[PATCH] usb: gadget: f_mass_storage: fix null pointer dereference in fsg_common_set_num_buffers()

[Jeffin Philip <jeffinphilip14@gmail.com>]

In fsg_common_set_num_buffers(), n can be 0 as conversion to u8
using kstrtou8() in fsg_opts_num_buffers_store() can return
values from _0_ to 255. When passing 0 as "n" value to kzalloc_objs,
it can return a ZERO_SIZE_PTR, which passes the null check for
buffhds. This leads to a null pointer dereference later in bh->next
in the do while loop. Fix this by adding a check for n = 0 case and
returning -EINVAL if n is 0.

Reported-by: syzbot+791be35f1fbcc85d06d7@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=791be35f1fbcc85d06d7
Fixes: fe5a6c48fd95 ("usb: gadget: storage: get rid of fsg_num_buffers_validate()")
Cc: stable@vger.kernel.org
Signed-off-by: Jeffin Philip <jeffinphilip14@gmail.com>
---
 drivers/usb/gadget/function/f_mass_storage.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/usb/gadget/function/f_mass_storage.c b/drivers/usb/gadget/function/f_mass_storage.c
index a50743caf083..640d3bcb7bf0 100644
--- a/drivers/usb/gadget/function/f_mass_storage.c
+++ b/drivers/usb/gadget/function/f_mass_storage.c
@@ -2747,6 +2747,9 @@ int fsg_common_set_num_buffers(struct fsg_common *common, unsigned int n)
 	struct fsg_buffhd *bh, *buffhds;
 	int i;
 
+	if (!n)
+		return -EINVAL;
+
 	buffhds = kzalloc_objs(*buffhds, n);
 	if (!buffhds)
 		return -ENOMEM;
-- 
2.55.0