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Date: Thu, 23 Sep 2010 14:40:06 -0400
From: Jason Baron <jbaron@...hat.com>
To: Mathieu Desnoyers <compudj@...stal.dyndns.org>
Cc: Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
Ingo Molnar <mingo@...e.hu>,
Andrew Morton <akpm@...ux-foundation.org>,
Frederic Weisbecker <fweisbec@...il.com>,
Andi Kleen <andi@...stfloor.org>,
David Miller <davem@...emloft.net>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH 03/11] jump label: Base patch for jump label
On Thu, Sep 23, 2010 at 11:48:52AM -0400, Mathieu Desnoyers wrote:
> * Jason Baron (jbaron@...hat.com) wrote:
> > On Thu, Sep 23, 2010 at 10:37:58AM -0400, Mathieu Desnoyers wrote:
> > > * Steven Rostedt (rostedt@...dmis.org) wrote:
> > > > From: Jason Baron <jbaron@...hat.com>
> > > >
> > > > base patch to implement 'jump labeling'. Based on a new 'asm goto' inline
> > > > assembly gcc mechanism, we can now branch to labels from an 'asm goto'
> > > > statment. This allows us to create a 'no-op' fastpath, which can subsequently
> > > > be patched with a jump to the slowpath code. This is useful for code which
> > > > might be rarely used, but which we'd like to be able to call, if needed.
> > > > Tracepoints are the current usecase that these are being implemented for.
> > > >
> > > [...]
> > > > +/***
> > > > + * jump_label_update - update jump label text
> > > > + * @key - key value associated with a a jump label
> > > > + * @type - enum set to JUMP_LABEL_ENABLE or JUMP_LABEL_DISABLE
> > > > + *
> > > > + * Will enable/disable the jump for jump label @key, depending on the
> > > > + * value of @type.
> > > > + *
> > > > + */
> > > > +
> > > > +void jump_label_update(unsigned long key, enum jump_label_type type)
> > > > +{
> > > > + struct jump_entry *iter;
> > > > + struct jump_label_entry *entry;
> > > > + struct hlist_node *module_node;
> > > > + struct jump_label_module_entry *e_module;
> > > > + int count;
> > > > +
> > > > + mutex_lock(&jump_label_mutex);
> > > > + entry = get_jump_label_entry((jump_label_t)key);
> > > > + if (entry) {
> > > > + count = entry->nr_entries;
> > > > + iter = entry->table;
> > > > + while (count--) {
> > > > + if (kernel_text_address(iter->code))
> > >
> > > As I pointed out in another thread, I'm concerned about the use of
> > > kernel_text_address without module mutex here. kernel_text_address calls
> > > is_module_text_address(), which calls __module_text_address() with
> > > preemption off.
> > >
> > > __module_text_address() looks like:
> > >
> > > struct module *__module_address(unsigned long addr)
> > > {
> > > struct module *mod;
> > >
> > > if (addr < module_addr_min || addr > module_addr_max)
> > > return NULL;
> > >
> > > list_for_each_entry_rcu(mod, &modules, list)
> > > if (within_module_core(addr, mod)
> > > || within_module_init(addr, mod))
> > > return mod;
> > > return NULL;
> > > }
> > >
> > > struct module *__module_text_address(unsigned long addr)
> > > {
> > > struct module *mod = __module_address(addr);
> > > if (mod) {
> > > /* Make sure it's within the text section. */
> > > if (!within(addr, mod->module_init, mod->init_text_size)
> > > && !within(addr, mod->module_core, mod->core_text_size))
> > > mod = NULL;
> > > }
> > > return mod;
> > > }
> > >
> > > So the test for the address being in the module core is already
> > > problematic, since we hold preempt off only within
> > > is_module_text_address(). The is_module_text_address() caller is then
> > > free to write to this address even after the module has been unloaded
> > > and the module unload grace period ended.
