[RFC PATCH v8 2/3] selftests/x86: sysret_rip: Add more tests to verify the 'syscall' behavior

[Ammar Faizi <ammarfaizi2@gnuweeb.org>]

There are two cases:

  A) 'syscall' in a FRED system preserves %rcx and %r11.

  B) 'syscall' in a non-FRED system sets %rcx=%rip and %r11=%rflags.

When the do_syscall() function is called for the first time, it will
memorize the behavior, either (A) or (B). Then, the next do_syscall()
call will verify that the 'syscall' behavior is the same.

Test them with trivial system calls like __NR_getppid and friends, which
are highly likely to return with SYSRET on an IDT system.

The purposes of this test are:

  - Ensure that the syscall behavior is consistent. It must always be
    (A) or always be (B). Not a mix of them.

  - Ensure that the kernel doesn't leak its internal data when returning
    to userspace.

Cc: Xin Li <xin3.li@intel.com>
Link: https://lore.kernel.org/lkml/25b96960-a07e-a952-5c23-786b55054126@zytor.com
Co-developed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
---
 tools/testing/selftests/x86/sysret_rip.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/x86/sysret_rip.c b/tools/testing/selftests/x86/sysret_rip.c
index 300104900192d396..1531593b50d02150 100644
--- a/tools/testing/selftests/x86/sysret_rip.c
+++ b/tools/testing/selftests/x86/sysret_rip.c
@@ -266,8 +266,21 @@ static void test_syscall_fallthrough_to(unsigned long ip)
 	printf("[OK]\tWe survived\n");
 }
 
+/* See the comment in do_syscall(). */
+static void test_syscall_rcx_r11_consistent(void)
+{
+	do_syscall(__NR_getpid, 0, 0, 0, 0, 0, 0);
+	do_syscall(__NR_gettid, 0, 0, 0, 0, 0, 0);
+	do_syscall(__NR_getppid, 0, 0, 0, 0, 0, 0);
+}
+
 int main()
 {
+	int i;
+
+	for (i = 0; i < 32; i++)
+		test_syscall_rcx_r11_consistent();
+
 	/*
 	 * When the kernel returns from a slow-path syscall, it will
 	 * detect whether SYSRET is appropriate.  If it incorrectly
@@ -275,7 +288,7 @@ int main()
 	 * it'll crash on Intel CPUs.
 	 */
 	sethandler(SIGUSR1, sigusr1, 0);
-	for (int i = 47; i < 64; i++)
+	for (i = 47; i < 64; i++)
 		test_sigreturn_to(1UL<<i);
 
 	clearhandler(SIGUSR1);
@@ -286,7 +299,7 @@ int main()
 	test_syscall_fallthrough_to((1UL << 47) - 2*PAGE_SIZE);
 
 	/* These are the interesting cases. */
-	for (int i = 47; i < 64; i++) {
+	for (i = 47; i < 64; i++) {
 		test_syscall_fallthrough_to((1UL<<i) - PAGE_SIZE);
 		test_syscall_fallthrough_to(1UL<<i);
 	}
-- 
Ammar Faizi