linux-kernel - [PATCH 0/3] selinux: Fix and clean policydb->cond

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Message-Id: <20220128202858.96935-1-vbendel@redhat.com>
Date:   Fri, 28 Jan 2022 21:28:55 +0100
From:   vbendel@...hat.com
To:     paul@...l-moore.com, stephen.smalley.work@...il.com,
        eparis@...isplace.org
Cc:     omosnace@...hat.com, selinux@...r.kernel.org,
        linux-kernel@...r.kernel.org, Vratislav Bendel <vbendel@...hat.com>
Subject: [PATCH 0/3] selinux: Fix and clean policydb->cond_list error paths 

There are two users of policydb->cond_list: cond_read_list()
and duplicate_policydb_cond_list(). If any of them gets an error,
usually an -ENOMEM, the error-path-cleanup *_destroy() functions
get called twice: firstly from these two and secondly from
the caller functions' error paths.

In case such -ENOMEM happens while assigning cond_node data, i.e.
while ->cond_list_len is already non-zero, it leads to inappropriate
dereferencing of policydb->cond_list[] data in the second called
cond_list_destroy() from the caller functions' error paths, resulting
with:
- NULL pointer deref from cond_read_list();
- use-after-free + double-free from duplicate_policydb_cond_list().
(the cond_read_list() manages to set ->cond_list to NULL)

Patch 1/3 simply makes the error behavior consistent by always setting
->cond_list to NULL.

Patch 2/3 fixes the actual bug by resetting ->cond_list_len to 0,
so any subsequent cond_list_destroy() calls would become noop.

Patch 3/3 cleans up the duplicate *_destroy calls on these error paths,
albeit it's a bit questionable and I'm looking for feedback on it:
- on one hand the idea is that the caller functions call the *_destroy()
bits anyway, hence removing duplicate efforts (which also fixes the bug,
but I'd still prefer to apply patches 1 and 2 regardless);
- on the other hand it's appropriate and more bug-proof for a function
to clean everything it allocated on error.
Hence I'm looking forward to seeing what approach the upstream would find
more appropriate.

Signed-off-by: Vratislav Bendel <vbendel@...hat.com>