selftests/powerpc: Add a test of 4PB SLB handling

Add a test for a bug we had in the 4PB address space SLB handling. It
was fixed in commit 4c2de74c ("powerpc/64: Interrupts save PPR on
stack rather than thread_struct").
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220317143925.1030447-1-mpe@ellerman.id.au

tools/testing/selftests/powerpc/mm/.gitignore

@@ -12,3 +12,4 @@ pkey_exec_prot
 pkey_siginfo
 stack_expansion_ldst
 stack_expansion_signal
+large_vm_gpr_corruption

tools/testing/selftests/powerpc/mm/Makefile

@@ -4,7 +4,8 @@ noarg:
 
 TEST_GEN_PROGS := hugetlb_vs_thp_test subpage_prot prot_sao segv_errors wild_bctr \
 		  large_vm_fork_separation bad_accesses pkey_exec_prot \
-		  pkey_siginfo stack_expansion_signal stack_expansion_ldst
+		  pkey_siginfo stack_expansion_signal stack_expansion_ldst \
+		  large_vm_gpr_corruption
 TEST_PROGS := stress_code_patching.sh
 
 TEST_GEN_PROGS_EXTENDED := tlbie_test
@@ -19,6 +20,7 @@ $(OUTPUT)/prot_sao: ../utils.c
 
 $(OUTPUT)/wild_bctr: CFLAGS += -m64
 $(OUTPUT)/large_vm_fork_separation: CFLAGS += -m64
+$(OUTPUT)/large_vm_gpr_corruption: CFLAGS += -m64
 $(OUTPUT)/bad_accesses: CFLAGS += -m64
 $(OUTPUT)/pkey_exec_prot: CFLAGS += -m64
 $(OUTPUT)/pkey_siginfo: CFLAGS += -m64

