[RFC UKL 07/10] x86/signal: Adjust signal handler register values and return frame

[Ali Raza <aliraza@bu.edu>]

For a UKL thread, returning to a signal handler is not done with iret or
sysret.  This means we need to adjust the way the return stack frame is
handled for these threads.  When constructing the signal frame, we leave
the previous frame in place because we will return to it from the signal
handler.  We also leave space for pushing eflags and the return address.
UKL threads will only use the __KERNEL_DS value in the ss register and 0xC3
in the cs register.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Michal Marek <michal.lkml@markovi.net>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>

Co-developed-by: Eric B Munson <munsoner@bu.edu>
Signed-off-by: Eric B Munson <munsoner@bu.edu>
Co-developed-by: Ali Raza <aliraza@bu.edu>
Signed-off-by: Ali Raza <aliraza@bu.edu>
---
 arch/x86/kernel/signal.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index 9c7265b524c7..a95c12f6dac6 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -121,8 +121,10 @@ static bool restore_sigcontext(struct pt_regs *regs,
 #endif /* CONFIG_X86_64 */
 
 	/* Get CS/SS and force CPL3 */
-	regs->cs = sc.cs | 0x03;
-	regs->ss = sc.ss | 0x03;
+	if (!is_ukl_thread()) {
+		regs->cs = sc.cs | 0x03;
+		regs->ss = sc.ss | 0x03;
+	}
 
 	regs->flags = (regs->flags & ~FIX_EFLAGS) | (sc.flags & FIX_EFLAGS);
 	/* disable syscall checks */
@@ -522,10 +524,15 @@ static int __setup_rt_frame(int sig, struct ksignal *ksig,
 	 * a trampoline.)  So we do our best: if the old SS was valid,
 	 * we keep it.  Otherwise we replace it.
 	 */
-	regs->cs = __USER_CS;
+	if (!is_ukl_thread()) {
+		regs->cs = __USER_CS;
 
-	if (unlikely(regs->ss != __USER_DS))
-		force_valid_ss(regs);
+		if (unlikely(regs->ss != __USER_DS))
+			force_valid_ss(regs);
+	} else {
+		regs->cs = 0xC3;
+		regs->ss = __KERNEL_DS;
+	}
 
 	return 0;
 
@@ -662,7 +669,10 @@ SYSCALL_DEFINE0(rt_sigreturn)
 	sigset_t set;
 	unsigned long uc_flags;
 
-	frame = (struct rt_sigframe __user *)(regs->sp - sizeof(long));
+	if (is_ukl_thread())
+		frame = (struct rt_sigframe __user *)(regs->sp + sizeof(long));
+	else
+		frame = (struct rt_sigframe __user *)(regs->sp - sizeof(long));
 	if (!access_ok(frame, sizeof(*frame)))
 		goto badframe;
 	if (__get_user(*(__u64 *)&set, (__u64 __user *)&frame->uc.uc_sigmask))
-- 
2.21.3