On Thu, Mar 21, 2024 at 01:03:10AM +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.
>
> in file.c:
> s_last_mounted is marked as __nonstring meaning it does not need to be
> NUL-terminated. Let's instead use strtomem_pad() to copy bytes from the
> string source to the byte array destination -- while also ensuring to
> pad with zeroes.
>
> in ioctl.c:
> We can drop the memset and size argument in favor of using the new
> 2-argument version of strscpy_pad() -- which was introduced with Commit
> e6584c3964f2f ("string: Allow 2-argument strscpy()"). This guarantees
> NUL-termination and NUL-padding on the destination buffer -- which seems
> to be a requirement judging from this comment:
>
> | static int ext4_ioctl_getlabel(struct ext4_sb_info *sbi, char __user
> *user_label)
> | {
> | char label[EXT4_LABEL_MAX + 1];
> |
> | /*
> | * EXT4_LABEL_MAX must always be smaller than FSLABEL_MAX
> because
> | * FSLABEL_MAX must include terminating null byte, while
> s_volume_name
> | * does not have to.
> | */
>
> in super.c:
> s_first_error_func is marked as __nonstring meaning we can take the same
> approach as in file.c; just use strtomem_pad()
>
> 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: [email protected]
> Signed-off-by: Justin Stitt <[email protected]>
> ---
> Note: build-tested only.
>
> Found with: $ rg "strncpy\("
> ---
> fs/ext4/file.c | 3 +--
> fs/ext4/ioctl.c | 3 +--
> fs/ext4/super.c | 7 +++----
> 3 files changed, 5 insertions(+), 8 deletions(-)
>
> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> index 54d6ff22585c..c675c0eb5f7e 100644
> --- a/fs/ext4/file.c
> +++ b/fs/ext4/file.c
> @@ -844,8 +844,7 @@ static int ext4_sample_last_mounted(struct super_block
> *sb,
> if (err)
> goto out_journal;
> lock_buffer(sbi->s_sbh);
> - strncpy(sbi->s_es->s_last_mounted, cp,
> - sizeof(sbi->s_es->s_last_mounted));
> + strtomem_pad(sbi->s_es->s_last_mounted, cp, 0);
> ext4_superblock_csum_set(sb);
> unlock_buffer(sbi->s_sbh);
> ext4_handle_dirty_metadata(handle, NULL, sbi->s_sbh);
> diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
> index 7160a71044c8..dab7acd49709 100644
> --- a/fs/ext4/ioctl.c
> +++ b/fs/ext4/ioctl.c
> @@ -1150,9 +1150,8 @@ static int ext4_ioctl_getlabel(struct ext4_sb_info
> *sbi, char __user *user_label
> */
> BUILD_BUG_ON(EXT4_LABEL_MAX >= FSLABEL_MAX);
>
> - memset(label, 0, sizeof(label));
> lock_buffer(sbi->s_sbh);
> - strncpy(label, sbi->s_es->s_volume_name, EXT4_LABEL_MAX);
> + strscpy_pad(label, sbi->s_es->s_volume_name);
> unlock_buffer(sbi->s_sbh);
The only reason I can imagine the memset() being split here is to keep
it out of the spinlock. For a non-fast-path ioctl, I can't imagine this
being a meaningful reduction in lock contention, though.
Reviewed-by: Kees Cook <[email protected]>
--
Kees Cook
