Oops (request_key_auth_describe) while running cve-2016-7042 from LTP

[David Howells]

Hi Hillf,

Would you like to me to put you down as the author of this patch?  If so, I'll
need a Signed-off-by from you.

David
---
commit df882ad6d4e24a3763719c1798ea58e87d56c2d7
Author: Hillf Danton <hdanton at sina.com>
Date:   Fri Aug 30 15:54:33 2019 +0100

    keys: Fix missing null pointer check in request_key_auth_describe()
    
    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.
    
    Fixes: 04c567d9313e ("[PATCH] Keys: Fix race between two instantiators of a key")
    Reported-by: Sachin Sant <sachinp at linux.vnet.ibm.com>
    Signed-off-by: David Howells <dhowells at redhat.com>
    Tested-by: Sachin Sant <sachinp at linux.vnet.ibm.com>

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;