Re: [Linux-kernel-mentees] [PATCH] block/floppy: Prevent kernel-infoleak in raw_cmd_copyout()

[Denis Efremov <efremov@linux.com>]

Hi,

On 7/28/20 5:19 PM, Peilin Ye wrote:
> raw_cmd_copyout() is potentially copying uninitialized kernel stack memory
> since it is initializing `cmd` by assignment, which may cause the compiler
> to leave uninitialized holes in this structure. Fix it by using memcpy()
> instead.
> 
> Cc: stable@vger.kernel.org
> Fixes: 2145e15e0557 ("floppy: don't write kernel-only members to FDRAWCMD ioctl output")
> Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
> Suggested-by: Arnd Bergmann <arnd@arndb.de>
> Signed-off-by: Peilin Ye <yepeilin.cs@gmail.com>

Reviewed-by: Denis Efremov <efremov@linux.com>

ptr comes from raw_cmd_copyin and it should be ok to use memcpy.

Jens, could you please take this one to your 5.9 branch?


> ---
> $ pahole -C "floppy_raw_cmd" drivers/block/floppy.o
> struct floppy_raw_cmd {
> 	unsigned int               flags;                /*     0     4 */
> 
> 	/* XXX 4 bytes hole, try to pack */
> 
> 	void *                     data;                 /*     8     8 */
> 	char *                     kernel_data;          /*    16     8 */
> 	struct floppy_raw_cmd *    next;                 /*    24     8 */
> 	long int                   length;               /*    32     8 */
> 	long int                   phys_length;          /*    40     8 */
> 	int                        buffer_length;        /*    48     4 */
> 	unsigned char              rate;                 /*    52     1 */
> 	unsigned char              cmd_count;            /*    53     1 */
> 	union {
> 		struct {
> 			unsigned char cmd[16];           /*    54    16 */
> 			/* --- cacheline 1 boundary (64 bytes) was 6 bytes ago --- */
> 			unsigned char reply_count;       /*    70     1 */
> 			unsigned char reply[16];         /*    71    16 */
> 		};                                       /*    54    33 */
> 		unsigned char      fullcmd[33];          /*    54    33 */
> 	};                                               /*    54    33 */
> 
> 	/* XXX 1 byte hole, try to pack */
> 
> 	/* --- cacheline 1 boundary (64 bytes) was 24 bytes ago --- */
> 	int                        track;                /*    88     4 */
> 	int                        resultcode;           /*    92     4 */
> 	int                        reserved1;            /*    96     4 */
> 	int                        reserved2;            /*   100     4 */
> 
> 	/* size: 104, cachelines: 2, members: 14 */
> 	/* sum members: 99, holes: 2, sum holes: 5 */
> 	/* last cacheline: 40 bytes */
> };
> 

It would be nice to add lkml links with discussion on the issue or
https://www.nccgroup.com/us/about-us/newsroom-and-events/blog/2019/october/padding-the-struct-how-a-compiler-optimization-can-disclose-stack-memory/
in addition to pahole output.

>  drivers/block/floppy.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/block/floppy.c b/drivers/block/floppy.c
> index 09079aee8dc4..b8ea98f7a9cb 100644
> --- a/drivers/block/floppy.c
> +++ b/drivers/block/floppy.c
> @@ -3126,7 +3126,9 @@ static int raw_cmd_copyout(int cmd, void __user *param,
>  	int ret;
>  
>  	while (ptr) {
> -		struct floppy_raw_cmd cmd = *ptr;
> +		struct floppy_raw_cmd cmd;
> +
> +		memcpy(&cmd, ptr, sizeof(cmd))>  		cmd.next = NULL;
>  		cmd.kernel_data = NULL;
>  		ret = copy_to_user(param, &cmd, sizeof(cmd));
> 

Thanks,
Denis