[PATCH] udf: replace deprecated strncpy/strcpy with strscpy

[Justin Stitt <justinstitt@google.com>]

strncpy() is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces. Also replace an instance of strcpy() which is also
deprecated.

s_volume_ident is a NUL-terminated string which is evident from its
usage in udf_debug:
|	udf_debug("volIdent[] = '%s'\n", UDF_SB(sb)->s_volume_ident);

s_volume_ident should also be NUL-padded as it is copied out to
userspace:
|	if (copy_to_user((char __user *)arg,
|				UDF_SB(inode->i_sb)->s_volume_ident, 32))
|		return -EFAULT;

Considering the above, a suitable replacement is `strscpy_pad` [2] due
to the fact that it guarantees both NUL-termination and NUL-padding on
the destination buffer.

To simplify the code, let's use the new 2-argument version of
strscpy_pad() introduced in Commit e6584c3964f2f ("string: Allow
2-argument strscpy()"). Also zero-allocate @outstr so we can safely use
a non-@ret length argument. This is just in case udf_dstrCS0toChar()
doesn't include the NUL-byte in its return length, we won't truncate
@outstr or write garbage bytes either.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
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/udf/super.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/udf/super.c b/fs/udf/super.c
index 2217f7ed7a49..77d32fe14434 100644
--- a/fs/udf/super.c
+++ b/fs/udf/super.c
@@ -895,7 +895,7 @@ static int udf_load_pvoldesc(struct super_block *sb, sector_t block)
 	int ret;
 	struct timestamp *ts;
 
-	outstr = kmalloc(128, GFP_KERNEL);
+	outstr = kzalloc(128, GFP_KERNEL);
 	if (!outstr)
 		return -ENOMEM;
 
@@ -921,11 +921,11 @@ static int udf_load_pvoldesc(struct super_block *sb, sector_t block)
 
 	ret = udf_dstrCS0toChar(sb, outstr, 31, pvoldesc->volIdent, 32);
 	if (ret < 0) {
-		strcpy(UDF_SB(sb)->s_volume_ident, "InvalidName");
+		strscpy_pad(UDF_SB(sb)->s_volume_ident, "InvalidName");
 		pr_warn("incorrect volume identification, setting to "
 			"'InvalidName'\n");
 	} else {
-		strncpy(UDF_SB(sb)->s_volume_ident, outstr, ret);
+		strscpy_pad(UDF_SB(sb)->s_volume_ident, outstr);
 	}
 	udf_debug("volIdent[] = '%s'\n", UDF_SB(sb)->s_volume_ident);
 

---
base-commit: 928a87efa42302a23bb9554be081a28058495f22
change-id: 20240401-strncpy-fs-udf-super-c-983457eb4ac3

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