tools/testing/selftests/powerpc/mm/large_vm_gpr_corruption.c

+// SPDX-License-Identifier: GPL-2.0+
+//
+// Copyright 2022, Michael Ellerman, IBM Corp.
+//
+// Test that the 4PB address space SLB handling doesn't corrupt userspace registers
+// (r9-r13) due to a SLB fault while saving the PPR.
+//
+// The bug was introduced in f384796c4 ("powerpc/mm: Add support for handling > 512TB
+// address in SLB miss") and fixed in 4c2de74cc869 ("powerpc/64: Interrupts save PPR on
+// stack rather than thread_struct").
+//
+// To hit the bug requires the task struct and kernel stack to be in different segments.
+// Usually that requires more than 1TB of RAM, or if that's not practical, boot the kernel
+// with "disable_1tb_segments".
+//
+// The test works by creating mappings above 512TB, to trigger the large address space
+// support. It creates 64 mappings, double the size of the SLB, to cause SLB faults on
+// each access (assuming naive replacement). It then loops over those mappings touching
+// each, and checks that r9-r13 aren't corrupted.
+//
+// It then forks another child and tries again, because a new child process will get a new
+// kernel stack and thread struct allocated, which may be more optimally placed to trigger
+// the bug. It would probably be better to leave the previous child processes hanging
+// around, so that kernel stack & thread struct allocations are not reused, but that would
+// amount to a 30 second fork bomb. The current design reliably triggers the bug on
+// unpatched kernels.
+
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#include "utils.h"
+
+#ifndef MAP_FIXED_NOREPLACE
+#define MAP_FIXED_NOREPLACE MAP_FIXED // "Should be safe" above 512TB
+#endif
+
+#define BASE_ADDRESS (1ul << 50) // 1PB
+#define STRIDE	     (2ul << 40) // 2TB
+#define SLB_SIZE     32
+#define NR_MAPPINGS  (SLB_SIZE * 2)
+
+static volatile sig_atomic_t signaled;
+
+static void signal_handler(int sig)
+{
+	signaled = 1;
+}
+
+#define CHECK_REG(_reg)                                                                \
+	if (_reg != _reg##_orig) {                                                     \
+		printf(str(_reg) " corrupted! Expected 0x%lx != 0x%lx\n", _reg##_orig, \
+		       _reg);                                                          \
+		_exit(1);                                                              \
+	}
+
+static int touch_mappings(void)
+{
+	unsigned long r9_orig, r10_orig, r11_orig, r12_orig, r13_orig;
+	unsigned long r9, r10, r11, r12, r13;
+	unsigned long addr, *p;
+	int i;
+
+	for (i = 0; i < NR_MAPPINGS; i++) {
+		addr = BASE_ADDRESS + (i * STRIDE);
+		p = (unsigned long *)addr;
+
+		asm volatile("mr   %0, %%r9	;" // Read original GPR values
+			     "mr   %1, %%r10	;"
+			     "mr   %2, %%r11	;"
+			     "mr   %3, %%r12	;"
+			     "mr   %4, %%r13	;"
+			     "std %10, 0(%11)   ;" // Trigger SLB fault
+			     "mr   %5, %%r9	;" // Save possibly corrupted values
+			     "mr   %6, %%r10	;"
+			     "mr   %7, %%r11	;"
+			     "mr   %8, %%r12	;"
+			     "mr   %9, %%r13	;"
+			     "mr   %%r9,  %0	;" // Restore original values
+			     "mr   %%r10, %1	;"
+			     "mr   %%r11, %2	;"
+			     "mr   %%r12, %3	;"
+			     "mr   %%r13, %4	;"
+			     : "=&b"(r9_orig), "=&b"(r10_orig), "=&b"(r11_orig),
+			       "=&b"(r12_orig), "=&b"(r13_orig), "=&b"(r9), "=&b"(r10),
+			       "=&b"(r11), "=&b"(r12), "=&b"(r13)
+			     : "b"(i), "b"(p)
+			     : "r9", "r10", "r11", "r12", "r13");
+
+		CHECK_REG(r9);
+		CHECK_REG(r10);
+		CHECK_REG(r11);
+		CHECK_REG(r12);
+		CHECK_REG(r13);
+	}
+
+	return 0;
+}
+
+static int test(void)
+{
+	unsigned long page_size, addr, *p;
+	struct sigaction action;
+	bool hash_mmu;
+	int i, status;
+	pid_t pid;
+
+	// This tests a hash MMU specific bug.
+	FAIL_IF(using_hash_mmu(&hash_mmu));
+	SKIP_IF(!hash_mmu);
+
+	page_size = sysconf(_SC_PAGESIZE);
+
+	for (i = 0; i < NR_MAPPINGS; i++) {
+		addr = BASE_ADDRESS + (i * STRIDE);
+
+		p = mmap((void *)addr, page_size, PROT_READ | PROT_WRITE,
+			 MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED_NOREPLACE, -1, 0);
+		if (p == MAP_FAILED) {
+			perror("mmap");
+			printf("Error: couldn't mmap(), confirm kernel has 4PB support?\n");
+			return 1;
+		}
+	}
+
+	action.sa_handler = signal_handler;
+	action.sa_flags = SA_RESTART;
+	FAIL_IF(sigaction(SIGALRM, &action, NULL) < 0);
+
+	// Seen to always crash in under ~10s on affected kernels.
+	alarm(30);
+
+	while (!signaled) {
+		// Fork new processes, to increase the chance that we hit the case where
+		// the kernel stack and task struct are in different segments.
+		pid = fork();
+		if (pid == 0)
+			exit(touch_mappings());
+
+		FAIL_IF(waitpid(-1, &status, 0) == -1);
+		FAIL_IF(WIFSIGNALED(status));
+		FAIL_IF(!WIFEXITED(status));
+		FAIL_IF(WEXITSTATUS(status));
+	}
+
+	return 0;
+}
+
+int main(void)
+{
+	return test_harness(test, "large_vm_gpr_corruption");
+}