Messages in this thread Patch in this message | | | Date | Thu, 23 Sep 2010 14:40:06 -0400 | From | Jason Baron <> | Subject | Re: [PATCH 03/11] jump label: Base patch for jump label |
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On Thu, Sep 23, 2010 at 11:48:52AM -0400, Mathieu Desnoyers wrote: > * Jason Baron (jbaron@redhat.com) wrote: > > On Thu, Sep 23, 2010 at 10:37:58AM -0400, Mathieu Desnoyers wrote: > > > * Steven Rostedt (rostedt@goodmis.org) wrote: > > > > From: Jason Baron <jbaron@redhat.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@redhat.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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