powerpc/signal64: Remove non-inline calls from setup_sigcontext()

The majority of setup_sigcontext() can be refactored to execute in an
"unsafe" context assuming an open uaccess window except for some
non-inline function calls. Move these out into a separate
prepare_setup_sigcontext() function which must be called first and
before opening up a uaccess window. Non-inline function calls should be
avoided during a uaccess window for a few reasons:

	- KUAP should be enabled for as much kernel code as possible.
	  Opening a uaccess window disables KUAP which means any code
	  executed during this time contributes to a potential attack
	  surface.

	- Non-inline functions default to traceable which means they are
	  instrumented for ftrace. This adds more code which could run
	  with KUAP disabled.

	- Powerpc does not currently support the objtool UACCESS checks.
	  All code running with uaccess must be audited manually which
	  means: less code -> less work -> fewer problems (in theory).

A follow-up commit converts setup_sigcontext() to be "unsafe".

Signed-off-by: Christopher M. Riedl <cmr@codefail.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210227011259.11992-4-cmr@codefail.de

diff --git a/arch/powerpc/kernel/signal_64.c b/arch/powerpc/kernel/signal_64.c
index f9e4a1ac440fb094718ee6402f38762a6e9c45de..6ca546192cbfbe5ab60ac889f1fd1b433bc91b07 100644 (file)
--- a/arch/powerpc/kernel/signal_64.c
+++ b/arch/powerpc/kernel/signal_64.c
@@ -79,6 +79,24 @@ static elf_vrreg_t __user *sigcontext_vmx_regs(struct sigcontext __user *sc)
 }
 #endif
 
+static void prepare_setup_sigcontext(struct task_struct *tsk)
+{
+#ifdef CONFIG_ALTIVEC
+       /* save altivec registers */
+       if (tsk->thread.used_vr)
+               flush_altivec_to_thread(tsk);
+       if (cpu_has_feature(CPU_FTR_ALTIVEC))
+               tsk->thread.vrsave = mfspr(SPRN_VRSAVE);
+#endif /* CONFIG_ALTIVEC */
+
+       flush_fp_to_thread(tsk);
+
+#ifdef CONFIG_VSX
+       if (tsk->thread.used_vsr)
+               flush_vsx_to_thread(tsk);
+#endif /* CONFIG_VSX */
+}
+
 /*
  * Set up the sigcontext for the signal frame.
  */
@@ -97,7 +115,6 @@ static long setup_sigcontext(struct sigcontext __user *sc,
         */
 #ifdef CONFIG_ALTIVEC
        elf_vrreg_t __user *v_regs = sigcontext_vmx_regs(sc);
-       unsigned long vrsave;
 #endif
        struct pt_regs *regs = tsk->thread.regs;
        unsigned long msr = regs->msr;
@@ -112,7 +129,6 @@ static long setup_sigcontext(struct sigcontext __user *sc,
 
        /* save altivec registers */
        if (tsk->thread.used_vr) {
-               flush_altivec_to_thread(tsk);
                /* Copy 33 vec registers (vr0..31 and vscr) to the stack */
                err |= __copy_to_user(v_regs, &tsk->thread.vr_state,
                                      33 * sizeof(vector128));
@@ -124,17 +140,10 @@ static long setup_sigcontext(struct sigcontext __user *sc,
        /* We always copy to/from vrsave, it's 0 if we don't have or don't
         * use altivec.
         */
-       vrsave = 0;
-       if (cpu_has_feature(CPU_FTR_ALTIVEC)) {
-               vrsave = mfspr(SPRN_VRSAVE);
-               tsk->thread.vrsave = vrsave;
-       }
-
-       err |= __put_user(vrsave, (u32 __user *)&v_regs[33]);
+       err |= __put_user(tsk->thread.vrsave, (u32 __user *)&v_regs[33]);
 #else /* CONFIG_ALTIVEC */
        err |= __put_user(0, &sc->v_regs);
 #endif /* CONFIG_ALTIVEC */
-       flush_fp_to_thread(tsk);
        /* copy fpr regs and fpscr */
        err |= copy_fpr_to_user(&sc->fp_regs, tsk);
 
@@ -150,7 +159,6 @@ static long setup_sigcontext(struct sigcontext __user *sc,
         * VMX data.
         */
        if (tsk->thread.used_vsr && ctx_has_vsx_region) {
-               flush_vsx_to_thread(tsk);
                v_regs += ELF_NVRREG;
                err |= copy_vsx_to_user(v_regs, tsk);
                /* set MSR_VSX in the MSR value in the frame to
@@ -655,6 +663,7 @@ SYSCALL_DEFINE3(swapcontext, struct ucontext __user *, old_ctx,
                ctx_has_vsx_region = 1;
 
        if (old_ctx != NULL) {
+               prepare_setup_sigcontext(current);
                if (!access_ok(old_ctx, ctx_size)
                    || setup_sigcontext(&old_ctx->uc_mcontext, current, 0, NULL, 0,
                                        ctx_has_vsx_region)
@@ -842,6 +851,7 @@ int handle_rt_signal64(struct ksignal *ksig, sigset_t *set,
 #endif
        {
                err |= __put_user(0, &frame->uc.uc_link);
+               prepare_setup_sigcontext(tsk);
                err |= setup_sigcontext(&frame->uc.uc_mcontext, tsk, ksig->sig,
                                        NULL, (unsigned long)ksig->ka.sa.sa_handler,
                                        1);