drivers/char/random.c: fix boot id uniqueness race

/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>

1 files changed, 8 insertions, 3 deletions

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 54ca8b23cde3..4ec04a754733 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -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);