[PATCH v29 04/10] x86/cet/ibt: Disable IBT for ia32

[Yu-cheng Yu]

In a signal, a task's IBT status needs to be saved to the signal frame, and
later restored in sigreturn.  For the purpose, previous versions of the
series add a new struct to the signal frame.  However, a new signal frame
format (or re-using a reserved space) introduces complex compatibility
issues.

In the discussion (see link below), Andy Lutomirski proposed using a
ucontext flag.  The approach is clean and eliminates most compatibility
issues.

However, a legacy IA32 signal frame does not have ucontext and cannot
support a uc flag.  Thus,

- Disable IBT for ia32.
- In ia32 sigreturn, verify ibt is disabled.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Acked-by: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Cyrill Gorcunov <gorcunov@xxxxxxxxx>
Cc: Florian Weimer <fweimer@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Link: https://lore.kernel.org/linux-api/f6e61dae-9805-c855-8873-7481ceb7ea79@xxxxxxxxx/
---
 arch/x86/ia32/ia32_signal.c |  7 +++++++
 arch/x86/include/asm/elf.h  | 13 ++++++++++++-
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/arch/x86/ia32/ia32_signal.c b/arch/x86/ia32/ia32_signal.c
index 77d0fa90cc19..946039cb3150 100644
--- a/arch/x86/ia32/ia32_signal.c
+++ b/arch/x86/ia32/ia32_signal.c
@@ -104,6 +104,13 @@ COMPAT_SYSCALL_DEFINE0(sigreturn)
 	struct sigframe_ia32 __user *frame = (struct sigframe_ia32 __user *)(regs->sp-8);
 	sigset_t set;
 
+	/*
+	 * Verify legacy sigreturn does not have IBT enabled.
+	 */
+#ifdef CONFIG_X86_IBT
+	if (current->thread.shstk.ibt)
+		goto badframe;
+#endif
 	if (!access_ok(frame, sizeof(*frame)))
 		goto badframe;
 	if (__get_user(set.sig[0], &frame->sc.oldmask)
diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index 3281a3d01bd2..cf9eeb30c00c 100644
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -6,6 +6,7 @@
  * ELF register definitions..
  */
 #include <linux/thread_info.h>
+#include <uapi/linux/elf.h>
 
 #include <asm/ptrace.h>
 #include <asm/user.h>
@@ -403,7 +404,17 @@ struct arch_elf_state {
 }
 
 #define arch_elf_pt_proc(ehdr, phdr, elf, interp, state) (0)
-#define arch_check_elf(ehdr, interp, interp_ehdr, state) (0)
+static inline int arch_check_elf(void *ehdr, bool interp,
+				 void *interp_ehdr,
+				 struct arch_elf_state *state)
+{
+	/*
+	 * Disable IBT for ia32
+	 */
+	if (elf_check_arch_ia32((struct elf32_hdr *)ehdr))
+		state->gnu_property &= ~GNU_PROPERTY_X86_FEATURE_1_IBT;
+	return 0;
+}
 
 /* Do not change the values. See get_align_mask() */
 enum align_flags {
-- 
2.21.0