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Petersen" , Justin Stitt , Andy Shevchenko , linux-hardening@vger.kernel.org, Charles Bertsch , Bart Van Assche , Sathya Prakash , Sreekanth Reddy , Suganath Prabu Subramani , "James E.J. Bottomley" , Kashyap Desai , Sumit Saxena , Nilesh Javali , Andrew Morton , Himanshu Madhani , linux-kernel@vger.kernel.org, MPT-FusionLinux.pdl@broadcom.com, linux-scsi@vger.kernel.org, mpi3mr-linuxdrv.pdl@broadcom.com, GR-QLogic-Storage-Upstream@marvell.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Wed, Apr 10, 2024 at 5:31=E2=80=AFAM Kees Cook w= rote: > > Another ambiguous use of strncpy() is to copy from strings that may not > be NUL-terminated. These cases depend on having the destination buffer > be explicitly larger than the source buffer's maximum size, having > the size of the copy exactly match the source buffer's maximum size, > and for the destination buffer to get explicitly NUL terminated. > > This usually happens when parsing protocols or hardware character arrays > that are not guaranteed to be NUL-terminated. The code pattern is > effectively this: > > char dest[sizeof(src) + 1]; > > strncpy(dest, src, sizeof(src)); > dest[sizeof(dest) - 1] =3D '\0'; > > In practice it usually looks like: > > struct from_hardware { > ... > char name[HW_NAME_SIZE] __nonstring; > ... > }; > > struct from_hardware *p =3D ...; > char name[HW_NAME_SIZE + 1]; > > strncpy(name, p->name, HW_NAME_SIZE); > name[NW_NAME_SIZE] =3D '\0'; > > This cannot be replaced with: > > strscpy(name, p->name, sizeof(name)); > > because p->name is smaller and not NUL-terminated, so FORTIFY will > trigger when strnlen(p->name, sizeof(name)) is used. And it cannot be > replaced with: > > strscpy(name, p->name, sizeof(p->name)); > > because then "name" may contain a 1 character early truncation of > p->name. > > Provide an unambiguous interface for converting a maybe not-NUL-terminate= d > string to a NUL-terminated string, with compile-time buffer size checking > so that it can never fail at runtime: memtostr() and memtostr_pad(). Also > add KUnit tests for both. Obvious question, why can't strscpy() be fixed for this corner case? --=20 With Best Regards, Andy Shevchenko