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@xxxxxxxxx>
Link: https://lore.kernel.org/lkml/25b96960-a07e-a952-5c23-786b55054126@xxxxxxxxx
Co-developed-by: H. Peter Anvin (Intel) <hpa@xxxxxxxxx>
Signed-off-by: H. Peter Anvin (Intel) <hpa@xxxxxxxxx>
Signed-off-by: Ammar Faizi <ammarfaizi2@xxxxxxxxxxx>
---
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
