linux-kernel - re: misc: alcor_pci: fix null-ptr-deref when there is no PCI bridge

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Message-ID: <cb099c69-0d59-7a12-b0bc-2ce71264363e@canonical.com>
Date:   Tue, 18 May 2021 19:32:11 +0100
From:   Colin Ian King <colin.king@...onical.com>
To:     Tong Zhang <ztong0001@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: re: misc: alcor_pci: fix null-ptr-deref when there is no PCI bridge

Hi,

Static analysis on linux-next with Coverity has detected an issue in
drivers/misc/cardreader/alcor_pci.c in function
alcor_pci_init_check_aspm  with the following commit:

commit 3ce3e45cc333da707d4d6eb433574b990bcc26f5
Author: Tong Zhang <ztong0001@...il.com>
Date:   Thu May 13 00:07:33 2021 -0400

    misc: alcor_pci: fix null-ptr-deref when there is no PCI bridge

The analysis is as follows:

135 static void alcor_pci_init_check_aspm(struct alcor_pci_priv *priv)
136 {
137        struct pci_dev *pci;
138        int where;
139        u32 val32;
140
141        priv->pdev_cap_off    = alcor_pci_find_cap_offset(priv,
priv->pdev);
142        /*
143         * A device might be attached to root complex directly and
144         * priv->parent_pdev will be NULL. In this case we don't
check its
145         * capability and disable ASPM completely.
146         */

   1. Condition !priv->parent_pdev, taking true branch.
   2. var_compare_op: Comparing priv->parent_pdev to null implies that
priv->parent_pdev might be null.

147        if (!priv->parent_pdev)

   Dereference after null check (FORWARD_NULL)
   3. var_deref_model: Passing null pointer priv->parent_pdev to
alcor_pci_find_cap_offset, which dereferences it.

148                priv->parent_cap_off = alcor_pci_find_cap_offset(priv,
149
priv->parent_pdev);

When !priv->parent_pdev is true, then priv->parent_pdev is NULL and
hence the call to alcor_pci_find_cap_offset() is dereferencing a null
pointer in the priv->parent_pdev argument.

I suspect the logic in the if statement is inverted, the ! should be
removed. This seems too trivial to be wrong. Maybe I'm missing something
deeper.

Colin