->debug_id, 1, netfs_rreq_trace_new);
 	netfs_proc_add_rreq(rreq);
 	netfs_stat(&netfs_n_rh_rreq);
@@ -124,6 +125,7 @@ static void netfs_free_request(struct work_struct *wor=
k)
 {
 	struct netfs_io_request *rreq =3D
 		container_of(work, struct netfs_io_request, work);
+	struct netfs_inode *ictx =3D netfs_inode(rreq->inode);
 	unsigned int i;
 =

 	trace_netfs_rreq(rreq, netfs_rreq_trace_free);
@@ -142,6 +144,9 @@ static void netfs_free_request(struct work_struct *wor=
k)
 		}
 		kvfree(rreq->direct_bv);
 	}
+
+	if (atomic_dec_and_test(&ictx->io_count))
+		wake_up_var(&ictx->io_count);
 	call_rcu(&rreq->rcu, netfs_free_request_rcu);
 }
 =

diff --git a/fs/smb/client/cifsfs.c b/fs/smb/client/cifsfs.c
index ec5b639f421a..14810ffd15c8 100644
--- a/fs/smb/client/cifsfs.c
+++ b/fs/smb/client/cifsfs.c
@@ -431,6 +431,7 @@ cifs_free_inode(struct inode *inode)
 static void
 cifs_evict_inode(struct inode *inode)
 {
+	netfs_wait_for_outstanding_io(inode);
 	truncate_inode_pages_final(&inode->i_data);
 	if (inode->i_state & I_PINNING_NETFS_WB)
 		cifs_fscache_unuse_inode_cookie(inode, true);
diff --git a/include/linux/netfs.h b/include/linux/netfs.h
index d2d291a9cdad..3ca3906bb8da 100644
--- a/include/linux/netfs.h
+++ b/include/linux/netfs.h
@@ -68,6 +68,7 @@ struct netfs_inode {
 	loff_t			remote_i_size;	/* Size of the remote file */
 	loff_t			zero_point;	/* Size after which we assume there's no data
 						 * on the server */
+	atomic_t		io_count;	/* Number of outstanding reqs */
 	unsigned long		flags;
 #define NETFS_ICTX_ODIRECT	0		/* The file has DIO in progress */
 #define NETFS_ICTX_UNBUFFERED	1		/* I/O should not use the pagecache */
@@ -474,6 +475,7 @@ static inline void netfs_inode_init(struct netfs_inode=
 *ctx,
 	ctx->remote_i_size =3D i_size_read(&ctx->inode);
 	ctx->zero_point =3D LLONG_MAX;
 	ctx->flags =3D 0;
+	atomic_set(&ctx->io_count, 0);
 #if IS_ENABLED(CONFIG_FSCACHE)
 	ctx->cache =3D NULL;
 #endif
@@ -517,4 +519,20 @@ static inline struct fscache_cookie *netfs_i_cookie(s=
truct netfs_inode *ctx)
 #endif
 }
 =

+/**
+ * netfs_wait_for_outstanding_io - Wait for outstanding I/O to complete
+ * @ctx: The netfs inode to wait on
+ *
+ * Wait for outstanding I/O requests of any type to complete.  This is in=
tended
+ * to be called from inode eviction routines.  This makes sure that any
+ * resources held by those requests are cleaned up before we let the inod=
e get
+ * cleaned up.
+ */
+static inline void netfs_wait_for_outstanding_io(struct inode *inode)
+{
+	struct netfs_inode *ictx =3D netfs_inode(inode);
+
+	wait_var_event(&ictx->io_count, atomic_read(&ictx->io_count) =3D=3D 0);
+}
+
 #endif /* _LINUX_NETFS_H */