linux.git/security/keys, branch v4.14-rc4

security/keys: rewrite all of big_key crypto

2017-09-25T22:31:58+00:00

This started out as just replacing the use of crypto/rng with
get_random_bytes_wait, so that we wouldn't use bad randomness at boot
time. But, upon looking further, it appears that there were even deeper
underlying cryptographic problems, and that this seems to have been
committed with very little crypto review. So, I rewrote the whole thing,
trying to keep to the conventions introduced by the previous author, to
fix these cryptographic flaws.

It makes no sense to seed crypto/rng at boot time and then keep
using it like this, when in fact there's already get_random_bytes_wait,
which can ensure there's enough entropy and be a much more standard way
of generating keys. Since this sensitive material is being stored
untrusted, using ECB and no authentication is simply not okay at all. I
find it surprising and a bit horrifying that this code even made it past
basic crypto review, which perhaps points to some larger issues. This
patch moves from using AES-ECB to using AES-GCM. Since keys are uniquely
generated each time, we can set the nonce to zero. There was also a race
condition in which the same key would be reused at the same time in
different threads. A mutex fixes this issue now.

So, to summarize, this commit fixes the following vulnerabilities:

  * Low entropy key generation, allowing an attacker to potentially
    guess or predict keys.
  * Unauthenticated encryption, allowing an attacker to modify the
    cipher text in particular ways in order to manipulate the plaintext,
    which is is even more frightening considering the next point.
  * Use of ECB mode, allowing an attacker to trivially swap blocks or
    compare identical plaintext blocks.
  * Key re-use.
  * Faulty memory zeroing.

Signed-off-by: Jason A. Donenfeld 
Reviewed-by: Eric Biggers 
Signed-off-by: David Howells 
Cc: Herbert Xu 
Cc: Kirill Marinushkin 
Cc: security@kernel.org
Cc: stable@vger.kernel.org

security/keys: properly zero out sensitive key material in big_key

2017-09-25T22:31:41+00:00

Error paths forgot to zero out sensitive material, so this patch changes
some kfrees into a kzfrees.

Signed-off-by: Jason A. Donenfeld 
Signed-off-by: David Howells 
Reviewed-by: Eric Biggers 
Cc: Herbert Xu 
Cc: Kirill Marinushkin 
Cc: security@kernel.org
Cc: stable@vger.kernel.org

KEYS: use kmemdup() in request_key_auth_new()

2017-09-25T14:19:57+00:00

kmemdup() is preferred to kmalloc() followed by memcpy().

Signed-off-by: Eric Biggers 
Signed-off-by: David Howells

KEYS: restrict /proc/keys by credentials at open time

2017-09-25T14:19:57+00:00

When checking for permission to view keys whilst reading from
/proc/keys, we should use the credentials with which the /proc/keys file
was opened.  This is because, in a classic type of exploit, it can be
possible to bypass checks for the *current* credentials by passing the
file descriptor to a suid program.

Following commit 34dbbcdbf633 ("Make file credentials available to the
seqfile interfaces") we can finally fix it.  So let's do it.

Signed-off-by: Eric Biggers 
Signed-off-by: David Howells

KEYS: reset parent each time before searching key_user_tree

2017-09-25T14:19:57+00:00

In key_user_lookup(), if there is no key_user for the given uid, we drop
key_user_lock, allocate a new key_user, and search the tree again.  But
we failed to set 'parent' to NULL at the beginning of the second search.
If the tree were to be empty for the second search, the insertion would
be done with an invalid 'parent', scribbling over freed memory.

Fortunately this can't actually happen currently because the tree always
contains at least the root_key_user.  But it still should be fixed to
make the code more robust.

Signed-off-by: Eric Biggers 
Signed-off-by: David Howells

KEYS: prevent KEYCTL_READ on negative key

2017-09-25T14:19:57+00:00

Because keyctl_read_key() looks up the key with no permissions
requested, it may find a negatively instantiated key.  If the key is
also possessed, we went ahead and called ->read() on the key.  But the
key payload will actually contain the ->reject_error rather than the
normal payload.  Thus, the kernel oopses trying to read the
user_key_payload from memory address (int)-ENOKEY = 0x00000000ffffff82.

Fortunately the payload data is stored inline, so it shouldn't be
possible to abuse this as an arbitrary memory read primitive...

Reproducer:
    keyctl new_session
    keyctl request2 user desc '' @s
    keyctl read $(keyctl show | awk '/user: desc/ {print $1}')

