Re: [PATCH] char random: fix boot id uniqueness race (v2)

[Eric Dumazet <eric.dumazet@gmail.com>]

Le mardi 14 février 2012 à 23:10 -0500, Mathieu Desnoyers a écrit :
> The proc file /proc/sys/kernel/random/boot_id can be read concurrently
> by user-space 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 mutex, and introduce a
> boot_id_generated flag, along with appropriate memory barriers, to let
> the fast-path know if the boot ID has been generated without having to
> hold the mutex.
> 
> I propose this approach rather than setting it up within an initcall(),
> because letting execution randomness add to entropy before populating
> the boot id seems to be a wanted property. Also, populating it lazily
> rather than at boot time only makes the performance hit be taken when
> boot_id is being read.
> 
> 
> Q: Why are these memory barriers required ? Aren't the mutexes already
>    dealing with ordering ?
> 
> The need for memory barriers is a consequence of letting the fast-path
> run without holding this mutex.
> 
> Here is the race dealt with by the smp_rmb()/smp_wmb(). I'm showing the
> result of reversed write order here:
> 
> CPU A                             CPU B
> 
> Load boot_id_generated            
>   (test -> false)
> mutex_lock(&boot_id_mutex)
>   (implied memory barrier
>    with acquire semantic)
> Load boot_id_generated again
>    (test -> false)
> boot_id_generated = 1
>   (both the compiler and
>    CPU are free to reorder
>    the boot_id_generated
>    store before uuid stores)
>                                   Load boot_id_generated
>                                     (test -> true)
>                                   Load uuid content
>                                     (races with generate_random_uuid:
>                                      result either 0 or corrupted)
>                                   Return corrupted uuid.
> generate_random_uuid(uuid)
> mutex_unlock(&boot_id_mutex)
> 
> I prefer not requiring the fast-path to take a mutex, because this
> would transform a read-mostly operation into an operation that
> requires cache-line exchanges (the mutex). However, if we want the
> fast-path to be mutex-free, we need to enforce order with
> memory barriers: smp_rmb on the read-side, smp_wmb on the
> update-side. Failure to do so leads to the race shown above, where
> a corrupted boot_id can be returned.
> 
> 
> * Changelog since v1:
> - boot_id_mutex is now declared within the proc_do_uuid scope.
> - added explanation for memory barriers.
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> CC: "Theodore Ts'o" <tytso@mit.edu>
> CC: Matt Mackall <mpm@selenic.com>
> CC: Greg Kroah-Hartman <greg@kroah.com>
> ---
>  drivers/char/random.c |   20 +++++++++++++++++---
>  1 file changed, 17 insertions(+), 3 deletions(-)
> 
> Index: linux-2.6-lttng/drivers/char/random.c
> ===================================================================
> --- linux-2.6-lttng.orig/drivers/char/random.c
> +++ linux-2.6-lttng/drivers/char/random.c
> @@ -1244,16 +1244,30 @@ static char sysctl_bootid[16];
>  static int proc_do_uuid(ctl_table *table, int write,
>  			void __user *buffer, size_t *lenp, loff_t *ppos)
>  {
> +	static int boot_id_generated;
> +	static DEFINE_MUTEX(boot_id_mutex);
>  	ctl_table fake_table;
>  	unsigned char buf[64], tmp_uuid[16], *uuid;
>  
>  	uuid = table->data;
>  	if (!uuid) {
>  		uuid = tmp_uuid;
> -		uuid[8] = 0;
> -	}
> -	if (uuid[8] == 0)
>  		generate_random_uuid(uuid);
> +	} else {
> +		if (unlikely(!ACCESS_ONCE(boot_id_generated))) {
> +			mutex_lock(&boot_id_mutex);
> +			if (!boot_id_generated) {
> +				generate_random_uuid(uuid);
> +				/* Store uuid before boot_id_generated. */
> +				smp_wmb();
> +				boot_id_generated = 1;
> +			}
> +			mutex_unlock(&boot_id_mutex);
> +		} else {
> +			/* Load boot_id_generated before uuid */
> +			smp_rmb();
> +		}
> +	}
>  
>  	sprintf(buf, "%pU", uuid);
>  
> 

This seems overly complex to me.

I doubt this is performance critical path ?

What about a basic patch like :

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 54ca8b2..af6040d 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1260,11 +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);
 
 	fake_table.data = buf;