[PATCH] usb: gadget: f_midi2: fix use-after-free in string attribute show path

[Ivy Lopez <skunkolee@gmail.com>]

f_midi2_opts_str_show() takes the string lock internally, but its
callers dereference the opts->info.<field> pointer before calling it,
outside the lock. This races with f_midi2_opts_str_store(), which
frees the old string under opts->lock when the attribute is written
concurrently, the show path can read a pointer that gets freed
before the lock inside str_show() is even taken.

Change f_midi2_opts_str_show() to take a pointer to the string field,
matching the existing pattern in f_midi2_opts_str_store(), and
dereference it only after the lock is held. Update all three callers
(iface_name, block name, and the EP string option macro) accordingly.

Reported-by: syzbot+2280f1cca5e6b0c353e4@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=2280f1cca5e6b0c353e4
Signed-off-by: Ivy Lopez <skunkolee@gmail.com>
---
 drivers/usb/gadget/function/f_midi2.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/usb/gadget/function/f_midi2.c b/drivers/usb/gadget/function/f_midi2.c
index 19fdac024343..6bfd171b96a9 100644
--- a/drivers/usb/gadget/function/f_midi2.c
+++ b/drivers/usb/gadget/function/f_midi2.c
@@ -2178,13 +2178,13 @@ static ssize_t f_midi2_opts_bool_store(struct f_midi2_opts *opts,
 
 /* generic show/store for string */
 static ssize_t f_midi2_opts_str_show(struct f_midi2_opts *opts,
-				     const char *str, char *page)
+				     const char **strp, char *page)
 {
 	int result = 0;
 
 	mutex_lock(&opts->lock);
-	if (str)
-		result = scnprintf(page, PAGE_SIZE, "%s\n", str);
+	if (*strp)
+		result = scnprintf(page, PAGE_SIZE, "%s\n", *strp);
 	mutex_unlock(&opts->lock);
 	return result;
 }
@@ -2278,7 +2278,7 @@ static ssize_t f_midi2_block_opts_name_show(struct config_item *item,
 {
 	struct f_midi2_block_opts *opts = to_f_midi2_block_opts(item);
 
-	return f_midi2_opts_str_show(opts->ep->opts, opts->info.name, page);
+	return f_midi2_opts_str_show(opts->ep->opts, &opts->info.name, page);
 }
 
 static ssize_t f_midi2_block_opts_name_store(struct config_item *item,
@@ -2435,7 +2435,7 @@ static ssize_t f_midi2_ep_opts_##name##_show(struct config_item *item,	\
 					     char *page)		\
 {									\
 	struct f_midi2_ep_opts *opts = to_f_midi2_ep_opts(item);	\
-	return f_midi2_opts_str_show(opts->opts, opts->info.name, page);\
+	return f_midi2_opts_str_show(opts->opts, &opts->info.name, page);\
 }									\
 									\
 static ssize_t f_midi2_ep_opts_##name##_store(struct config_item *item,	\
@@ -2589,7 +2589,7 @@ static ssize_t f_midi2_opts_iface_name_show(struct config_item *item,
 {
 	struct f_midi2_opts *opts = to_f_midi2_opts(item);
 
-	return f_midi2_opts_str_show(opts, opts->info.iface_name, page);
+	return f_midi2_opts_str_show(opts, &opts->info.iface_name, page);
 }
 
 static ssize_t f_midi2_opts_iface_name_store(struct config_item *item,
-- 
2.55.0