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Message-Id: <20231230174131.fe6dc271f126d0e17662514c@kernel.org>
Date: Sat, 30 Dec 2023 17:41:31 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Linux Trace Kernel
<linux-trace-kernel@...r.kernel.org>, Masami Hiramatsu
<mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>, Mathieu
Desnoyers <mathieu.desnoyers@...icios.com>, Jiri Olsa <jolsa@...nel.org>,
Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann
<daniel@...earbox.net>, bpf@...r.kernel.org
Subject: Re: [PATCH] ftrace: Fix modification of direct_function hash while
in use
On Fri, 29 Dec 2023 11:51:34 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:
> From: "Steven Rostedt (Google)" <rostedt@...dmis.org>
>
> Masami Hiramatsu reported a memory leak in register_ftrace_direct() where
> if the number of new entries are added is large enough to cause two
> allocations in the loop:
>
> for (i = 0; i < size; i++) {
> hlist_for_each_entry(entry, &hash->buckets[i], hlist) {
> new = ftrace_add_rec_direct(entry->ip, addr, &free_hash);
> if (!new)
> goto out_remove;
> entry->direct = addr;
> }
> }
>
> Where ftrace_add_rec_direct() has:
>
> if (ftrace_hash_empty(direct_functions) ||
> direct_functions->count > 2 * (1 << direct_functions->size_bits)) {
> struct ftrace_hash *new_hash;
> int size = ftrace_hash_empty(direct_functions) ? 0 :
> direct_functions->count + 1;
>
> if (size < 32)
> size = 32;
>
> new_hash = dup_hash(direct_functions, size);
> if (!new_hash)
> return NULL;
>
> *free_hash = direct_functions;
> direct_functions = new_hash;
> }
>
> The "*free_hash = direct_functions;" can happen twice, losing the previous
> allocation of direct_functions.
>
> But this also exposed a more serious bug.
>
> The modification of direct_functions above is not safe. As
> direct_functions can be referenced at any time to find what direct caller
> it should call, the time between:
>
> new_hash = dup_hash(direct_functions, size);
> and
> direct_functions = new_hash;
>
> can have a race with another CPU (or even this one if it gets interrupted),
> and the entries being moved to the new hash are not referenced.
>
> That's because the "dup_hash()" is really misnamed and is really a
> "move_hash()". It moves the entries from the old hash to the new one.
>
> Now even if that was changed, this code is not proper as direct_functions
> should not be updated until the end. That is the best way to handle
> function reference changes, and is the way other parts of ftrace handles
> this.
Oops, I also misunderstood.
>
> The following is done:
>
> 1. Change add_hash_entry() to return the entry it created and inserted
> into the hash, and not just return success or not.
>
> 2. Replace ftrace_add_rec_direct() with add_hash_entry(), and remove
> the former.
>
> 3. Allocate a "new_hash" at the start that is made for holding both the
> new hash entries as well as the existing entries in direct_functions.
>
> 4. Copy (not move) the direct_function entries over to the new_hash.
>
> 5. Copy the entries of the added hash to the new_hash.
>
> 6. If everything succeeds, then use rcu_pointer_assign() to update the
> direct_functions with the new_hash.
>
> This simplifies the code and fixes both the memory leak as well as the
> race condition mentioned above.
This looks good to me.
Acked-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
Thank you,
>
> Link: https://lore.kernel.org/all/170368070504.42064.8960569647118388081.stgit@devnote2/
>
> Cc: stable@...r.kernel.org
> Fixes: 763e34e74bb7d ("ftrace: Add register_ftrace_direct()")
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
> kernel/trace/ftrace.c | 100 ++++++++++++++++++++----------------------
> 1 file changed, 47 insertions(+), 53 deletions(-)
>
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 8de8bec5f366..b01ae7d36021 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -1183,18 +1183,19 @@ static void __add_hash_entry(struct ftrace_hash *hash,
> hash->count++;
> }
>
> -static int add_hash_entry(struct ftrace_hash *hash, unsigned long ip)
> +static struct ftrace_func_entry *
> +add_hash_entry(struct ftrace_hash *hash, unsigned long ip)
> {
> struct ftrace_func_entry *entry;
>
> entry = kmalloc(sizeof(*entry), GFP_KERNEL);
> if (!entry)
> - return -ENOMEM;
> + return NULL;
>
> entry->ip = ip;
> __add_hash_entry(hash, entry);
>
> - return 0;
> + return entry;
> }
>
> static void
> @@ -1349,7 +1350,6 @@ alloc_and_copy_ftrace_hash(int size_bits, struct ftrace_hash *hash)
> struct ftrace_func_entry *entry;
> struct ftrace_hash *new_hash;
> int size;
> - int ret;
> int i;
>
> new_hash = alloc_ftrace_hash(size_bits);
> @@ -1366,8 +1366,7 @@ alloc_and_copy_ftrace_hash(int size_bits, struct ftrace_hash *hash)
> size = 1 << hash->size_bits;
> for (i = 0; i < size; i++) {
> hlist_for_each_entry(entry, &hash->buckets[i], hlist) {
