Re: [PATCH v4] usb: gadget: configfs: Fix KASAN use-after-free

[Macpaul Lin <macpaul.lin@mediatek.com>]

On Thu, 2021-03-11 at 14:42 +0800, Macpaul Lin wrote:
> From: Jim Lin <jilin@nvidia.com>
> 
> 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
> 
> 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>
> Signed-off-by: Macpaul Lin <macpaul.lin@mediatek.com>
> Cc: stable@vger.kernel.org
> ---
> Changes in v2:
> Changes in v3:
>  - Change commit description
> Changes in v4:
>  - Fix build error and adapt patch to kernel-5.12-rc1.
>    Replace definition "MAX_USB_STRING_WITH_NULL_LEN" with
>    "USB_MAX_STRING_WITH_NULL_LEN".
>  - Note: The patch v2 and v3 has been verified by
>    Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
>    http://spinics.net/lists/kernel/msg3840792.html

Dear Cascardo,

Would you please help to confirm if you've tested it on Linux PC,
Chrome OS, or an Android OS?

Thanks!
Macpaul Lin

>    and
>    Macpaul Lin <macpaul.lin@mediatek.com> on Android kernels.
>    http://lkml.org/lkml/2020/6/11/8
>  - The patch is suggested to be applied to LTS versions.
> 
>  drivers/usb/gadget/configfs.c |   14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/usb/gadget/configfs.c b/drivers/usb/gadget/configfs.c
> index 0d56f33..15a607c 100644
> --- a/drivers/usb/gadget/configfs.c
> +++ b/drivers/usb/gadget/configfs.c
> @@ -97,6 +97,8 @@ struct gadget_config_name {
>  	struct list_head list;
>  };
>  
> +#define USB_MAX_STRING_WITH_NULL_LEN	(USB_MAX_STRING_LEN+1)
> +
>  static int usb_string_copy(const char *s, char **s_copy)
>  {
>  	int ret;
> @@ -106,12 +108,16 @@ static int usb_string_copy(const char *s, char **s_copy)
>  	if (ret > USB_MAX_STRING_LEN)
>  		return -EOVERFLOW;
>  
> -	str = kstrdup(s, GFP_KERNEL);
> -	if (!str)
> -		return -ENOMEM;
> +	if (copy) {
> +		str = copy;
> +	} else {
> +		str = kmalloc(USB_MAX_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;
>  }

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