[PATCH] keys: Fix missing null pointer check in request_key_auth_describe()

[David Howells]

From: Hillf Danton <hdan...@sina.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 <sach...@linux.vnet.ibm.com>
Signed-off-by: Hillf Danton <hdan...@sina.com>
Signed-off-by: David Howells <dhowe...@redhat.com>
Tested-by: Sachin Sant <sach...@linux.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;