On Wed, May 11, 2022 at 11:45 AM Florian Weimer <fwei...@redhat.com> wrote: > > * H. J. Lu: > > >> NOTRACK avoids the need for ENDBR instructions, right? That's a > >> hardening improvement, so it should be used by default. > > > > NOTRACK weakens IBT since it disables IBT on the indirect jump instruction. > > GCC uses it in the jump table to avoid ENDBR. > > Typical jump table code looks like this: > > Dump of assembler code for function __cache_sysconf: > 0x00000000000f7a80 <+0>: endbr64 > 0x00000000000f7a84 <+4>: sub $0xb9,%edi > 0x00000000000f7a8a <+10>: cmp $0xc,%edi > 0x00000000000f7a8d <+13>: ja 0xf7b70 <__cache_sysconf+240> > 0x00000000000f7a93 <+19>: lea 0xba926(%rip),%rdx # 0x1b23c0 > 0x00000000000f7a9a <+26>: movslq (%rdx,%rdi,4),%rax > 0x00000000000f7a9e <+30>: add %rdx,%rax > 0x00000000000f7aa1 <+33>: notrack jmp *%rax > > There's no ENDBR instruction between range check, the address > computation, and the NOTRACK JMP, so it's not possible to redirect that > JMP to some other place.
That is the assumption we made. RAX will always point to the valid address. > I don't know if GCC systematically enforces this in its optimizers, > though. > > Thanks, > Florian > -- H.J.
