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Date: Thu, 14 Apr 2022 11:25:05 -0700
From: Josh Poimboeuf <jpoimboe@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
Miroslav Benes <mbenes@...e.cz>
Subject: Re: [PATCH 10/18] objtool: Extricate ibt from stack validation
On Thu, Apr 14, 2022 at 10:05:53AM -0700, Josh Poimboeuf wrote:
> On Thu, Apr 14, 2022 at 06:38:16PM +0200, Peter Zijlstra wrote:
> > On Thu, Apr 14, 2022 at 08:44:49AM -0700, Josh Poimboeuf wrote:
> >
> > > Ok. That was subtle, it needs a comment or two. I had the distinct
> > > feeling I was introducing a bug, then I got distracted ;-)
> >
> > Right, lemme try and not forget to write one ;-)
>
> I'm rewriting the code anyway, I'll add some comments.
>
> > > Doesn't the compiler give those special cases ENDBR anyway? Just
> > > wondering why we avoid the warning for those.
> >
> > Sure, but this is about not scribbling that ENDBR with a NOP.
>
> Right, but it only prints warnings for data sections, despite marking
> others:
>
> - dest = validate_ibt_reloc(file, reloc);
> - if (is_data && dest && !dest->noendbr)
> - warn_noendbr("data ", sec, reloc->offset, dest);
How about this? This way it doesn't silence warnings for non
.data/.rodata, as other sections (including ksymtab) can also have valid
function pointers. It also caught a bug(?) in putuser.S, as some of the
exported inner labels didn't have ENDBR.
diff --git a/arch/x86/include/asm/static_call.h b/arch/x86/include/asm/static_call.h
index 2455d721503e..2d8dacd02643 100644
--- a/arch/x86/include/asm/static_call.h
+++ b/arch/x86/include/asm/static_call.h
@@ -26,6 +26,7 @@
".align 4 \n" \
".globl " STATIC_CALL_TRAMP_STR(name) " \n" \
STATIC_CALL_TRAMP_STR(name) ": \n" \
+ ANNOTATE_NOENDBR \
insns " \n" \
".byte 0x53, 0x43, 0x54 \n" \
".type " STATIC_CALL_TRAMP_STR(name) ", @function \n" \
diff --git a/arch/x86/lib/putuser.S b/arch/x86/lib/putuser.S
index ecb2049c1273..b7dfd60243b7 100644
--- a/arch/x86/lib/putuser.S
+++ b/arch/x86/lib/putuser.S
@@ -48,6 +48,7 @@ SYM_FUNC_START(__put_user_1)
cmp %_ASM_BX,%_ASM_CX
jae .Lbad_put_user
SYM_INNER_LABEL(__put_user_nocheck_1, SYM_L_GLOBAL)
+ ENDBR
ASM_STAC
1: movb %al,(%_ASM_CX)
xor %ecx,%ecx
@@ -62,6 +63,7 @@ SYM_FUNC_START(__put_user_2)
cmp %_ASM_BX,%_ASM_CX
jae .Lbad_put_user
SYM_INNER_LABEL(__put_user_nocheck_2, SYM_L_GLOBAL)
+ ENDBR
ASM_STAC
2: movw %ax,(%_ASM_CX)
xor %ecx,%ecx
@@ -76,6 +78,7 @@ SYM_FUNC_START(__put_user_4)
cmp %_ASM_BX,%_ASM_CX
jae .Lbad_put_user
SYM_INNER_LABEL(__put_user_nocheck_4, SYM_L_GLOBAL)
+ ENDBR
ASM_STAC
3: movl %eax,(%_ASM_CX)
xor %ecx,%ecx
@@ -90,6 +93,7 @@ SYM_FUNC_START(__put_user_8)
cmp %_ASM_BX,%_ASM_CX
jae .Lbad_put_user
SYM_INNER_LABEL(__put_user_nocheck_8, SYM_L_GLOBAL)
+ ENDBR
ASM_STAC
4: mov %_ASM_AX,(%_ASM_CX)
#ifdef CONFIG_X86_32
diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index 5f87bab4fb8d..b2b2366885a2 100644
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -31,6 +31,7 @@
.align RETPOLINE_THUNK_SIZE
SYM_INNER_LABEL(__x86_indirect_thunk_\reg, SYM_L_GLOBAL)
UNWIND_HINT_EMPTY
+ ANNOTATE_NOENDBR
ALTERNATIVE_2 __stringify(ANNOTATE_RETPOLINE_SAFE; jmp *%\reg), \
__stringify(RETPOLINE \reg), X86_FEATURE_RETPOLINE, \
@@ -55,7 +56,6 @@ SYM_INNER_LABEL(__x86_indirect_thunk_\reg, SYM_L_GLOBAL)
.align RETPOLINE_THUNK_SIZE
SYM_CODE_START(__x86_indirect_thunk_array)
- ANNOTATE_NOENDBR // apply_retpolines
#define GEN(reg) THUNK reg
#include <asm/GEN-for-each-reg.h>
diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
index ac17196e2518..3a2cd93bf059 100644
--- a/arch/x86/xen/xen-head.S
+++ b/arch/x86/xen/xen-head.S
@@ -45,6 +45,7 @@ SYM_CODE_END(hypercall_page)
__INIT
SYM_CODE_START(startup_xen)
UNWIND_HINT_EMPTY
+ ANNOTATE_NOENDBR
cld
/* Clear .bss */
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index b09c416432b6..10e375c84088 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -3794,7 +3794,10 @@ static int validate_ibt_data_reloc(struct objtool_file *file,
return 1;
}
-
+/*
+ * Validate IBT rules and mark used ENDBR instructions so the non-used ones can
+ * be sealed later by create_ibt_endbr_seal_sections().
+ */
static int validate_ibt(struct objtool_file *file)
{
struct section *sec;
@@ -3807,12 +3810,36 @@ static int validate_ibt(struct objtool_file *file)
for_each_sec(file, sec) {
- if (!strstr(sec->name, ".data") && !strstr(sec->name, ".rodata"))
+ /* Already done by validate_ibt_insn() */
+ if (sec->sh.sh_flags & SHF_EXECINSTR)
continue;
if (!sec->reloc)
continue;
+ /*
+ * These sections can reference text addresses, but not with
+ * the intent to indirect branch to them.
+ */
+ if (!strncmp(sec->name, ".discard", 8) ||
+ !strncmp(sec->name, ".debug", 6) ||
+ !strcmp(sec->name, ".altinstructions") ||
+ !strcmp(sec->name, ".ibt_endbr_seal") ||
+ !strcmp(sec->name, ".orc_unwind_ip") ||
+ !strcmp(sec->name, ".parainstructions") ||
+ !strcmp(sec->name, ".retpoline_sites") ||
+ !strcmp(sec->name, ".smp_locks") ||
+ !strcmp(sec->name, ".static_call_sites") ||
+ !strcmp(sec->name, ".static_call_tramp_key") ||
+ !strcmp(sec->name, "_error_injection_whitelist") ||
+ !strcmp(sec->name, "_kprobe_blacklist") ||
+ !strcmp(sec->name, "__bug_table") ||
+ !strcmp(sec->name, "__ex_table") ||
+ !strcmp(sec->name, "__jump_table") ||
+ !strcmp(sec->name, "__mcount_loc") ||
+ !strcmp(sec->name, "__tracepoints"))
+ continue;
+
list_for_each_entry(reloc, &sec->reloc->reloc_list, list)
warnings += validate_ibt_data_reloc(file, reloc);
}
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