On Tue, Jun 5, 2018 at 8:44 PM, Eric Sandeen <sandeen@xxxxxxxxxxx> wrote:
> On 5/25/18 10:14 AM, Arnd Bergmann wrote:
>> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
>> index 84fbf164cbc3..eb79f2bc4dcc 100644
>> --- a/fs/xfs/xfs_ioctl.c
>> +++ b/fs/xfs/xfs_ioctl.c
>> @@ -1819,12 +1819,12 @@ xfs_ioc_getlabel(
>> BUILD_BUG_ON(sizeof(sbp->sb_fname) > FSLABEL_MAX);
>>
>> spin_lock(&mp->m_sb_lock);
>> - strncpy(label, sbp->sb_fname, sizeof(sbp->sb_fname));
>> + strncpy(label, sbp->sb_fname, XFSLABEL_MAX);
>> spin_unlock(&mp->m_sb_lock);
>>
>> /* xfs on-disk label is 12 chars, be sure we send a null to user */
>> label[XFSLABEL_MAX] = '\0';
>> - if (copy_to_user(user_label, label, sizeof(sbp->sb_fname)))
>> + if (copy_to_user(user_label, label, sizeof(label)))
>
> I /think/ this also runs the risk of copying out stack memory.
>
> I'll send another proposal based on this with slight modifications.
I assumed it's safe since the earlier strncpy() pads the local 'label'
with zero bytes up the XFSLABEL_MAX, and the last byte
is explicitly set to guarantee zero padding.
Using strlcpy() or strscpy() would guarantee a zero-terminated
string without the explicit ='\0' assignment but would risk
the data leak you were probably thinking of.
Arnd
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