[PATCH] vmcore: replace strncpy with strtomem

[Justin Stitt <justinstitt@google.com>]

strncpy() is in the process of being replaced as it is deprecated in
some situations [1]. While the specific use of strncpy that this patch
targets is not exactly deprecated, the real mission is to rid the kernel
of all its uses.

Looking at vmcoredd_header's definition:
|	struct vmcoredd_header {
|		__u32 n_namesz; /* Name size */
|		__u32 n_descsz; /* Content size */
|		__u32 n_type;   /* NT_VMCOREDD */
|		__u8 name[8];   /* LINUX\0\0\0 */
|		__u8 dump_name[VMCOREDD_MAX_NAME_BYTES]; /* Device dump's name */
|	};
... we can see that both `name` and `dump_name` are u8s. It seems `name`
wants to be NUL-padded (based on the comment above), but for the sake of
symmetry lets NUL-pad both of these.

Mark these buffers as __nonstring and use strtomem_pad.

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
Signed-off-by: Justin Stitt <justinstitt@google.com>
---
Note: build-tested only.

Found with: $ rg "strncpy\("
---
 fs/proc/vmcore.c            | 5 ++---
 include/uapi/linux/vmcore.h | 4 ++--
 2 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
index 1fb213f379a5..5d7ecf3b75e8 100644
--- a/fs/proc/vmcore.c
+++ b/fs/proc/vmcore.c
@@ -1370,9 +1370,8 @@ static void vmcoredd_write_header(void *buf, struct vmcoredd_data *data,
 	vdd_hdr->n_descsz = size + sizeof(vdd_hdr->dump_name);
 	vdd_hdr->n_type = NT_VMCOREDD;
 
-	strncpy((char *)vdd_hdr->name, VMCOREDD_NOTE_NAME,
-		sizeof(vdd_hdr->name));
-	memcpy(vdd_hdr->dump_name, data->dump_name, sizeof(vdd_hdr->dump_name));
+	strtomem_pad(vdd_hdr->name, VMCOREDD_NOTE_NAME, 0);
+	strtomem_pad(vdd_hdr->dump_name, data->dump_name, 0);
 }
 
 /**
diff --git a/include/uapi/linux/vmcore.h b/include/uapi/linux/vmcore.h
index 3e9da91866ff..7053e2b62fa0 100644
--- a/include/uapi/linux/vmcore.h
+++ b/include/uapi/linux/vmcore.h
@@ -11,8 +11,8 @@ struct vmcoredd_header {
 	__u32 n_namesz; /* Name size */
 	__u32 n_descsz; /* Content size */
 	__u32 n_type;   /* NT_VMCOREDD */
-	__u8 name[8];   /* LINUX\0\0\0 */
-	__u8 dump_name[VMCOREDD_MAX_NAME_BYTES]; /* Device dump's name */
+	__u8 name[8] __nonstring;   /* LINUX\0\0\0 */
+	__u8 dump_name[VMCOREDD_MAX_NAME_BYTES] __nonstring; /* Device dump's name */
 };
 
 #endif /* _UAPI_VMCORE_H */

---
base-commit: 928a87efa42302a23bb9554be081a28058495f22
change-id: 20240327-strncpy-fs-proc-vmcore-c-b18d761feaef

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