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From:   Willem de Bruijn <willemdebruijn.kernel@gmail.com>
To:     linux-kernel@vger.kernel.org
Cc:     fenghua.yu@intel.com, reinette.chatre@intel.com,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, x86@kernel.org,
        hpa@zytor.com, Willem de Bruijn <willemb@google.com>
Subject: [PATCH] x86/resctrl: Fix memory leak on kernfs dir removal
Date:   Mon, 26 Oct 2020 11:09:13 -0400
Message-Id: <20201026150913.2485171-1-willemdebruijn.kernel@gmail.com>
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From: Willem de Bruijn <willemb@google.com>

Resctrl takes an extra kernfs ref on directory entries, to access
the entry on cleanup in rdtgroup_kn_unlock after removing the entire
subtree with kfree_remove.

But the path takes an extra ref both on mkdir and on rmdir.

The kernfs_get on mkdir causes a memleak in the unlikely exit with
error in the same function, as no extra kernfs_put exists and no extra
rdtgroup_kn_unlock occurs.

More importantly, essentially the same happens in the normal path, as
this simple program demonstrates:

    for i in {1..200000}; do
      mkdir /sys/fs/resctrl/task1
      rmdir /sys/fs/resctrl/task1
    done
    slabtop

When taking an extra ref for the duration of kernfs_remove, it is
easiest to reason about when holding this extra ref as short as
possible. For that, the refcnt on error reason and free on umount
(rmdir_all_sub), remove the first kernfs_get on mkdir, leaving the
other on rmdir.

As the caller of rdtgroup_rmdir, kernfs_iop_rmdir, itself takes a
reference on the kernfs object, the extra reference is possibly not
needed at all.

Fixes: 60cf5e101fd4 ("x86/intel_rdt: Add mkdir to resctrl file system")
Signed-off-by: Willem de Bruijn <willemb@google.com>
---
 arch/x86/kernel/cpu/resctrl/rdtgroup.c | 8 --------
 1 file changed, 8 deletions(-)

diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
index 4d02ec8f371e..115a86bf6bd8 100644
--- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
+++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
@@ -2839,14 +2839,6 @@ static int mkdir_rdt_prepare(struct kernfs_node *parent_kn,
 	}
 	rdtgrp->kn = kn;
 
-	/*
-	 * kernfs_remove() will drop the reference count on "kn" which
-	 * will free it. But we still need it to stick around for the
-	 * rdtgroup_kn_unlock(kn} call below. Take one extra reference
-	 * here, which will be dropped inside rdtgroup_kn_unlock().
-	 */
-	kernfs_get(kn);
-
 	ret = rdtgroup_kn_set_ugid(kn);
 	if (ret) {
 		rdt_last_cmd_puts("kernfs perm error\n");
-- 
2.29.0.rc1.297.gfa9743e501-goog