[PATCH AUTOSEL 5.2 10/12] keys: Fix missing null pointer check in request_key_auth_describe()

From: Sasha Levin
Date: Mon Sep 09 2019 - 18:16:46 EST

From: Hillf Danton <hdanton@xxxxxxxx>

[ 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@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Hillf Danton <hdanton@xxxxxxxx>
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
Tested-by: Sachin Sant <sachinp@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
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 e45b5cf3b97fd..8491becb57270 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 = get_request_key_auth(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;