[PATCH AUTOSEL 5.15 4/9] gfs2: fix an oops in gfs2_permission

[Sasha Levin <sashal@kernel.org>]

From: Al Viro <viro@zeniv.linux.org.uk>

[ Upstream commit 0abd1557e21c617bd13fc18f7725fc6363c05913 ]

In RCU mode, we might race with gfs2_evict_inode(), which zeroes
->i_gl.  Freeing of the object it points to is RCU-delayed, so
if we manage to fetch the pointer before it's been replaced with
NULL, we are fine.  Check if we'd fetched NULL and treat that
as "bail out and tell the caller to get out of RCU mode".

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/gfs2/inode.c | 11 +++++++++--
 fs/gfs2/super.c |  2 +-
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/fs/gfs2/inode.c b/fs/gfs2/inode.c
index 97ee17843b4d0..682418d9c8e72 100644
--- a/fs/gfs2/inode.c
+++ b/fs/gfs2/inode.c
@@ -1850,14 +1850,21 @@ int gfs2_permission(struct user_namespace *mnt_userns, struct inode *inode,
 {
 	struct gfs2_inode *ip;
 	struct gfs2_holder i_gh;
+	struct gfs2_glock *gl;
 	int error;
 
 	gfs2_holder_mark_uninitialized(&i_gh);
 	ip = GFS2_I(inode);
-	if (gfs2_glock_is_locked_by_me(ip->i_gl) == NULL) {
+	gl = rcu_dereference(ip->i_gl);
+	if (unlikely(!gl)) {
+		/* inode is getting torn down, must be RCU mode */
+		WARN_ON_ONCE(!(mask & MAY_NOT_BLOCK));
+		return -ECHILD;
+        }
+	if (gfs2_glock_is_locked_by_me(gl) == NULL) {
 		if (mask & MAY_NOT_BLOCK)
 			return -ECHILD;
-		error = gfs2_glock_nq_init(ip->i_gl, LM_ST_SHARED, LM_FLAG_ANY, &i_gh);
+		error = gfs2_glock_nq_init(gl, LM_ST_SHARED, LM_FLAG_ANY, &i_gh);
 		if (error)
 			return error;
 	}
diff --git a/fs/gfs2/super.c b/fs/gfs2/super.c
index 51b44da4a0d64..268651ac9fc84 100644
--- a/fs/gfs2/super.c
+++ b/fs/gfs2/super.c
@@ -1436,7 +1436,7 @@ static void gfs2_evict_inode(struct inode *inode)
 		wait_on_bit_io(&ip->i_flags, GIF_GLOP_PENDING, TASK_UNINTERRUPTIBLE);
 		gfs2_glock_add_to_lru(ip->i_gl);
 		gfs2_glock_put_eventually(ip->i_gl);
-		ip->i_gl = NULL;
+		rcu_assign_pointer(ip->i_gl, NULL);
 	}
 }
 
-- 
2.42.0