It causes a crash like the following:
     BUG: unable to handle kernel paging request at 00000000ffffff92
     IP: user_read+0x33/0xa0
     PGD 36a54067 P4D 36a54067 PUD 0
     Oops: 0000 [#1] SMP
     CPU: 0 PID: 211 Comm: keyctl Not tainted 4.14.0-rc1 #337
     Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-20170228_101828-anatol 04/01/2014
     task: ffff90aa3b74c3c0 task.stack: ffff9878c0478000
     RIP: 0010:user_read+0x33/0xa0
     RSP: 0018:ffff9878c047bee8 EFLAGS: 00010246
     RAX: 0000000000000001 RBX: ffff90aa3d7da340 RCX: 0000000000000017
     RDX: 0000000000000000 RSI: 00000000ffffff82 RDI: ffff90aa3d7da340
     RBP: ffff9878c047bf00 R08: 00000024f95da94f R09: 0000000000000000
     R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000000
     R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
     FS:  00007f58ece69740(0000) GS:ffff90aa3e200000(0000) knlGS:0000000000000000
     CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
     CR2: 00000000ffffff92 CR3: 0000000036adc001 CR4: 00000000003606f0
     Call Trace:
      keyctl_read_key+0xac/0xe0
      SyS_keyctl+0x99/0x120
      entry_SYSCALL_64_fastpath+0x1f/0xbe
     RIP: 0033:0x7f58ec787bb9
     RSP: 002b:00007ffc8d401678 EFLAGS: 00000206 ORIG_RAX: 00000000000000fa
     RAX: ffffffffffffffda RBX: 00007ffc8d402800 RCX: 00007f58ec787bb9
     RDX: 0000000000000000 RSI: 00000000174a63ac RDI: 000000000000000b
     RBP: 0000000000000004 R08: 00007ffc8d402809 R09: 0000000000000020
     R10: 0000000000000000 R11: 0000000000000206 R12: 00007ffc8d402800
     R13: 00007ffc8d4016e0 R14: 0000000000000000 R15: 0000000000000000
     Code: e5 41 55 49 89 f5 41 54 49 89 d4 53 48 89 fb e8 a4 b4 ad ff 85 c0 74 09 80 3d b9 4c 96 00 00 74 43 48 8b b3 20 01 00 00 4d 85 ed <0f> b7 5e 10 74 29 4d 85 e4 74 24 4c 39 e3 4c 89 e2 4c 89 ef 48
     RIP: user_read+0x33/0xa0 RSP: ffff9878c047bee8
     CR2: 00000000ffffff92

Fixes: 61ea0c0ba904 ("KEYS: Skip key state checks when checking for possession")
Cc: 	[v3.13+]
Signed-off-by: Eric Biggers 
Signed-off-by: David Howells

KEYS: prevent creating a different user's keyrings

2017-09-25T14:19:57+00:00

It was possible for an unprivileged user to create the user and user
session keyrings for another user.  For example:

    sudo -u '#3000' sh -c 'keyctl add keyring _uid.4000 "" @u
                           keyctl add keyring _uid_ses.4000 "" @u
                           sleep 15' &
    sleep 1
    sudo -u '#4000' keyctl describe @u
    sudo -u '#4000' keyctl describe @us

This is problematic because these "fake" keyrings won't have the right
permissions.  In particular, the user who created them first will own
them and will have full access to them via the possessor permissions,
which can be used to compromise the security of a user's keys:

    -4: alswrv-----v------------  3000     0 keyring: _uid.4000
    -5: alswrv-----v------------  3000     0 keyring: _uid_ses.4000

Fix it by marking user and user session keyrings with a flag
KEY_FLAG_UID_KEYRING.  Then, when searching for a user or user session
keyring by name, skip all keyrings that don't have the flag set.

Fixes: 69664cf16af4 ("keys: don't generate user and user session keyrings unless they're accessed")
Cc: 	[v2.6.26+]
Signed-off-by: Eric Biggers 
Signed-off-by: David Howells

KEYS: fix writing past end of user-supplied buffer in keyring_read()

2017-09-25T14:19:57+00:00

Userspace can call keyctl_read() on a keyring to get the list of IDs of
keys in the keyring.  But if the user-supplied buffer is too small, the
kernel would write the full list anyway --- which will corrupt whatever
userspace memory happened to be past the end of the buffer.  Fix it by
only filling the space that is available.

Fixes: b2a4df200d57 ("KEYS: Expand the capacity of a keyring")
Cc: 	[v3.13+]
Signed-off-by: Eric Biggers 
Signed-off-by: David Howells

KEYS: fix key refcount leak in keyctl_read_key()

2017-09-25T14:19:57+00:00

In keyctl_read_key(), if key_permission() were to return an error code
other than EACCES, we would leak a the reference to the key.  This can't
actually happen currently because key_permission() can only return an
error code other than EACCES if security_key_permission() does, only
SELinux and Smack implement that hook, and neither can return an error
code other than EACCES.  But it should still be fixed, as it is a bug
waiting to happen.

Fixes: 29db91906340 ("[PATCH] Keys: Add LSM hooks for key management [try #3]")
Signed-off-by: Eric Biggers 
Signed-off-by: David Howells

KEYS: fix key refcount leak in keyctl_assume_authority()

2017-09-25T14:19:57+00:00

In keyctl_assume_authority(), if keyctl_change_reqkey_auth() were to
fail, we would leak the reference to the 'authkey'.  Currently this can
only happen if prepare_creds() fails to allocate memory.  But it still
should be fixed, as it is a more severe bug waiting to happen.

This patch also moves the read of 'authkey->serial' to before the
reference to the authkey is dropped.  Doing the read after dropping the
reference is very fragile because it assumes we still hold another
reference to the key.  (Which we do, in current->cred->request_key_auth,
but there's no reason not to write it in the "obviously correct" way.)

Fixes: d84f4f992cbd ("CRED: Inaugurate COW credentials")
Signed-off-by: Eric Biggers 
Signed-off-by: David Howells