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