[PATCH] null_blk: refactor deprecated strncpy

[PATCH] null_blk: refactor deprecated strncpy

[Justin Stitt]

`strncpy` is deprecated for use on NUL-terminated destination strings [1].

We should favor a more robust and less ambiguous interface.

We know `nullb->disk_name` is NUL-terminated and we expect that
`disk->disk_name` also be NUL-terminated:
|     snprintf(nullb->disk_name, sizeof(nullb->disk_name),
|              "%s", config_item_name(&dev->group.cg_item));

It seems like NUL-padding may be required due to __assign_disk_name()
utilizing a memcpy as opposed to a `str*cpy` api.
| static inline void __assign_disk_name(char *name, struct gendisk *disk)
| {
| 	if (disk)
| 		memcpy(name, disk->disk_name, DISK_NAME_LEN);
| 	else
| 		memset(name, 0, DISK_NAME_LEN);
| }

This puzzles me as immediately after assigning the disk name we
go and print it with `__print_disk_name` which wraps `nullb_trace_disk_name()`.
| #define __print_disk_name(name) nullb_trace_disk_name(p, name)

This function obviously expects a NUL-terminated string.
| const char *nullb_trace_disk_name(struct trace_seq *p, char *name)
| {
| 	const char *ret = trace_seq_buffer_ptr(p);
|
| 	if (name && *name)
| 		trace_seq_printf(p, "disk=%s, ", name);
| 	trace_seq_putc(p, 0);
|
| 	return ret;
| }

I suspect that NUL-padding is _not_ needed but to be overly cautious
let's use `strscpy_pad` (although I strongly suspect that whole memcpy
thing should be changed).

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@vger.kernel.org
Cc: Kees Cook <keescook@chromium.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Justin Stitt <justinstitt@google.com>
---
Note: build-tested
---
 drivers/block/null_blk/main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c
index 864013019d6b..994919c63f05 100644
--- a/drivers/block/null_blk/main.c
+++ b/drivers/block/null_blk/main.c
@@ -1938,7 +1938,7 @@ static int null_gendisk_register(struct nullb *nullb)
 	else
 		disk->fops		= &null_bio_ops;
 	disk->private_data	= nullb;
-	strncpy(disk->disk_name, nullb->disk_name, DISK_NAME_LEN);
+	strscpy(disk->disk_name, nullb->disk_name, DISK_NAME_LEN);
 
 	if (nullb->dev->zoned) {
 		int ret = null_register_zoned_dev(nullb);

---
base-commit: 2dde18cd1d8fac735875f2e4987f11817cc0bc2c
change-id: 20230911-strncpy-drivers-block-null_blk-main-c-7349153e1c6a

Best regards,
--
Justin Stitt <justinstitt@google.com>

Re: [PATCH] null_blk: refactor deprecated strncpy

[Kees Cook]

On Mon, Sep 11, 2023 at 09:46:47PM +0000, Justin Stitt wrote:
> `strncpy` is deprecated for use on NUL-terminated destination strings [1].
> 
> We should favor a more robust and less ambiguous interface.
> 
> We know `nullb->disk_name` is NUL-terminated and we expect that
> `disk->disk_name` also be NUL-terminated:
> |     snprintf(nullb->disk_name, sizeof(nullb->disk_name),
> |              "%s", config_item_name(&dev->group.cg_item));

More accurately, it's has uses that require a %NUL-terminated string:

	pr_info("disk %s created\n", nullb->disk_name);

(i.e. showing the consumer's use is better evidence than the producer's
use.)

But I do think the above is good evidence that truncation is tolerated.

> 
> It seems like NUL-padding may be required due to __assign_disk_name()
> utilizing a memcpy as opposed to a `str*cpy` api.
> | static inline void __assign_disk_name(char *name, struct gendisk *disk)
> | {
> | 	if (disk)
> | 		memcpy(name, disk->disk_name, DISK_NAME_LEN);
> | 	else
> | 		memset(name, 0, DISK_NAME_LEN);
> | }

I does look like it expects 0-fill. Looking at it with more context,
this appears to be a trace buffer:

TRACE_EVENT(nullb_zone_op,
            TP_PROTO(struct nullb_cmd *cmd, unsigned int zone_no,
                     unsigned int zone_cond),
            TP_ARGS(cmd, zone_no, zone_cond),
            TP_STRUCT__entry(
                __array(char, disk, DISK_NAME_LEN)
                __field(enum req_op, op)
                __field(unsigned int, zone_no)
                __field(unsigned int, zone_cond)
            ),
            TP_fast_assign(
                __entry->op = req_op(cmd->rq);
                __entry->zone_no = zone_no;
                __entry->zone_cond = zone_cond;
                __assign_disk_name(__entry->disk, cmd->rq->q->disk);
            ),

This should probably have been a dynamic string, but it's not. So let's
make sure this stays padded. Can you send this again but use
strscpy_pad() instead?

-- 
Kees Cook