> - ret = add_hash_entry(new_hash, entry->ip);
> - if (ret < 0)
> + if (add_hash_entry(new_hash, entry->ip) == NULL)
> goto free_hash;
> }
> }
> @@ -2536,7 +2535,7 @@ ftrace_find_unique_ops(struct dyn_ftrace *rec)
>
> #ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
> /* Protected by rcu_tasks for reading, and direct_mutex for writing */
> -static struct ftrace_hash *direct_functions = EMPTY_HASH;
> +static struct ftrace_hash __rcu *direct_functions = EMPTY_HASH;
> static DEFINE_MUTEX(direct_mutex);
> int ftrace_direct_func_count;
>
> @@ -2555,39 +2554,6 @@ unsigned long ftrace_find_rec_direct(unsigned long ip)
> return entry->direct;
> }
>
> -static struct ftrace_func_entry*
> -ftrace_add_rec_direct(unsigned long ip, unsigned long addr,
> - struct ftrace_hash **free_hash)
> -{
> - struct ftrace_func_entry *entry;
> -
> - if (ftrace_hash_empty(direct_functions) ||
> - direct_functions->count > 2 * (1 << direct_functions->size_bits)) {
> - struct ftrace_hash *new_hash;
> - int size = ftrace_hash_empty(direct_functions) ? 0 :
> - direct_functions->count + 1;
> -
> - if (size < 32)
> - size = 32;
> -
> - new_hash = dup_hash(direct_functions, size);
> - if (!new_hash)
> - return NULL;
> -
> - *free_hash = direct_functions;
> - direct_functions = new_hash;
> - }
> -
> - entry = kmalloc(sizeof(*entry), GFP_KERNEL);
> - if (!entry)
> - return NULL;
> -
> - entry->ip = ip;
> - entry->direct = addr;
> - __add_hash_entry(direct_functions, entry);
> - return entry;
> -}
> -
> static void call_direct_funcs(unsigned long ip, unsigned long pip,
> struct ftrace_ops *ops, struct ftrace_regs *fregs)
> {
> @@ -4223,8 +4189,8 @@ enter_record(struct ftrace_hash *hash, struct dyn_ftrace *rec, int clear_filter)
> /* Do nothing if it exists */
> if (entry)
> return 0;
> -
> - ret = add_hash_entry(hash, rec->ip);
> + if (add_hash_entry(hash, rec->ip) == NULL)
> + ret = -ENOMEM;
> }
> return ret;
> }
> @@ -5266,7 +5232,8 @@ __ftrace_match_addr(struct ftrace_hash *hash, unsigned long ip, int remove)
> return 0;
> }
>
> - return add_hash_entry(hash, ip);
> + entry = add_hash_entry(hash, ip);
> + return entry ? 0 : -ENOMEM;
> }
>
> static int
> @@ -5410,7 +5377,7 @@ static void remove_direct_functions_hash(struct ftrace_hash *hash, unsigned long
> */
> int register_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
> {
> - struct ftrace_hash *hash, *free_hash = NULL;
> + struct ftrace_hash *hash, *new_hash = NULL, *free_hash = NULL;
> struct ftrace_func_entry *entry, *new;
> int err = -EBUSY, size, i;
>
> @@ -5436,17 +5403,44 @@ int register_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
> }
> }
>
> - /* ... and insert them to direct_functions hash. */
> err = -ENOMEM;
> +
> + /* Make a copy hash to place the new and the old entries in */
> + size = hash->count + direct_functions->count;
> + if (size > 32)
> + size = 32;
> + new_hash = alloc_ftrace_hash(fls(size));
> + if (!new_hash)
> + goto out_unlock;
> +
> + /* Now copy over the existing direct entries */
> + size = 1 << direct_functions->size_bits;
> + for (i = 0; i < size; i++) {
> + hlist_for_each_entry(entry, &direct_functions->buckets[i], hlist) {
> + new = add_hash_entry(new_hash, entry->ip);
> + if (!new)
> + goto out_unlock;
> + new->direct = entry->direct;
> + }
> + }
> +
> + /* ... and add the new entries */
> + size = 1 << hash->size_bits;
> for (i = 0; i < size; i++) {
> hlist_for_each_entry(entry, &hash->buckets[i], hlist) {
> - new = ftrace_add_rec_direct(entry->ip, addr, &free_hash);
> + new = add_hash_entry(new_hash, entry->ip);
> if (!new)
> - goto out_remove;
> + goto out_unlock;
> + /* Update both the copy and the hash entry */
> + new->direct = addr;
> entry->direct = addr;
> }
> }
>
> + free_hash = direct_functions;
> + rcu_assign_pointer(direct_functions, new_hash);
> + new_hash = NULL;
> +
> ops->func = call_direct_funcs;
> ops->flags = MULTI_FLAGS;
> ops->trampoline = FTRACE_REGS_ADDR;
> @@ -5454,17 +5448,17 @@ int register_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
>
> err = register_ftrace_function_nolock(ops);
>
> - out_remove:
> - if (err)
> - remove_direct_functions_hash(hash, addr);
> -
> out_unlock:
> mutex_unlock(&direct_mutex);
>
> - if (free_hash) {
> + if (free_hash && free_hash != EMPTY_HASH) {
> synchronize_rcu_tasks();
> free_ftrace_hash(free_hash);
> }
> +
> + if (new_hash)
> + free_ftrace_hash(new_hash);
> +
> return err;
> }
> EXPORT_SYMBOL_GPL(register_ftrace_direct);
> @@ -6309,7 +6303,7 @@ ftrace_graph_set_hash(struct ftrace_hash *hash, char *buffer)
>
> if (entry)
> continue;
> - if (add_hash_entry(hash, rec->ip) < 0)
> + if (add_hash_entry(hash, rec->ip) == NULL)
> goto out;
> } else {
> if (entry) {
> --
> 2.42.0
>
--
Masami Hiramatsu (Google) <mhiramat@...nel.org>
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