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From f144220f72062ed5359e0211f130670c915a12dd Mon Sep 17 00:00:00 2001
From: David Howells <dhowells@redhat.com>
Date: Mon, 14 Dec 2015 10:36:31 -0500
Subject: [PATCH] KEYS: Fix race between read and revoke
There's a race between keyctl_read() and keyctl_revoke(). If the revoke
happens between keyctl_read() checking the validity of a key and the key's
semaphore being taken, then the key type read method will see a revoked key.
This causes a problem for the user-defined key type because it assumes in
its read method that there will always be a payload in a non-revoked key
and doesn't check for a NULL pointer.
Fix this by making keyctl_read() check the validity of a key after taking
semaphore instead of before.
This was discovered by a multithreaded test program generated by syzkaller
(http://github.com/google/syzkaller). Here's a cleaned up version:
#include <sys/types.h>
#include <keyutils.h>
#include <pthread.h>
void *thr0(void *arg)
{
key_serial_t key = (unsigned long)arg;
keyctl_revoke(key);
return 0;
}
void *thr1(void *arg)
{
key_serial_t key = (unsigned long)arg;
char buffer[16];
keyctl_read(key, buffer, 16);
return 0;
}
int main()
{
key_serial_t key = add_key("user", "%", "foo", 3, KEY_SPEC_USER_KEYRING);
pthread_t th[5];
pthread_create(&th[0], 0, thr0, (void *)(unsigned long)key);
pthread_create(&th[1], 0, thr1, (void *)(unsigned long)key);
pthread_create(&th[2], 0, thr0, (void *)(unsigned long)key);
pthread_create(&th[3], 0, thr1, (void *)(unsigned long)key);
pthread_join(th[0], 0);
pthread_join(th[1], 0);
pthread_join(th[2], 0);
pthread_join(th[3], 0);
return 0;
}
Build as:
cc -o keyctl-race keyctl-race.c -lkeyutils -lpthread
Run as:
while keyctl-race; do :; done
as it may need several iterations to crash the kernel. The crash can be
summarised as:
BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
IP: [<ffffffff81279b08>] user_read+0x56/0xa3
...
Call Trace:
[<ffffffff81276aa9>] keyctl_read_key+0xb6/0xd7
[<ffffffff81277815>] SyS_keyctl+0x83/0xe0
[<ffffffff815dbb97>] entry_SYSCALL_64_fastpath+0x12/0x6f
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
---
security/keys/keyctl.c | 18 +++++++++---------
1 file changed, 9 insertions(+), 9 deletions(-)
diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index fb111eafcb89..1c3872aeed14 100644
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -751,16 +751,16 @@ long keyctl_read_key(key_serial_t keyid, char __user *buffer, size_t buflen)
/* the key is probably readable - now try to read it */
can_read_key:
- ret = key_validate(key);
- if (ret == 0) {
- ret = -EOPNOTSUPP;
- if (key->type->read) {
- /* read the data with the semaphore held (since we
- * might sleep) */
- down_read(&key->sem);
+ ret = -EOPNOTSUPP;
+ if (key->type->read) {
+ /* Read the data with the semaphore held (since we might sleep)
+ * to protect against the key being updated or revoked.
+ */
+ down_read(&key->sem);
+ ret = key_validate(key);
+ if (ret == 0)
ret = key->type->read(key, buffer, buflen);
- up_read(&key->sem);
- }
+ up_read(&key->sem);
}
error2:
--
2.5.0
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