linux-kernel - [PATCH] afs: Fix use-after-loss-of-ref

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Message-ID: <157901858546.1172.10059769927247020179.stgit@warthog.procyon.org.uk>
Date:   Tue, 14 Jan 2020 16:16:25 +0000
From:   David Howells <dhowells@...hat.com>
To:     torvalds@...ux-foundation.org
Cc:     Al Viro <viro@...iv.linux.org.uk>, linux-afs@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] afs: Fix use-after-loss-of-ref

afs_lookup() has a tracepoint to indicate the outcome of d_splice_alias(),
passing it the inode to retrieve the fid from.  However, the function gave
up its ref on that inode when it called d_splice_alias(), which may have
failed and dropped the inode.

Fix this by caching the fid.

Fixes: 80548b03991f ("afs: Add more tracepoints")
Reported-by: Al Viro <viro@...iv.linux.org.uk>
Signed-off-by: David Howells <dhowells@...hat.com>
---

 fs/afs/dir.c               |   12 +++++++-----
 include/trace/events/afs.h |   12 +++---------
 2 files changed, 10 insertions(+), 14 deletions(-)

diff --git a/fs/afs/dir.c b/fs/afs/dir.c
index 497f979018c2..813db1708494 100644
--- a/fs/afs/dir.c
+++ b/fs/afs/dir.c
@@ -908,6 +908,7 @@ static struct dentry *afs_lookup(struct inode *dir, struct dentry *dentry,
 				 unsigned int flags)
 {
 	struct afs_vnode *dvnode = AFS_FS_I(dir);
+	struct afs_fid fid = {};
 	struct inode *inode;
 	struct dentry *d;
 	struct key *key;
@@ -957,15 +958,16 @@ static struct dentry *afs_lookup(struct inode *dir, struct dentry *dentry,
 		dentry->d_fsdata =
 			(void *)(unsigned long)dvnode->status.data_version;
 	}
+
+	if (!IS_ERR_OR_NULL(inode))
+		fid = AFS_FS_I(inode)->fid;
+
 	d = d_splice_alias(inode, dentry);
 	if (!IS_ERR_OR_NULL(d)) {
 		d->d_fsdata = dentry->d_fsdata;
-		trace_afs_lookup(dvnode, &d->d_name,
-				 inode ? AFS_FS_I(inode) : NULL);
+		trace_afs_lookup(dvnode, &d->d_name, &fid);
 	} else {
-		trace_afs_lookup(dvnode, &dentry->d_name,
-				 IS_ERR_OR_NULL(inode) ? NULL
-				 : AFS_FS_I(inode));
+		trace_afs_lookup(dvnode, &dentry->d_name, &fid);
 	}
 	return d;
 }
diff --git a/include/trace/events/afs.h b/include/trace/events/afs.h
index d5ec4fac82ae..564ba1b5cf57 100644
--- a/include/trace/events/afs.h
+++ b/include/trace/events/afs.h
@@ -915,9 +915,9 @@ TRACE_EVENT(afs_call_state,
 
 TRACE_EVENT(afs_lookup,
 	    TP_PROTO(struct afs_vnode *dvnode, const struct qstr *name,
-		     struct afs_vnode *vnode),
+		     struct afs_fid *fid),
 
-	    TP_ARGS(dvnode, name, vnode),
+	    TP_ARGS(dvnode, name, fid),
 
 	    TP_STRUCT__entry(
 		    __field_struct(struct afs_fid,	dfid		)
@@ -928,13 +928,7 @@ TRACE_EVENT(afs_lookup,
 	    TP_fast_assign(
 		    int __len = min_t(int, name->len, 23);
 		    __entry->dfid = dvnode->fid;
-		    if (vnode) {
-			    __entry->fid = vnode->fid;
-		    } else {
-			    __entry->fid.vid = 0;
-			    __entry->fid.vnode = 0;
-			    __entry->fid.unique = 0;
-		    }
+		    __entry->fid = *fid;
 		    memcpy(__entry->name, name->name, __len);
 		    __entry->name[__len] = 0;
 			   ),