> > >
> > > Even worse, such grace period is not waited for at module load time
> > > within:
> > >
> > > init_module()
> > > module_free(mod, mod->module_init);
> > > mod->module_init = NULL;
> > > mod->init_size = 0;
> > > mod->init_text_size = 0;
> > > (done with module_mutex held, while the module is already in the
> > > module list)
> > >
> > > We'd probably have to hold the module mutex around the
> > > is_module_text_address() call and address use (which can be a pain), or
> > > to correctly address this part of init_module() with RCU and require
> > > that preempt off is held across both __module_text_address() call site
> > > and the actual use of that pointer (which does not fit with jump label,
> > > which need to sleep, so we'd have to move module.c to a preemptable
> > > rcu_read_lock/synchronize_rcu() C.S.).
> > >
> > > Thoughts ?
> > >
> >
> > I was thinking about the rcu_read_lock/synchronize_rcu() for this race.
> > We can hold the rcu_read_lock() across the is_module_text_address()
> > check in the jump label code, and then we can do in module.c:
> >
> > mod->module_init = NULL;
> > synchronize_rcu();
> > module_free(mod, mod->module_init);
>
> Beware that you need to copy the module_init address. Also make sure you
> audit the "module_free" (per-architecture) to make sure they don't use
> "mod" in ways you did not foresee.
>
> > .
> > .
> > .
> >
> > or we could push the rcu_read_lock() further down into
> > is_module_address()?
>
> We need to pull rcu_read_lock further _up_. It needs to be held across
> both is_module_address() and the actual use of the address, otherwise
> the memory mapping can be removed underneath us.
>
> You can see the rcu read lock as keeping the memory mapping alive for as
> long as the rcu read lock is held.
>
> We'd also need to add a synchronize_rcu() in module removal.
>
I agree that we this synchronization for the module __init section.
However, I believe we are ok for module removal case. free_module() is
called *after* blocking_notifier_call_chain() call. The
blocking_notifier_call_chain() is going to call back into the jump label
code, grab the jump_label_mutex and remove the reference to the module that
is about to freed. Thus, the jump label code can no longer reference it.
So I think the following patch is all that is required here (lightly
tested).
Steve, I'll re-post as a separate patch, if we agree on this fix.
thanks,
-Jason
jump label: fix __init module section race
Jump label uses is_module_text_address() to ensure that the module
__init sections are valid before updating them. However, between the
check for a valid module __init section and the subsequent jump
label update, the module's __init section could be free out from under
us.
We fix this potential race putting the address check *and* the jump
label update under a rcu_read_lock(), and making sure a grace period
has completed before we free the __init section.
Thanks to Mathieu Desnoyers for pointing out this race condition.
Signed-off-by: Jason Baron <jbaron@...hat.com>
---
kernel/jump_label.c | 2 ++
kernel/module.c | 5 ++++-
2 files changed, 6 insertions(+), 1 deletions(-)
diff --git a/kernel/jump_label.c b/kernel/jump_label.c
index f82878b..7830bfb 100644
--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -160,6 +160,7 @@ void jump_label_update(unsigned long key, enum jump_label_type type)
iter++;
}
/* eanble/disable jump labels in modules */
+ rcu_read_lock();
hlist_for_each_entry(e_module, module_node, &(entry->modules),
hlist) {
count = e_module->nr_entries;
@@ -170,6 +171,7 @@ void jump_label_update(unsigned long key, enum jump_label_type type)
iter++;
}
}
+ rcu_read_unlock();
}
mutex_unlock(&jump_label_mutex);
}
diff --git a/kernel/module.c b/kernel/module.c
index eba1341..09f7e9e 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2692,6 +2692,7 @@ SYSCALL_DEFINE3(init_module, void __user *, umod,
unsigned long, len, const char __user *, uargs)
{
struct module *mod;
+ void *init_code;
int ret = 0;
/* Must have permission */
@@ -2749,8 +2750,10 @@ SYSCALL_DEFINE3(init_module, void __user *, umod,
mod->symtab = mod->core_symtab;
mod->strtab = mod->core_strtab;
#endif
- module_free(mod, mod->module_init);
+ init_code = mod->module_init;
mod->module_init = NULL;
+ synchronize_rcu();
+ module_free(mod, init_code);
mod->init_size = 0;
mod->init_text_size = 0;
mutex_unlock(&module_mutex);
--
1.7.1
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