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SubjectRe: [PATCH v2] usb: gadget: configfs: Fix KASAN use-after-free
On Tue, Jan 17, 2017 at 1:29 AM, Jim Lin <jilin@nvidia.com> wrote:
> When gadget is disconnected, running sequence is like this.
> . composite_disconnect
> . Call trace:
> usb_string_copy+0xd0/0x128
> gadget_config_name_configuration_store+0x4
> gadget_config_name_attr_store+0x40/0x50
> configfs_write_file+0x198/0x1f4
> vfs_write+0x100/0x220
> SyS_write+0x58/0xa8
> . configfs_composite_unbind
> . configfs_composite_bind
>
> In configfs_composite_bind, it has
> "cn->strings.s = cn->configuration;"
>
> When usb_string_copy is invoked. it would
> allocate memory, copy input string, release previous pointed memory space,
> and use new allocated memory.
>
> When gadget is connected, host sends down request to get information.
> Call trace:
> usb_gadget_get_string+0xec/0x168
> lookup_string+0x64/0x98
> composite_setup+0xa34/0x1ee8
> android_setup+0xb4/0x140
>
> If gadget is disconnected and connected quickly, in the failed case,
> cn->configuration memory has been released by usb_string_copy kfree but
> configfs_composite_bind hasn't been run in time to assign new allocated
> "cn->configuration" pointer to "cn->strings.s".
>
> When "strlen(s->s) of usb_gadget_get_string is being executed, the dangling
> memory is accessed, "BUG: KASAN: use-after-free" error occurs.
>
> Signed-off-by: Jim Lin <jilin@nvidia.com>

Hi! What's the current state of this patch?

> ---
> Changes in v2:
> Rephrase commit description
>
> drivers/usb/gadget/configfs.c | 15 +++++++++++----
> 1 file changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/usb/gadget/configfs.c b/drivers/usb/gadget/configfs.c
> index 78c4497..39fea62 100644
> --- a/drivers/usb/gadget/configfs.c
> +++ b/drivers/usb/gadget/configfs.c
> @@ -106,6 +106,9 @@ struct gadget_config_name {
> struct list_head list;
> };
>
> +#define MAX_USB_STRING_LEN 126
> +#define MAX_USB_STRING_WITH_NULL_LEN (MAX_USB_STRING_LEN+1)
> +
> static int usb_string_copy(const char *s, char **s_copy)
> {
> int ret;
> @@ -115,12 +118,16 @@ static int usb_string_copy(const char *s, char **s_copy)
> if (ret > 126)

This should be MAX_USB_STRING_LEN, yes?

> return -EOVERFLOW;
>
> - str = kstrdup(s, GFP_KERNEL);
> - if (!str)
> - return -ENOMEM;
> + if (copy) {
> + str = copy;
> + } else {
> + str = kmalloc(MAX_USB_STRING_WITH_NULL_LEN, GFP_KERNEL);
> + if (!str)
> + return -ENOMEM;
> + }
> + strcpy(str, s);
> if (str[ret - 1] == '\n')
> str[ret - 1] = '\0';
> - kfree(copy);
> *s_copy = str;
> return 0;
> }
> --
> 2.7.4
>

-Kees


--
Kees Cook
Pixel Security

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