lkml.org 
[lkml]   [2012]   [Feb]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH] char random: fix boot id uniqueness race (v2)
    * Eric Dumazet (eric.dumazet@gmail.com) wrote:
    > 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 ?

    Fair point, I don't see many use-cases where an application would like
    to get the boot_id value (which stays constant over an entire kernel
    execution) very frequently, unlike /proc/sys/kernel/random/uuid.

    >
    > 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);

    That would make sense, as long as we're OK about turning a read-mostly
    operation into a fully serialized operation that requires to exchange
    the lock between processor cache-lines. But as you point out, it should
    be fairly unfrequently used.

    Any particular reason to use a spin lock rather than a mutex ? I did put
    a mutex in my implementation assuming that it would be a little more
    RT-friendly.

    Thanks,

    Mathieu

    > + }
    > sprintf(buf, "%pU", uuid);
    >
    > fake_table.data = buf;
    >
    >
    >

    --
    Mathieu Desnoyers
    Operating System Efficiency R&D Consultant
    EfficiOS Inc.
    http://www.efficios.com
    --
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/

    \
     
     \ /
      Last update: 2012-02-15 14:37    [W:7.866 / U:0.136 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site