[PATCH v14 3/7] x86/cet/ibt: Handle signals for Indirect Branch Tracking

Yu-cheng Yu
2020-10-12T15:46:13Z

An indirect CALL/JMP moves the indirect branch tracking (IBT) state machine
to WAIT_ENDBR status until the instruction reaches an ENDBR opcode. If the
CALL/JMP does not reach an ENDBR opcode, the processor raises a control-
protection fault. WAIT_ENDBR status can be read from MSR_IA32_U_CET.

WAIT_ENDBR is cleared for signal handling, and restored for sigreturn.

IBT state machine is described in Intel SDM Vol. 1, Sec. 18.3.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
---
arch/x86/kernel/cet.c | 27 +++++++++++++++++++++++++--
arch/x86/kernel/fpu/signal.c | 8 +++++---
2 files changed, 30 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
index 1dcd16aa4f90..0edcbd70f408 100644
--- a/arch/x86/kernel/cet.c
+++ b/arch/x86/kernel/cet.c
@@ -295,6 +295,13 @@ void cet_restore_signal(struct sc_ext *sc_ext)
msr_val |= CET_SHSTK_EN;
}

+ if (cet->ibt_enabled) {
+ msr_val |= (CET_ENDBR_EN | CET_NO_TRACK_EN);
+
+ if (sc_ext->wait_endbr)
+ msr_val |= CET_WAIT_ENDBR;
+ }
+
if (test_thread_flag(TIF_NEED_FPU_LOAD))
cet_user_state->user_cet = msr_val;
else
@@ -335,9 +342,25 @@ int cet_setup_signal(bool ia32, unsigned long rstor_addr, struct sc_ext *sc_ext)
sc_ext->ssp = new_ssp;
}

- if (ssp) {
+ if (ssp || cet->ibt_enabled) {
+
start_update_msrs();
- wrmsrl(MSR_IA32_PL3_SSP, ssp);
+
+ if (ssp)
+ wrmsrl(MSR_IA32_PL3_SSP, ssp);
+
+ if (cet->ibt_enabled) {
+ u64 r;
+
+ rdmsrl(MSR_IA32_U_CET, r);
+
+ if (r & CET_WAIT_ENDBR) {
+ sc_ext->wait_endbr = 1;
+ r &= ~CET_WAIT_ENDBR;
+ wrmsrl(MSR_IA32_U_CET, r);
+ }
+ }
+
end_update_msrs();
}

diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index c0c2141cb4b3..077853ef6f48 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -57,7 +57,8 @@ int save_cet_to_sigframe(int ia32, void __user *fp, unsigned long restorer)
{
int err = 0;

- if (!current->thread.cet.shstk_size)
+ if (!current->thread.cet.shstk_size &&
+ !current->thread.cet.ibt_enabled)
return 0;

if (fp) {
@@ -89,7 +90,8 @@ static int get_cet_from_sigframe(int ia32, void __user *fp, struct sc_ext *ext)

memset(ext, 0, sizeof(*ext));

- if (!current->thread.cet.shstk_size)
+ if (!current->thread.cet.shstk_size &&
+ !current->thread.cet.ibt_enabled)
return 0;

if (fp) {
@@ -577,7 +579,7 @@ static unsigned long fpu__alloc_sigcontext_ext(unsigned long sp)
* sigcontext_ext is at: fpu + fpu_user_xstate_size +
* FP_XSTATE_MAGIC2_SIZE, then aligned to 8.
*/
- if (cet->shstk_size)
+ if (cet->shstk_size || cet->ibt_enabled)
sp -= (sizeof(struct sc_ext) + 8);

return sp;
--
2.21.0