[ 010/123] arm64: signal: push the unwinding prologue on the signal stack

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

3.7-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Will Deacon <will.deacon@arm.com>

commit 304ef4e8367244b547734143c792a2ab764831e8 upstream.

To allow debuggers to unwind through signal frames, we create a fake
stack unwinding prologue containing the link register and frame pointer
of the interrupted context. The signal frame is then offset by 16 bytes
to make room for the two saved registers which are pushed onto the frame
of the *interrupted* context, rather than placed directly above the
signal stack.

This doesn't work when an alternative signal stack is set up for a SEGV
handler, which is raised in response to RLIMIT_STACK being reached. In
this case, we try to push the unwinding prologue onto the full stack and
subsequently take a fault which we fail to resolve, causing setup_return
to return -EFAULT and handle_signal to force_sigsegv on the current task.

This patch fixes the problem by including the unwinding prologue as part
of the rt_sigframe definition, which is populated during setup_sigframe,
ensuring that it always ends up on the signal stack.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 arch/arm64/kernel/signal.c |   37 ++++++++++++++++---------------------
 1 file changed, 16 insertions(+), 21 deletions(-)

--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -41,6 +41,8 @@
 struct rt_sigframe {
 	struct siginfo info;
 	struct ucontext uc;
+	u64 fp;
+	u64 lr;
 };
 
 static int preserve_fpsimd_context(struct fpsimd_context __user *ctx)
@@ -175,6 +177,10 @@ static int setup_sigframe(struct rt_sigf
 	struct aux_context __user *aux =
 		(struct aux_context __user *)sf->uc.uc_mcontext.__reserved;
 
+	/* set up the stack frame for unwinding */
+	__put_user_error(regs->regs[29], &sf->fp, err);
+	__put_user_error(regs->regs[30], &sf->lr, err);
+
 	for (i = 0; i < 31; i++)
 		__put_user_error(regs->regs[i], &sf->uc.uc_mcontext.regs[i],
 				 err);
@@ -210,9 +216,6 @@ static void __user *get_sigframe(struct
 	if ((ka->sa.sa_flags & SA_ONSTACK) && !sas_ss_flags(sp))
 		sp = sp_top = current->sas_ss_sp + current->sas_ss_size;
 
-	/* room for stack frame (FP, LR) */
-	sp -= 16;
-
 	sp = (sp - framesize) & ~15;
 	frame = (void __user *)sp;
 
@@ -225,20 +228,14 @@ static void __user *get_sigframe(struct
 	return frame;
 }
 
-static int setup_return(struct pt_regs *regs, struct k_sigaction *ka,
-			void __user *frame, int usig)
+static void setup_return(struct pt_regs *regs, struct k_sigaction *ka,
+			 void __user *frame, int usig)
 {
-	int err = 0;
 	__sigrestore_t sigtramp;
-	unsigned long __user *sp = (unsigned long __user *)regs->sp;
-
-	/* set up the stack frame */
-	__put_user_error(regs->regs[29], sp - 2, err);
-	__put_user_error(regs->regs[30], sp - 1, err);
 
 	regs->regs[0] = usig;
-	regs->regs[29] = regs->sp - 16;
 	regs->sp = (unsigned long)frame;
+	regs->regs[29] = regs->sp + offsetof(struct rt_sigframe, fp);
 	regs->pc = (unsigned long)ka->sa.sa_handler;
 
 	if (ka->sa.sa_flags & SA_RESTORER)
@@ -247,8 +244,6 @@ static int setup_return(struct pt_regs *
 		sigtramp = VDSO_SYMBOL(current->mm->context.vdso, sigtramp);
 
 	regs->regs[30] = (unsigned long)sigtramp;
-
-	return err;
 }
 
 static int setup_rt_frame(int usig, struct k_sigaction *ka, siginfo_t *info,
@@ -272,13 +267,13 @@ static int setup_rt_frame(int usig, stru
 	err |= __copy_to_user(&frame->uc.uc_stack, &stack, sizeof(stack));
 
 	err |= setup_sigframe(frame, regs, set);
-	if (err == 0)
-		err = setup_return(regs, ka, frame, usig);
-
-	if (err == 0 && ka->sa.sa_flags & SA_SIGINFO) {
-		err |= copy_siginfo_to_user(&frame->info, info);
-		regs->regs[1] = (unsigned long)&frame->info;
-		regs->regs[2] = (unsigned long)&frame->uc;
+	if (err == 0) {
+		setup_return(regs, ka, frame, usig);
+		if (ka->sa.sa_flags & SA_SIGINFO) {
+			err |= copy_siginfo_to_user(&frame->info, info);
+			regs->regs[1] = (unsigned long)&frame->info;
+			regs->regs[2] = (unsigned long)&frame->uc;
+		}
 	}
 
 	return err;