Provide support for randomized stack offsets per syscall to make
stack-based attacks harder which rely on the deterministic stack layout.
The feature is based on the original idea of PaX's RANDSTACK feature, but
uses a significantly different implementation.
The offset does not affect the pt_regs location on the task stack as this
was agreed on to be of dubious value. The offset is applied before the
actual syscall is invoked.
The offset is stored per cpu and the randomization happens at the end of
the syscall which is less predictable than on syscall entry.
The mechanism to apply the offset is via alloca(), i.e. abusing the
dispised VLAs. This comes with the drawback that stack-clash-protection
has to be disabled for the affected compilation units and there is also
a negative interaction with stack-protector.
Those downsides are traded with the advantage that this approach does not
require any intrusive changes to the low level assembly entry code, does
not affect the unwinder and the correct stack alignment is handled
automatically by the compiler.
The feature is guarded with a static branch which avoids the overhead when
disabled.
Currently this is supported for X86 and ARM64.