[PATCH] i386: Default to -mcet-switch [PR104816]

[PATCH] i386: Default to -mcet-switch [PR104816]

[Fangrui Song]

When -fcf-protection=branch is used, with the current -mno-cet-switch
default, a NOTRACK indirect jump is generated for jump tables, which can
target a non-ENDBR instruction.  However, the overwhelming opinion is to
avoid NOTRACK (PR104816) to improve safety.  Projects such as Linux
kernel and Xen even specify -fno-jump-table to avoid NOTRACK. Therefore,
let's change the default.

Note, for `default: __builtin_unreachable()`, LLVM AArch64 even made a
decision (https://reviews.llvm.org/D155485) to keep the range check,
which can otherwise be optimized out.  This reinforces the opinion that
people want protection for jump tables.

    #define DO A(0) A(1) A(2) A(3) A(4) A(5) A(6) A(7) A(8) A(9) A(10) A(11) A(12) A(13)
    #define A(i) void bar##i();
    DO
    #undef A
    void ext();
    void foo(int i) {
      switch (i) {
    #define A(i) case i: bar##i(); break;
        DO
    // -mbranch-protection=bti causes Clang AArch64 to keep the i <= 13 range check
      default: __builtin_unreachable();
      }
      ext();
    }

gcc/ChangeLog:

    PR target/104816
    * config/i386/i386.opt: Default to -mcet-switch.
    * doc/invoke.texi: Update doc.

gcc/testsuite/ChangeLog:

    * gcc.target/i386/cet-switch-1.c: Add -mno-cet-switch.
    * gcc.target/i386/cet-switch-2.c: Remove -mcet-switch to check the
      default.
---
 gcc/config/i386/i386.opt                     |  2 +-
 gcc/doc/invoke.texi                          | 19 +++++++++----------
 gcc/testsuite/gcc.target/i386/cet-switch-1.c |  2 +-
 gcc/testsuite/gcc.target/i386/cet-switch-2.c |  2 +-
 4 files changed, 12 insertions(+), 13 deletions(-)

diff --git a/gcc/config/i386/i386.opt b/gcc/config/i386/i386.opt
index 5b4f1bff25f..0e168f3c07a 100644
--- a/gcc/config/i386/i386.opt
+++ b/gcc/config/i386/i386.opt
@@ -1074,7 +1074,7 @@ Enable shadow stack built-in functions from Control-flow Enforcement
 Technology (CET).
 
 mcet-switch
-Target Var(flag_cet_switch) Init(0)
+Target Var(flag_cet_switch) Init(1)
 Turn on CET instrumentation for switch statements that use a jump table and
 an indirect jump.
 
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 16e31a3c6db..720be71f8fa 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -1455,7 +1455,7 @@ See RS/6000 and PowerPC Options.
 -msse4a  -m3dnow  -m3dnowa  -mpopcnt  -mabm  -mbmi  -mtbm  -mfma4  -mxop
 -madx  -mlzcnt  -mbmi2  -mfxsr  -mxsave  -mxsaveopt  -mrtm  -mhle  -mlwp
 -mmwaitx  -mclzero  -mpku  -mthreads  -mgfni  -mvaes  -mwaitpkg
--mshstk -mmanual-endbr -mcet-switch -mforce-indirect-call
+-mshstk -mmanual-endbr -mno-cet-switch -mforce-indirect-call
 -mavx512vbmi2 -mavx512bf16 -menqcmd
 -mvpclmulqdq  -mavx512bitalg  -mmovdiri  -mmovdir64b  -mavx512vpopcntdq
 -mavx5124fmaps  -mavx512vnni  -mavx5124vnniw  -mprfchw  -mrdpid
@@ -34886,16 +34886,15 @@ function attribute. This is useful when used with the option
 @option{-fcf-protection=branch} to control ENDBR insertion at the
 function entry.
 
+@opindex mno-cet-switch
 @opindex mcet-switch
-@item -mcet-switch
-By default, CET instrumentation is turned off on switch statements that
-use a jump table and indirect branch track is disabled.  Since jump
-tables are stored in read-only memory, this does not result in a direct
-loss of hardening.  But if the jump table index is attacker-controlled,
-the indirect jump may not be constrained by CET.  This option turns on
-CET instrumentation to enable indirect branch track for switch statements
-with jump tables which leads to the jump targets reachable via any indirect
-jumps.
+@item -mno-cet-switch
+When @option{-fcf-protection=branch} is enabled, by default, switch statements
+that use a jump table are instrumented to use ENDBR instructions and constrain
+the indirect jump with CET to protect against an attacker-controlled jump table
+index.  @option{-mno-cet-switch} generates a NOTRACK indirect jump and removes
+ENDBR instructions, which may make the jump table smaller at the cost of an
+unprotected indirect jump.
 
 @opindex mcall-ms2sysv-xlogues
 @opindex mno-call-ms2sysv-xlogues
diff --git a/gcc/testsuite/gcc.target/i386/cet-switch-1.c b/gcc/testsuite/gcc.target/i386/cet-switch-1.c
index afe5adc2f3d..4931c3ad1d2 100644
--- a/gcc/testsuite/gcc.target/i386/cet-switch-1.c
+++ b/gcc/testsuite/gcc.target/i386/cet-switch-1.c
@@ -1,6 +1,6 @@
 /* Verify that CET works.  */
 /* { dg-do compile } */
-/* { dg-options "-O -fcf-protection" } */
+/* { dg-options "-O -fcf-protection -mno-cet-switch" } */
 /* { dg-final { scan-assembler-times "endbr32" 1 { target ia32 } } } */
 /* { dg-final { scan-assembler-times "endbr64" 1 { target { ! ia32 } } } } */
 /* { dg-final { scan-assembler-times "notrack jmp\[ \t]+\[*]" 1 } } */
diff --git a/gcc/testsuite/gcc.target/i386/cet-switch-2.c b/gcc/testsuite/gcc.target/i386/cet-switch-2.c
index 69ddc6fd5b7..11578d1a30c 100644
--- a/gcc/testsuite/gcc.target/i386/cet-switch-2.c
+++ b/gcc/testsuite/gcc.target/i386/cet-switch-2.c
@@ -1,6 +1,6 @@
 /* Verify that CET works.  */
 /* { dg-do compile } */
-/* { dg-options "-O -fcf-protection -mcet-switch" } */
+/* { dg-options "-O -fcf-protection" } */
 /* { dg-final { scan-assembler-times "endbr32" 12 { target ia32 } } } */
 /* { dg-final { scan-assembler-times "endbr64" 12 { target { ! ia32 } } } } */
 /* { dg-final { scan-assembler-times "\[ \t]+jmp\[ \t]+\[*]" 1 } } */
-- 
2.43.0.381.gb435a96ce8-goog