drivers/char/random.c: fix boot id uniqueness race
Mathieu Desnoyers [Thu, 12 Apr 2012 19:49:12 +0000 (12:49 -0700)]
/proc/sys/kernel/random/boot_id can be read concurrently by userspace
processes.  If two (or more) user-space processes concurrently read
boot_id when sysctl_bootid is not yet assigned, a race can occur making
boot_id differ between the reads.  Because the whole point of the boot id
is to be unique across a kernel execution, fix this by protecting this
operation with a spinlock.

Given that this operation is not frequently used, hitting the spinlock
on each call should not be an issue.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Matt Mackall <mpm@selenic.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Greg Kroah-Hartman <greg@kroah.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/char/random.c

index 54ca8b2..4ec04a7 100644 (file)
@@ -1260,10 +1260,15 @@ static int proc_do_uuid(ctl_table *table, int write,
        uuid = table->data;
        if (!uuid) {
                uuid = tmp_uuid;
-               uuid[8] = 0;
-       }
-       if (uuid[8] == 0)
                generate_random_uuid(uuid);
+       } else {
+               static DEFINE_SPINLOCK(bootid_spinlock);
+
+               spin_lock(&bootid_spinlock);
+               if (!uuid[8])
+                       generate_random_uuid(uuid);
+               spin_unlock(&bootid_spinlock);
+       }
 
        sprintf(buf, "%pU", uuid);