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    SubjectRe: [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@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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