[PATCH 4/7] x86: ban 32-bit sigreturn when user IBT enabled

[Richard Patel <ripatel@wii.dev>]

IBT enforces that indirect branch targets land on an endbr
instruction. The CPU enforces this by setting the 'WAIT_FOR_ENDBR'
bit after executing an indirect branch/jump.

The only relevant edge case with user IBT is signal handling:
When entering/leaving a signal handler, the WAIT_FOR_ENDBR bit must
be backed up/restored.

IBT is not implemented for 32-bit and cannot be enabled using a
32-bit syscall. However, a 64-bit thread could far jump into 32-bit.
Therefore, 32-bit sigreturn must be banned until IBT supports that
environment.

Signed-off-by: Richard Patel <ripatel@wii.dev>
Based-on-patch-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Link: https://lwn.net/ml/linux-kernel/20210830182221.3535-5-yu-cheng.yu@intel.com/
---
 arch/x86/kernel/signal_32.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/x86/kernel/signal_32.c b/arch/x86/kernel/signal_32.c
index e55cf19e68fe..7cb76d794366 100644
--- a/arch/x86/kernel/signal_32.c
+++ b/arch/x86/kernel/signal_32.c
@@ -143,6 +143,11 @@ static bool ia32_restore_sigcontext(struct pt_regs *regs,
 	regs->ds = fixup_rpl(sc.ds);
 #endif
 
+#ifdef CONFIG_X86_USER_IBT
+	if (current->thread.ibt)
+		return false;
+#endif
+
 	return fpu__restore_sig(compat_ptr(sc.fpstate), 1);
 }
 
-- 
2.47.3