[RFC PATCH v2 15/15] s390/unwind_user/backchain: Enable HAVE_UNWIND_USER_BACKCHAIN

[Jens Remus <jremus@linux.ibm.com>]

Enable unwinding of user space using back chain on s390.  Based on
arch_stack_walk_user_common() in arch/s390/kernel/stacktrace.c.

Note that an invalid RA obtained from the stack frame pointed to by the
back chain is not a valid indication that the IP is still early in the
function prologue and a fallback to the RA and SP register r14 and r15
contents should be made.

Signed-off-by: Jens Remus <jremus@linux.ibm.com>
---

Notes (jremus):
    Changes in RFC v2:
    - Adjusted to latest unwind user changes.
    - Use struct stack_frame_user and struct stack_frame_vdso_wrapper from
      asm/stacktrace.h.
    - In topmost frame do not fallback to RA (and SP) register values if
      RA is invalid.  This is not a valid indication for early prologue.
    - In topmost frame use RA and SP register values if they match those
      saved in the frame.  This indicates early prologue.

 arch/s390/Kconfig                        |   1 +
 arch/s390/kernel/Makefile                |   2 +
 arch/s390/kernel/unwind_user_backchain.c | 112 +++++++++++++++++++++++
 3 files changed, 115 insertions(+)
 create mode 100644 arch/s390/kernel/unwind_user_backchain.c

diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 52d3f3b3e086..5aeb2abd390f 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -246,6 +246,7 @@ config S390
 	select HAVE_SETUP_PER_CPU_AREA
 	select HAVE_SOFTIRQ_ON_OWN_STACK
 	select HAVE_SYSCALL_TRACEPOINTS
+	select HAVE_UNWIND_USER_BACKCHAIN
 	select HAVE_UNWIND_USER_SFRAME
 	select HAVE_VIRT_CPU_ACCOUNTING
 	select HAVE_VIRT_CPU_ACCOUNTING_IDLE
diff --git a/arch/s390/kernel/Makefile b/arch/s390/kernel/Makefile
index eb06ff888314..eb662e95c5fd 100644
--- a/arch/s390/kernel/Makefile
+++ b/arch/s390/kernel/Makefile
@@ -83,6 +83,8 @@ obj-$(CONFIG_PERF_EVENTS)	+= perf_pai_crypto.o perf_pai_ext.o
 
 obj-$(CONFIG_TRACEPOINTS)	+= trace.o
 
+obj-$(CONFIG_HAVE_UNWIND_USER_BACKCHAIN)	+= unwind_user_backchain.o
+
 # vdso
 obj-y				+= vdso64/
 obj-$(CONFIG_COMPAT)		+= vdso32/
diff --git a/arch/s390/kernel/unwind_user_backchain.c b/arch/s390/kernel/unwind_user_backchain.c
new file mode 100644
index 000000000000..4e10ca43ea36
--- /dev/null
+++ b/arch/s390/kernel/unwind_user_backchain.c
@@ -0,0 +1,112 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#define pr_fmt(fmt)	"backchain: " fmt
+
+#include <asm/asm-offsets.h>
+#include <asm/stacktrace.h>
+#include <linux/security.h>
+#include <linux/unwind_user.h>
+#include <linux/unwind_user_backchain.h>
+
+/**
+ * ip_invalid - Perform some basic checks whether an instruction pointer (IP)
+ * taken from an unreliable source is invalid
+ * @ip: The instruction pointer to be validated
+ *
+ * returns whether the instruction pointer is invalid
+ */
+static inline bool ip_invalid(unsigned long ip)
+{
+	/* Architecture requires IP to be 2-byte aligned. */
+	if (ip & 1)
+		return true;
+	if (ip < mmap_min_addr)
+		return true;
+	if (ip >= current->mm->context.asce_limit)
+		return true;
+	return false;
+}
+
+/**
+ * ip_within_vdso - Check whether an instruction pointer (IP) is within vDSO
+ * @ip: The instruction pointer
+ *
+ * returns whether the instruction pointer is within vDSO
+ */
+static inline bool ip_within_vdso(unsigned long ip)
+{
+	return in_range(ip, current->mm->context.vdso_base, vdso_text_size());
+}
+
+/**
+ * arch_unwind_user_next_backchain - Unwind one frame using s390 back chain
+ * @state: The unwind user state
+ *
+ * returns zero when successful, otherwise -EINVAL.
+ */
+int arch_unwind_user_next_backchain(struct unwind_user_state *state)
+{
+	struct stack_frame_user __user *sf;
+	unsigned long sp, ra;
+
+	sf = (void __user *)state->sp;
+
+	/*
+	 * In topmost frame check whether IP in early prologue, RA and SP
+	 * registers saved, and no new stack frame allocated.
+	 */
+	if (state->topmost) {
+		unsigned long ra_reg;
+
+		if (__get_user(ra, (unsigned long __user *)&sf->gprs[8]))
+			return -EINVAL;
+		if (__get_user(sp, (unsigned long __user *)&sf->gprs[9]))
+			return -EINVAL;
+		if (unwind_user_get_ra_reg(&ra_reg))
+			return -EINVAL;
+		if (ra == ra_reg && sp == state->sp)
+			goto done;
+	}
+
+	if (__get_user(sp, (unsigned long __user *)&sf->back_chain))
+		return -EINVAL;
+	if (!sp && ip_within_vdso(state->ip)) {
+		/*
+		 * Assume non-standard vDSO user wrapper stack frame.
+		 * See vDSO user wrapper code for details.
+		 */
+		struct stack_frame_vdso_wrapper *sf_vdso = (void __user *)sf;
+
+		if (__get_user(ra, (unsigned long __user *)&sf_vdso->return_address))
+			return -EINVAL;
+		sf = (void __user *)((unsigned long)sf + STACK_FRAME_VDSO_OVERHEAD);
+		if (__get_user(sp, (unsigned long __user *)&sf->back_chain))
+			return -EINVAL;
+	} else if (!sp) {
+		/* Assume outermost frame reached. */
+		state->done = true;
+		return 0;
+	} else {
+		/*
+		 * Assume IP past prologue and new stack frame allocated.
+		 * Follow back chain, which then equals the SP at entry.
+		 * Skips caller if wrong in topmost frame.
+		 */
+		sf = (void __user *)sp;
+		if (__get_user(ra, (unsigned long __user *)&sf->gprs[8]))
+			return -EINVAL;
+		/* Skip validation: ABI requires SP to be saved as well. */
+	}
+
+done:
+	/* Validate SP and RA (ABI requires SP to be 8-byte aligned). */
+	if (sp & 7 || ip_invalid(ra))
+		return -EINVAL;
+
+	state->ip = ra;
+	state->sp = sp;
+	state->fp = 0;		/* Cannot unwind FP. */
+	state->topmost = false;
+
+	return 0;
+}
-- 
2.51.0