kselftest: arm64: fake_sigreturn_bad_size

Add a simple fake_sigreturn testcase which builds a ucontext_t with a
badly sized header that causes a overrun in the __reserved area and
place it onto the stack. Expects a SIGSEGV on test PASS.
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

tools/testing/selftests/arm64/signal/testcases/fake_sigreturn_bad_size.c

+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2019 ARM Limited
+ *
+ * Place a fake sigframe on the stack including a bad record overflowing
+ * the __reserved space: on sigreturn Kernel must spot this attempt and
+ * the test case is expected to be terminated via SEGV.
+ */
+
+#include <signal.h>
+#include <ucontext.h>
+
+#include "test_signals_utils.h"
+#include "testcases.h"
+
+struct fake_sigframe sf;
+
+#define MIN_SZ_ALIGN	16
+
+static int fake_sigreturn_bad_size_run(struct tdescr *td,
+				       siginfo_t *si, ucontext_t *uc)
+{
+	size_t resv_sz, need_sz, offset;
+	struct _aarch64_ctx *shead = GET_SF_RESV_HEAD(sf), *head;
+
+	/* just to fill the ucontext_t with something real */
+	if (!get_current_context(td, &sf.uc))
+		return 1;
+
+	resv_sz = GET_SF_RESV_SIZE(sf);
+	/* at least HDR_SZ + bad sized esr_context needed */
+	need_sz = sizeof(struct esr_context) + HDR_SZ;
+	head = get_starting_head(shead, need_sz, resv_sz, &offset);
+	if (!head)
+		return 0;
+
+	/*
+	 * Use an esr_context to build a fake header with a
+	 * size greater then the free __reserved area minus HDR_SZ;
+	 * using ESR_MAGIC here since it is not checked for size nor
+	 * is limited to one instance.
+	 *
+	 * At first inject an additional normal esr_context
+	 */
+	head->magic = ESR_MAGIC;
+	head->size = sizeof(struct esr_context);
+	/* and terminate properly */
+	write_terminator_record(GET_RESV_NEXT_HEAD(head));
+	ASSERT_GOOD_CONTEXT(&sf.uc);
+
+	/*
+	 * now mess with fake esr_context size: leaving less space than
+	 * needed while keeping size value 16-aligned
+	 *
+	 * It must trigger a SEGV from Kernel on:
+	 *
+	 *	resv_sz - offset < sizeof(*head)
+	 */
+	/* at first set the maximum good 16-aligned size */
+	head->size = (resv_sz - offset - need_sz + MIN_SZ_ALIGN) & ~0xfUL;
+	/* plus a bit more of 16-aligned sized stuff */
+	head->size += MIN_SZ_ALIGN;
+	/* and terminate properly */
+	write_terminator_record(GET_RESV_NEXT_HEAD(head));
+	ASSERT_BAD_CONTEXT(&sf.uc);
+	fake_sigreturn(&sf, sizeof(sf), 0);
+
+	return 1;
+}
+
+struct tdescr tde = {
+		.name = "FAKE_SIGRETURN_BAD_SIZE",
+		.descr = "Triggers a sigreturn with a overrun __reserved area",
+		.sig_ok = SIGSEGV,
+		.timeout = 3,
+		.run = fake_sigreturn_bad_size_run,
+};