md: replace deprecated strncpy with memcpy

Commit Message

Justin Stitt Sept. 25, 2023, 9:49 a.m. UTC

`strncpy` is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces.

There are three such strncpy uses that this patch addresses:

The respective destination buffers are:
1) mddev->clevel
2) clevel
3) mddev->metadata_type

We expect mddev->clevel to be NUL-terminated due to its use with format
strings:
|       ret = sprintf(page, "%s\n", mddev->clevel);

Furthermore, we can see that mddev->clevel is not expected to be
NUL-padded as `md_clean()` merely set's its first byte to NULL -- not
the entire buffer:
|       static void md_clean(struct mddev *mddev)
|       {
|       	mddev->array_sectors = 0;
|       	mddev->external_size = 0;
|               ...
|       	mddev->level = LEVEL_NONE;
|       	mddev->clevel[0] = 0;
|               ...

A suitable replacement for this instance is `memcpy` as we know the
number of bytes to copy and perform manual NUL-termination at a
specified offset. This really decays to just a byte copy from one buffer
to another. `strscpy` is also a considerable replacement but using
`slen` as the length argument would result in truncation of the last
byte unless something like `slen + 1` was provided which isn't the most
idiomatic strscpy usage.

For the next case, the destination buffer `clevel` is expected to be
NUL-terminated based on its usage within kstrtol() which expects
NUL-terminated strings. Note that, in context, this code removes a
trailing newline which is seemingly not required as kstrtol() can handle
trailing newlines implicitly. However, there exists further usage of
clevel (or buf) that would also like to have the newline removed. All in
all, with similar reasoning to the first case, let's just use memcpy as
this is just a byte copy and NUL-termination is handled manually.

The third and final case concerning `mddev->metadata_type` is more or
less the same as the other two. We expect that it be NUL-terminated
based on its usage with seq_printf:
|       seq_printf(seq, " super external:%s",
|       	   mddev->metadata_type);
... and we can surmise that NUL-padding isn't required either due to how
it is handled in md_clean():
|       static void md_clean(struct mddev *mddev)
|       {
|       ...
|       mddev->metadata_type[0] = 0;
|       ...

So really, all these instances have precisely calculated lengths and
purposeful NUL-termination so we can just use memcpy to remove ambiguity
surrounding strncpy.

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: [e-mail]
Signed-off-by: Justin Stitt <[e-mail]>
---
Note: build-tested only.
---
 drivers/md/md.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)


---
base-commit: 6465e260f48790807eef06b583b38ca9789b6072
change-id: 20230925-strncpy-drivers-md-md-c-e775504361ab

Best regards,
--
Justin Stitt <[e-mail]>

Comments

Kees Cook Sept. 25, 2023, 6:22 p.m. UTC | #1

On Mon, Sep 25, 2023 at 09:49:17AM +0000, Justin Stitt wrote:
> `strncpy` is deprecated for use on NUL-terminated destination strings
> [1] and as such we should prefer more robust and less ambiguous string
> interfaces.
> 
> There are three such strncpy uses that this patch addresses:
> 
> The respective destination buffers are:
> 1) mddev->clevel
> 2) clevel
> 3) mddev->metadata_type
> 
> We expect mddev->clevel to be NUL-terminated due to its use with format
> strings:
> |       ret = sprintf(page, "%s\n", mddev->clevel);
> 
> Furthermore, we can see that mddev->clevel is not expected to be
> NUL-padded as `md_clean()` merely set's its first byte to NULL -- not
> the entire buffer:
> |       static void md_clean(struct mddev *mddev)
> |       {
> |       	mddev->array_sectors = 0;
> |       	mddev->external_size = 0;
> |               ...
> |       	mddev->level = LEVEL_NONE;
> |       	mddev->clevel[0] = 0;
> |               ...
> 
> A suitable replacement for this instance is `memcpy` as we know the
> number of bytes to copy and perform manual NUL-termination at a
> specified offset. This really decays to just a byte copy from one buffer
> to another. `strscpy` is also a considerable replacement but using
> `slen` as the length argument would result in truncation of the last
> byte unless something like `slen + 1` was provided which isn't the most
> idiomatic strscpy usage.
> 
> For the next case, the destination buffer `clevel` is expected to be
> NUL-terminated based on its usage within kstrtol() which expects
> NUL-terminated strings. Note that, in context, this code removes a
> trailing newline which is seemingly not required as kstrtol() can handle
> trailing newlines implicitly. However, there exists further usage of
> clevel (or buf) that would also like to have the newline removed. All in
> all, with similar reasoning to the first case, let's just use memcpy as
> this is just a byte copy and NUL-termination is handled manually.
> 
> The third and final case concerning `mddev->metadata_type` is more or
> less the same as the other two. We expect that it be NUL-terminated
> based on its usage with seq_printf:
> |       seq_printf(seq, " super external:%s",
> |       	   mddev->metadata_type);
> ... and we can surmise that NUL-padding isn't required either due to how
> it is handled in md_clean():
> |       static void md_clean(struct mddev *mddev)
> |       {
> |       ...
> |       mddev->metadata_type[0] = 0;
> |       ...
> 
> So really, all these instances have precisely calculated lengths and
> purposeful NUL-termination so we can just use memcpy to remove ambiguity
> surrounding strncpy.
> 
> 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: [e-mail]
> Signed-off-by: Justin Stitt <[e-mail]>

I agree on the analysis of the replacements. Thanks for all the detail!

Reviewed-by: Kees Cook <[e-mail]>

Song Liu Sept. 26, 2023, 11:17 p.m. UTC | #2

On Mon, Sep 25, 2023 at 11:22 AM Kees Cook <[e-mail]> wrote:
>
[...]
> >
> > So really, all these instances have precisely calculated lengths and
> > purposeful NUL-termination so we can just use memcpy to remove ambiguity
> > surrounding strncpy.
> >
> > 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: [e-mail]
> > Signed-off-by: Justin Stitt <[e-mail]>
>
> I agree on the analysis of the replacements. Thanks for all the detail!
>
> Reviewed-by: Kees Cook <[e-mail]>

Applied to md-next.  Thanks!

Song

Patch

diff --git a/drivers/md/md.c b/drivers/md/md.c
index a104a025084d..73846c275880 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -3879,7 +3879,7 @@  level_store(struct mddev *mddev, const char *buf, size_t len)
 		return rv;
 
 	if (mddev->pers == NULL) {
-		strncpy(mddev->clevel, buf, slen);
+		memcpy(mddev->clevel, buf, slen);
 		if (mddev->clevel[slen-1] == '\n')
 			slen--;
 		mddev->clevel[slen] = 0;
@@ -3912,7 +3912,7 @@  level_store(struct mddev *mddev, const char *buf, size_t len)
 	}
 
 	/* Now find the new personality */
-	strncpy(clevel, buf, slen);
+	memcpy(clevel, buf, slen);
 	if (clevel[slen-1] == '\n')
 		slen--;
 	clevel[slen] = 0;
@@ -4698,7 +4698,7 @@  metadata_store(struct mddev *mddev, const char *buf, size_t len)
 		size_t namelen = len-9;
 		if (namelen >= sizeof(mddev->metadata_type))
 			namelen = sizeof(mddev->metadata_type)-1;
-		strncpy(mddev->metadata_type, buf+9, namelen);
+		memcpy(mddev->metadata_type, buf+9, namelen);
 		mddev->metadata_type[namelen] = 0;
 		if (namelen && mddev->metadata_type[namelen-1] == '\n')
 			mddev->metadata_type[--namelen] = 0;