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[PATCH 3.16.y-ckt 105/107] KEYS: Fix race between key destruction and finding a keyring by name

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3.16.7-ckt19 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: David Howells <dhowells@redhat.com>

commit 94c4554ba07adbdde396748ee7ae01e86cf2d8d7 upstream.

There appears to be a race between:

 (1) key_gc_unused_keys() which frees key->security and then calls
     keyring_destroy() to unlink the name from the name list

 (2) find_keyring_by_name() which calls key_permission(), thus accessing
     key->security, on a key before checking to see whether the key usage is 0
     (ie. the key is dead and might be cleaned up).

Fix this by calling ->destroy() before cleaning up the core key data -
including key->security.

Reported-by: Petr Matousek <pmatouse@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Cc: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
---
 security/keys/gc.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/security/keys/gc.c b/security/keys/gc.c
index 009d9370c8fd..38676fac7943 100644
--- a/security/keys/gc.c
+++ b/security/keys/gc.c
@@ -143,6 +143,10 @@ static noinline void key_gc_unused_keys(struct list_head *keys)
 		kdebug("- %u", key->serial);
 		key_check(key);
 
+		/* Throw away the key data */
+		if (key->type->destroy)
+			key->type->destroy(key);
+
 		security_key_free(key);
 
 		/* deal with the user's key tracking and quota */
@@ -157,10 +161,6 @@ static noinline void key_gc_unused_keys(struct list_head *keys)
 		if (test_bit(KEY_FLAG_INSTANTIATED, &key->flags))
 			atomic_dec(&key->user->nikeys);
 
-		/* now throw away the key memory */
-		if (key->type->destroy)
-			key->type->destroy(key);
-
 		key_user_put(key->user);
 
 		kfree(key->description);
--
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[3.16.y-ckt stable] Linux 3.16.7-ckt19 stable review Luis Henriques <luis.henriques@canonical.com> - 2015-10-26 15:30 +0100
  [PATCH 3.16.y-ckt 005/104] arm64: head.S: initialise mdcr_el2 in el2_setup Luis Henriques <luis.henriques@canonical.com> - 2015-10-26 15:30 +0100
  [PATCH 3.16.y-ckt 038/104] usb: xhci: Clear XHCI_STATE_DYING on start Luis Henriques <luis.henriques@canonical.com> - 2015-10-26 15:30 +0100
  [PATCH 3.16.y-ckt 006/104] kvm: don't try to register to KVM_FAST_MMIO_BUS for non mmio eventfd Luis Henriques <luis.henriques@canonical.com> - 2015-10-26 15:30 +0100
  [PATCH 3.16.y-ckt 027/104] drm/vmwgfx: Fix up user_dmabuf refcounting Luis Henriques <luis.henriques@canonical.com> - 2015-10-26 15:30 +0100
  [PATCH 3.16.y-ckt 001/104] ARM: 8429/1: disable GCC SRA optimization Luis Henriques <luis.henriques@canonical.com> - 2015-10-26 15:30 +0100
  [PATCH 3.16.y-ckt 015/104] arm64: errata: add module build workaround for erratum #843419 Luis Henriques <luis.henriques@canonical.com> - 2015-10-26 15:30 +0100
  [PATCH 3.16.y-ckt 029/104] Btrfs: fix read corruption of compressed and shared extents Luis Henriques <luis.henriques@canonical.com> - 2015-10-26 15:30 +0100
  [PATCH 3.16.y-ckt 105/107] KEYS: Fix race between key destruction and finding a keyring by name Luis Henriques <luis.henriques@canonical.com> - 2015-10-26 20:10 +0100
  [PATCH 3.16.y-ckt 107/107] KEYS: Don't permit request_key() to construct a new keyring Luis Henriques <luis.henriques@canonical.com> - 2015-10-26 20:10 +0100
  Re: [3.16.y-ckt stable] Linux 3.16.7-ckt19 stable review Luis Henriques <luis.henriques@canonical.com> - 2015-10-26 20:10 +0100
    Re: [3.16.y-ckt stable] Linux 3.16.7-ckt19 stable review Luis Henriques <luis.henriques@canonical.com> - 2015-10-28 18:00 +0100
  [PATCH 3.16.y-ckt 106/107] KEYS: Fix crash when attempt to garbage collect an uninstantiated keyring Luis Henriques <luis.henriques@canonical.com> - 2015-10-26 20:10 +0100
  [PATCH 3.16.y-ckt 108/108] Btrfs: update fix for read corruption of compressed and shared extents Luis Henriques <luis.henriques@canonical.com> - 2015-10-28 18:00 +0100

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