keys: Fix missing null pointer check in request_key_auth_describe()

BugLink: https://bugs.launchpad.net/bugs/1845266
[ Upstream commit d41a3effbb53b1bcea41e328d16a4d046a508381 ]

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@linux.vnet.ibm.com>
Signed-off-by: Hillf Danton <hdanton@sina.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Sachin Sant <sachinp@linux.vnet.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

diff --git a/security/keys/request_key_auth.c b/security/keys/request_key_auth.c
index 5e515791ccd119d89150f6ede3f0a2a91eccbf9f..1d34b2a5f485e2380e8b092cfb62955434c3f908 100644 (file)
--- a/security/keys/request_key_auth.c
+++ b/security/keys/request_key_auth.c
@@ -71,6 +71,9 @@ static void request_key_auth_describe(const struct key *key,
 {
        struct request_key_auth *rka = get_request_key_auth(key);
 
+       if (!rka)
+               return;
+
        seq_puts(m, "key:");
        seq_puts(m, key->description);
        if (key_is_positive(key))
@@ -88,6 +91,9 @@ 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;