linux-kernel - [PATCH v3 8/8] crypto: x86/crc32c-intel - Tweaks to make objtool's life harder

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Message-ID: <20241011170847.334429-18-ardb+git@google.com>
Date: Fri, 11 Oct 2024 19:08:56 +0200
From: Ard Biesheuvel <ardb+git@...gle.com>
To: linux-kernel@...r.kernel.org
Cc: llvm@...ts.linux.dev, keescook@...omium.org, 
	linux-hardening@...r.kernel.org, nathan@...nel.org, 
	Ard Biesheuvel <ardb@...nel.org>, Josh Poimboeuf <jpoimboe@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Jan Beulich <jbeulich@...e.com>, 
	"Jose E. Marchesi" <jemarch@....org>, Kees Cook <kees@...nel.org>
Subject: [PATCH v3 8/8] crypto: x86/crc32c-intel - Tweaks to make objtool's
 life harder

From: Ard Biesheuvel <ardb@...nel.org>

Tweak the asm crc32c asm code so that the jump table is more difficult
to decipher for objtool:

- load the address in the prologue
- move the jump table to the middle of .rodata, so that the section
  offset and the symbol offset differ
- emit an entry following the jump table that is unrelated
- add unconditional ENDBRs so we can test --ibt regardless of the kernel
  config

Test code only. Not intended for merging.
---
 arch/x86/crypto/crc32c-pcl-intel-asm_64.S | 50 +++++++++++---------
 1 file changed, 28 insertions(+), 22 deletions(-)

diff --git a/arch/x86/crypto/crc32c-pcl-intel-asm_64.S b/arch/x86/crypto/crc32c-pcl-intel-asm_64.S
index 7292090e76dd..cbedf5820e30 100644
--- a/arch/x86/crypto/crc32c-pcl-intel-asm_64.S
+++ b/arch/x86/crypto/crc32c-pcl-intel-asm_64.S
@@ -93,10 +93,14 @@ SYM_FUNC_START(crc_pcl)
 #define    crc1		%r9
 #define    crc2		%r10
 
+	endbr64
+	pushq	%rbp
 	pushq   %rbx
 	pushq   %rdi
 	pushq   %rsi
 
+	leaq	jump_table(%rip), %rbp
+
 	## Move crc_init for Linux to a different
 	mov     crc_init_arg, crc_init
 
@@ -168,9 +172,8 @@ SYM_FUNC_START(crc_pcl)
 	xor     crc2, crc2
 
 	## branch into array
-	leaq	jump_table(%rip), %bufp
-	movslq	(%bufp,%rax,4), len
-	addq	len, %bufp
+	movslq	(%rbp,%rax,4), %bufp
+	addq	%rbp, %bufp
 	.reloc	., R_X86_64_NONE, jump_table
 	JMP_NOSPEC bufp
 
@@ -197,7 +200,7 @@ SYM_FUNC_START(crc_pcl)
 .altmacro
 LABEL crc_ %i
 .noaltmacro
-	ENDBR
+	endbr64
 	crc32q   -i*8(block_0), crc_init
 	crc32q   -i*8(block_1), crc1
 	crc32q   -i*8(block_2), crc2
@@ -207,7 +210,7 @@ LABEL crc_ %i
 .altmacro
 LABEL crc_ %i
 .noaltmacro
-	ENDBR
+	endbr64
 	crc32q   -i*8(block_0), crc_init
 	crc32q   -i*8(block_1), crc1
 # SKIP  crc32  -i*8(block_2), crc2 ; Don't do this one yet
@@ -241,7 +244,7 @@ LABEL crc_ %i
 	################################################################
 
 LABEL crc_ 0
-	ENDBR
+	endbr64
 	mov     tmp, len
 	cmp     $128*24, tmp
 	jae     .Lfull_block
@@ -311,26 +314,11 @@ LABEL less_than_ %j			# less_than_j: Length should be in
 	popq    %rsi
 	popq    %rdi
 	popq    %rbx
+	popq    %rbp
         RET
 SYM_FUNC_END(crc_pcl)
 
 .section	.rodata, "a", @progbits
-        ################################################################
-        ## jump table        Table is 129 entries x 2 bytes each
-        ################################################################
-.align 4
-jump_table:
-	i=0
-.rept 129
-.altmacro
-JMPTBL_ENTRY %i
-.noaltmacro
-	i=i+1
-.endr
-
-.size	jump_table, . - jump_table
-.type	jump_table, @object
-
 	################################################################
 	## PCLMULQDQ tables
 	## Table is 128 entries x 2 words (8 bytes) each
@@ -465,3 +453,21 @@ K_table:
 	.long 0x45cddf4e, 0xe0ac139e
 	.long 0xacfa3103, 0x6c23e841
 	.long 0xa51b6135, 0x170076fa
+
+        ################################################################
+        ## jump table        Table is 129 entries x 2 bytes each
+        ################################################################
+.align 4
+jump_table:
+	i=0
+.rept 129
+.altmacro
+JMPTBL_ENTRY %i
+.noaltmacro
+	i=i+1
+.endr
+
+.size	jump_table, . - jump_table
+.type	jump_table, @object
+
+	.long	crc_pcl - .
-- 
2.47.0.rc1.288.g06298d1525-goog