[PATCH v27 04/10] x86/cet/ibt: Disable IBT for ia32
From: Yu-cheng Yu
Date: Fri May 21 2021 - 18:17:45 EST
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>
Cc: 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 2fe94efe1c0c..eef78b795b1e 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>
@@ -399,7 +400,17 @@ 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