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    Subject[PATCH 4.4 53/56] keys: Fix missing null pointer check in request_key_auth_describe()
    Date
    From: Hillf Danton <hdanton@sina.com>

    [ 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>
    ---
    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 8882b729924dd..976deea0569e3 100644
    --- 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 = key->payload.data[0];

    + 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;

    --
    2.20.1


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