Messages in this thread Patch in this message | | | Date | Sat, 26 Feb 2022 19:13:48 -0800 | From | Josh Poimboeuf <> | Subject | Re: [PATCH v2 34/39] objtool: Validate IBT assumptions |
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On Thu, Feb 24, 2022 at 03:52:12PM +0100, Peter Zijlstra wrote: > +++ b/tools/objtool/check.c > @@ -380,6 +380,7 @@ static int decode_instructions(struct ob > memset(insn, 0, sizeof(*insn)); > INIT_LIST_HEAD(&insn->alts); > INIT_LIST_HEAD(&insn->stack_ops); > + INIT_LIST_HEAD(&insn->call_node);
Is this needed? 'call_node' isn't actually a list head, otherwise this would presumably be fixing a major bug.
> insn->sec = sec; > insn->offset = offset; > @@ -1176,6 +1177,14 @@ static int add_jump_destinations(struct > unsigned long dest_off; > > for_each_insn(file, insn) { > + if (insn->type == INSN_ENDBR && insn->func) { > + if (insn->offset == insn->func->offset) { > + file->nr_endbr++; > + } else { > + file->nr_endbr_int++; > + } > + } > +
This doesn't have much to do with adding jump destinations. I'm thinking this would fit better in decode_instructions() in the sym_for_each_insn() loop.
> if (!is_static_jump(insn)) > continue; > > @@ -1192,10 +1201,14 @@ static int add_jump_destinations(struct > } else if (insn->func) { > /* internal or external sibling call (with reloc) */ > add_call_dest(file, insn, reloc->sym, true); > - continue; > + > + dest_sec = reloc->sym->sec; > + dest_off = reloc->sym->offset + > + arch_dest_reloc_offset(reloc->addend); > + > } else if (reloc->sym->sec->idx) { > dest_sec = reloc->sym->sec; > - dest_off = reloc->sym->sym.st_value + > + dest_off = reloc->sym->offset + > arch_dest_reloc_offset(reloc->addend); > } else { > /* non-func asm code jumping to another file */ > @@ -1205,6 +1218,10 @@ static int add_jump_destinations(struct > insn->jump_dest = find_insn(file, dest_sec, dest_off); > if (!insn->jump_dest) { > > + /* external symbol */ > + if (!vmlinux && insn->func) > + continue; > + > /* > * This is a special case where an alt instruction > * jumps past the end of the section. These are > @@ -1219,6 +1236,16 @@ static int add_jump_destinations(struct > return -1; > } > > + if (ibt && insn->jump_dest->type == INSN_ENDBR && > + insn->jump_dest->func && > + insn->jump_dest->offset == insn->jump_dest->func->offset) { > + if (reloc) { > + WARN_FUNC("Direct RELOC jump to ENDBR", insn->sec, insn->offset); > + } else { > + WARN_FUNC("Direct IMM jump to ENDBR", insn->sec, insn->offset); > + } > + } > +
I have several concerns about all the above (and corresponding changes elsewhere), but it looks like this was moved to separate patches, for ease of NACKing :-)
> /* > * Cross-function jump. > */ > @@ -1246,7 +1273,8 @@ static int add_jump_destinations(struct > insn->jump_dest->func->pfunc = insn->func; > > } else if (insn->jump_dest->func->pfunc != insn->func->pfunc && > - insn->jump_dest->offset == insn->jump_dest->func->offset) { > + ((insn->jump_dest->offset == insn->jump_dest->func->offset) || > + (insn->jump_dest->offset == insn->jump_dest->func->offset + 4))) { > /* internal sibling call (without reloc) */ > add_call_dest(file, insn, insn->jump_dest->func, true);
How about something more precise/readable/portable:
static bool same_func(struct instruction *insn1, struct instruction *insn2) { return insn1->func->pfunc == insn2->func->pfunc; }
static bool is_first_func_insn(struct instruction *insn) { return insn->offset == insn->func->offset || (insn->type == INSN_ENDBR && insn->offset == insn->func->offset + insn->len); }
...
} else if (!same_func(insn, insn->jump_dest) && is_first_func_insn(insn->jump_dest))
> +static void validate_ibt_insn(struct objtool_file *file, struct instruction *insn);
I'd rather avoid forward declares and stay with the existing convention.
> + > /* > * Follow the branch starting at the given instruction, and recursively follow > * any other branches (jumps). Meanwhile, track the frame pointer state at > @@ -3101,6 +3164,17 @@ static int validate_branch(struct objtoo > > if (insn->hint) { > state.cfi = *insn->cfi; > + if (ibt) { > + struct symbol *sym; > + > + if (insn->cfi->type == UNWIND_HINT_TYPE_REGS_PARTIAL && > + (sym = find_symbol_by_offset(insn->sec, insn->offset)) && > + insn->type != INSN_ENDBR && !insn->noendbr) { > + WARN_FUNC("IRET_REGS hint without ENDBR: %s", > + insn->sec, insn->offset, > + sym->name); > + }
No need to print sym->name here, WARN_FUNC() already does it?
