tjh.dev / kernel
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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.

[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>

authored by

Hillf Danton and committed by
Linus Torvalds d41a3eff a3f5e1f5

+6
+6
security/keys/request_key_auth.c
··· 66 66 { 67 67 struct request_key_auth *rka = dereference_key_rcu(key); 68 68 69 + if (!rka) 70 + return; 71 + 69 72 seq_puts(m, "key:"); 70 73 seq_puts(m, key->description); 71 74 if (key_is_positive(key)) ··· 85 82 struct request_key_auth *rka = dereference_key_locked(key); 86 83 size_t datalen; 87 84 long ret; 85 + 86 + if (!rka) 87 + return -EKEYREVOKED; 88 88 89 89 datalen = rka->callout_len; 90 90 ret = datalen;