* H. J. Lu: > 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.
Which means that NOTRACK should not weaken anything here. What am I missing? Thanks, Florian
