linux-kernel - [PATCH] keys: Fix missing null pointer check in request_key_auth

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Message-ID: <156742784963.20310.17991758398365196262.stgit@warthog.procyon.org.uk>
Date:   Mon, 02 Sep 2019 13:37:29 +0100
From:   David Howells <dhowells@...hat.com>
To:     torvalds@...ux-foundation.org
Cc:     Sachin Sant <sachinp@...ux.vnet.ibm.com>,
        Hillf Danton <hdanton@...a.com>,
        Sachin Sant <sachinp@...ux.vnet.ibm.com>, dhowells@...hat.com,
        keyrings@...r.kernel.org, linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] keys: Fix missing null pointer check in
 request_key_auth_describe()

From: Hillf Danton <hdanton@...a.com>

If a request_key authentication token key gets revoked, there's a window in
which request_key_auth_describe() can see it with a NULL payload - but it
makes no check for this and something like the following oops may occur:

	BUG: Kernel NULL pointer dereference at 0x00000038
	Faulting instruction address: 0xc0000000004ddf30
	Oops: Kernel access of bad area, sig: 11 [#1]
	...
	NIP [...] request_key_auth_describe+0x90/0xd0
	LR [...] request_key_auth_describe+0x54/0xd0
	Call Trace:
	[...] request_key_auth_describe+0x54/0xd0 (unreliable)
	[...] proc_keys_show+0x308/0x4c0
	[...] seq_read+0x3d0/0x540
	[...] proc_reg_read+0x90/0x110
	[...] __vfs_read+0x3c/0x70
	[...] vfs_read+0xb4/0x1b0
	[...] ksys_read+0x7c/0x130
	[...] system_call+0x5c/0x70

Fix this by checking for a NULL pointer when describing such a key.

Also make the read routine check for a NULL pointer to be on the safe side.

[DH: Modified to not take already-held rcu lock and modified to also check
 in the read routine]

Fixes: 04c567d9313e ("[PATCH] Keys: Fix race between two instantiators of a key")
Reported-by: Sachin Sant <sachinp@...ux.vnet.ibm.com>
Signed-off-by: Hillf Danton <hdanton@...a.com>
Signed-off-by: David Howells <dhowells@...hat.com>
Tested-by: Sachin Sant <sachinp@...ux.vnet.ibm.com>
---

 security/keys/request_key_auth.c |    6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/security/keys/request_key_auth.c b/security/keys/request_key_auth.c
index e73ec040e250..ecba39c93fd9 100644
--- a/security/keys/request_key_auth.c
+++ b/security/keys/request_key_auth.c
@@ -66,6 +66,9 @@ static void request_key_auth_describe(const struct key *key,
 {
 	struct request_key_auth *rka = dereference_key_rcu(key);
 
+	if (!rka)
+		return;
+
 	seq_puts(m, "key:");
 	seq_puts(m, key->description);
 	if (key_is_positive(key))
@@ -83,6 +86,9 @@ static long request_key_auth_read(const struct key *key,
 	size_t datalen;
 	long ret;
 
+	if (!rka)
+		return -EKEYREVOKED;
+
 	datalen = rka->callout_len;
 	ret = datalen;