On Fri, 27 Oct 2017 17:34:44 +0800
Zhou Chengming <zhouchengmi...@huawei.com> wrote:
> Fixes: 2cfa197 "ftrace/alternatives: Introducing *_text_reserved
> functions"
>
> We use alternatives_text_reserved() to check if the address is in
> the fixed pieces of alternative reserved, but the problem is that
> we don't hold the smp_alt mutex when call this function. So the list
> traversal may encounter a deleted list_head if another path is doing
> alternatives_smp_module_del().
>
> One solution is that we can hold smp_alt mutex before call this
> function, but the difficult point is that the callers of this
> functions, arch_prepare_kprobe() and arch_prepare_optimized_kprobe(),
> are called inside the text_mutex. So we must hold smp_alt mutex
> before we go into these arch dependent code. But we can't now,
> the smp_alt mutex is the arch dependent part, only x86 has it.
> Maybe we can export another arch dependent callback to solve this.
>
> But there is a simpler way to handle this problem. We can reuse the
> text_mutex to protect smp_alt_modules instead of using another mutex.
> And all the arch dependent checks of kprobes are inside the text_mutex,
> so it's safe now.
OK, I considered other ways but those may introduce unneeded
complexity. So this simple solution is better to fix this bug.
Reviewed-by: Masami Hiramatsu <mhira...@kernel.org>
Thank you,
>
> Signed-off-by: Zhou Chengming <zhouchengmi...@huawei.com>
> ---
> arch/x86/kernel/alternative.c | 24 +++++++++++-------------
> 1 file changed, 11 insertions(+), 13 deletions(-)
>
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 3344d33..55abbaa 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -442,7 +442,6 @@ static void alternatives_smp_lock(const s32 *start, const
> s32 *end,
> {
> const s32 *poff;
>
> - mutex_lock(&text_mutex);
> for (poff = start; poff < end; poff++) {
> u8 *ptr = (u8 *)poff + *poff;
>
> @@ -452,7 +451,6 @@ static void alternatives_smp_lock(const s32 *start, const
> s32 *end,
> if (*ptr == 0x3e)
> text_poke(ptr, ((unsigned char []){0xf0}), 1);
> }
> - mutex_unlock(&text_mutex);
> }
>
> static void alternatives_smp_unlock(const s32 *start, const s32 *end,
> @@ -460,7 +458,6 @@ static void alternatives_smp_unlock(const s32 *start,
> const s32 *end,
> {
> const s32 *poff;
>
> - mutex_lock(&text_mutex);
> for (poff = start; poff < end; poff++) {
> u8 *ptr = (u8 *)poff + *poff;
>
> @@ -470,7 +467,6 @@ static void alternatives_smp_unlock(const s32 *start,
> const s32 *end,
> if (*ptr == 0xf0)
> text_poke(ptr, ((unsigned char []){0x3E}), 1);
> }
> - mutex_unlock(&text_mutex);
> }
>
> struct smp_alt_module {
> @@ -489,8 +485,7 @@ struct smp_alt_module {
> struct list_head next;
> };
> static LIST_HEAD(smp_alt_modules);
> -static DEFINE_MUTEX(smp_alt);
> -static bool uniproc_patched = false; /* protected by smp_alt */
> +static bool uniproc_patched = false; /* protected by text_mutex */
>
> void __init_or_module alternatives_smp_module_add(struct module *mod,
> char *name,
> @@ -499,7 +494,7 @@ void __init_or_module alternatives_smp_module_add(struct
> module *mod,
> {
> struct smp_alt_module *smp;
>
> - mutex_lock(&smp_alt);
> + mutex_lock(&text_mutex);
> if (!uniproc_patched)
> goto unlock;
>
> @@ -526,14 +521,14 @@ void __init_or_module
> alternatives_smp_module_add(struct module *mod,
> smp_unlock:
> alternatives_smp_unlock(locks, locks_end, text, text_end);
> unlock:
> - mutex_unlock(&smp_alt);
> + mutex_unlock(&text_mutex);
> }
>
> void __init_or_module alternatives_smp_module_del(struct module *mod)
> {
> struct smp_alt_module *item;
>
> - mutex_lock(&smp_alt);
> + mutex_lock(&text_mutex);
> list_for_each_entry(item, &smp_alt_modules, next) {
> if (mod != item->mod)
> continue;
> @@ -541,7 +536,7 @@ void __init_or_module alternatives_smp_module_del(struct
> module *mod)
> kfree(item);
> break;
> }
> - mutex_unlock(&smp_alt);
> + mutex_unlock(&text_mutex);
> }
>
> void alternatives_enable_smp(void)
> @@ -551,7 +546,7 @@ void alternatives_enable_smp(void)
> /* Why bother if there are no other CPUs? */
> BUG_ON(num_possible_cpus() == 1);
>
> - mutex_lock(&smp_alt);
> + mutex_lock(&text_mutex);
>
> if (uniproc_patched) {
> pr_info("switching to SMP code\n");
> @@ -563,10 +558,13 @@ void alternatives_enable_smp(void)
> mod->text, mod->text_end);
> uniproc_patched = false;
> }
> - mutex_unlock(&smp_alt);
> + mutex_unlock(&text_mutex);
> }
>
> -/* Return 1 if the address range is reserved for smp-alternatives */
> +/*
> + * Return 1 if the address range is reserved for smp-alternatives.
> + * Must hold text_mutex.
> + */
> int alternatives_text_reserved(void *start, void *end)
> {
> struct smp_alt_module *mod;
> --
> 1.8.3.1
>
--
Masami Hiramatsu <mhira...@kernel.org>