> + } > } else { > /* XXX track if we actually changed state.cfi */ > > @@ -3260,7 +3334,12 @@ static int validate_branch(struct objtoo > state.df = false; > break; > > + case INSN_NOP: > + break; > + > default: > + if (ibt) > + validate_ibt_insn(file, insn);
This is kind of subtle. It would be more robust/clear to move this call out of the switch statement and check explicitly for the exclusion of jump/call instructions from within validate_ibt_insn().
> break; > } > > @@ -3506,6 +3585,130 @@ static int validate_functions(struct obj > return warnings; > } > > +static struct instruction * > +validate_ibt_reloc(struct objtool_file *file, struct reloc *reloc) > +{ > + struct instruction *dest; > + struct section *sec; > + unsigned long off; > + > + sec = reloc->sym->sec; > + off = reloc->sym->offset + reloc->addend;
This math assumes non-PC-relative. If it's R_X86_64_PC32 or R_X86_64_PLT32 then it needs +4 added.
There are actually a few cases of this in startup_64(). Those are harmless, but there might conceivably be other code which isn't?
> + > + dest = find_insn(file, sec, off); > + if (!dest) > + return NULL; > + > + if (dest->type == INSN_ENDBR) > + return NULL; > + > + if (reloc->sym->static_call_tramp) > + return NULL; > + > + return dest; > +} > + > +static void warn_noendbr(const char *msg, struct section *sec, unsigned long offset, > + struct instruction *target) > +{ > + WARN_FUNC("%srelocation to !ENDBR: %s+0x%lx", sec, offset, msg, > + target->func ? target->func->name : target->sec->name, > + target->func ? target->offset - target->func->offset : target->offset); > +} > + > +static void validate_ibt_target(struct objtool_file *file, struct instruction *insn, > + struct instruction *target) > +{ > + if (target->func && target->func == insn->func) {
(Here and elsewhere) Instead of 'target' can we call it 'dest' for consistency with existing code?
> + /* > + * Anything from->to self is either _THIS_IP_ or IRET-to-self. > + * > + * There is no sane way to annotate _THIS_IP_ since the compiler treats the > + * relocation as a constant and is happy to fold in offsets, skewing any > + * annotation we do, leading to vast amounts of false-positives. > + * > + * There's also compiler generated _THIS_IP_ through KCOV and > + * such which we have no hope of annotating. > + * > + * As such, blanked accept self-references without issue.
"blanket" > + */ > + return; > + } > + > + /* > + * Annotated non-control flow target. > + */ > + if (target->noendbr) > + return;
I don't think the comment really adds anything. What's a "non-control flow target" anyway...
> + > + warn_noendbr("", insn->sec, insn->offset, target); > +} > + > +static void validate_ibt_insn(struct objtool_file *file, struct instruction *insn) > +{ > + struct reloc *reloc = insn_reloc(file, insn); > + struct instruction *target; > + > + for (;;) { > + if (!reloc) > + return; > + > + target = validate_ibt_reloc(file, reloc); > + if (target) > + validate_ibt_target(file, insn, target); > + > + reloc = find_reloc_by_dest_range(file->elf, insn->sec, reloc->offset + 1, > + (insn->offset + insn->len) - (reloc->offset + 1)); > + }
I'm confused about what this loop is trying to do. Why would an instruction have more than one reloc? It at least needs a comment.
Also a proper for() loop would be easier to follow:
for (reloc = insn_reloc(file, insn); reloc; reloc = find_reloc_by_dest_range(file->elf, insn->sec, reloc->offset + 1, (insn->offset + insn->len) - (reloc->offset + 1)) { > +} > + > +static int validate_ibt(struct objtool_file *file) > +{ > + struct section *sec; > + struct reloc *reloc; > + > + for_each_sec(file, sec) { > + bool is_data; > + > + /* already done in validate_branch() */ > + if (sec->sh.sh_flags & SHF_EXECINSTR) > + continue; > + > + if (!sec->reloc) > + continue; > + > + if (!strncmp(sec->name, ".orc", 4)) > + continue; > + > + if (!strncmp(sec->name, ".discard", 8)) > + continue; > + > + if (!strncmp(sec->name, ".debug", 6)) > + continue; > + > + if (!strcmp(sec->name, "_error_injection_whitelist")) > + continue; > + > + if (!strcmp(sec->name, "_kprobe_blacklist")) > + continue; > + > + is_data = strstr(sec->name, ".data") || strstr(sec->name, ".rodata"); > + > + list_for_each_entry(reloc, &sec->reloc->reloc_list, list) { > + struct instruction *target; > + > + target = validate_ibt_reloc(file, reloc); > + if (is_data && target && !target->noendbr) { > + warn_noendbr("data ", reloc->sym->sec, > + reloc->sym->offset + reloc->addend,
Another case where the addend math would be wrong if it were pc-relative. Not sure if that's possible here or not.
-- Josh
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