[RFC PATCH v6 1/3] selftests/x86: sysret_rip: Handle syscall in a FRED system

Ammar Faizi
2023-01-25T21:17:45Z

The current selftest asserts (%r11 == %rflags) after the 'syscall'
returns to user. Such an assertion doesn't apply to the FRED system
because in that system the 'syscall' instruction does not set
%r11=%rflags and %rcx=%rip.

Handle the FRED case. Now, test that:

- "syscall" in a FRED system doesn't clobber %rcx and %r11.
- "syscall" in a non-FRED system sets %rcx=%rip and %r11=%rflags.

The 'raise()' function from libc can't be used to control those
registers. Therefore, create a syscall wrapper in inline Assembly to
fully control them.

Fixes: 660602140103 ("selftests/x86: Add a selftest for SYSRET to noncanonical addresses")
Link: https://lore.kernel.org/lkml/25b96960-a07e-a952-5c23-786b55054126@zytor.com
Reported-by: Xin Li <xin3.li@intel.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 | 120 +++++++++++++++++++++--
1 file changed, 113 insertions(+), 7 deletions(-)

diff --git a/tools/testing/selftests/x86/sysret_rip.c b/tools/testing/selftests/x86/sysret_rip.c
index 84d74be1d90207ab..100f55981d77a29b 100644
--- a/tools/testing/selftests/x86/sysret_rip.c
+++ b/tools/testing/selftests/x86/sysret_rip.c
@@ -39,6 +39,110 @@ asm (
extern const char test_page[];
static void const *current_test_page_addr = test_page;

+/* Arbitrary values */
+static const unsigned long r11_sentinel = 0xfeedfacedeadbeef;
+static const unsigned long rcx_sentinel = 0x5ca1ab1e0b57ac1e;
+
+/* An arbitrary *valid* RFLAGS value */
+static const unsigned long rflags_sentinel = 0x200a93;
+
+enum regs_ok {
+ REGS_UNDEFINED = -1, /* For consistency checker init, never returned */
+ REGS_SAVED = 0, /* Registers properly preserved */
+ REGS_SYSRET = 1 /* Registers match syscall/sysret */
+};
+
+/*
+ * REGS_SAVED = %rcx and %r11 preserved.
+ * REGS_SYSRET = %rcx and %r11 set to %rflags and %rip.
+ * REGS_ERROR = %rcx and/or %r11 set to any other values.
+ *
+ * @rbx should be set to the syscall return %rip.
+ */
+static void check_regs_result(unsigned long r11, unsigned long rcx,
+ unsigned long rbx)
+{
+ static enum regs_ok regs_ok_state = REGS_UNDEFINED;
+ enum regs_ok ret;
+
+ /*
+ * Test that:
+ *
+ * - "syscall" in a FRED system doesn't clobber %rcx and %r11.
+ * - "syscall" in a non-FRED system sets %rcx=%rip and %r11=%rflags.
+ */
+ if (r11 == r11_sentinel && rcx == rcx_sentinel) {
+ ret = REGS_SAVED;
+ } else if (r11 == rflags_sentinel && rcx == rbx) {
+ ret = REGS_SYSRET;
+ } else {
+ printf("[FAIL] check_regs_result\n");
+ printf(" r11_sentinel = %#lx; %%r11 = %#lx;\n", r11_sentinel, r11);
+ printf(" rcx_sentinel = %#lx; %%rcx = %#lx;\n", rcx_sentinel, rcx);
+ printf(" rflags_sentinel = %#lx\n", rflags_sentinel);
+ exit(1);
+ }
+
+
+ /*
+ * Test that we don't get a mix of REGS_SAVED and REGS_SYSRET.
+ * It needs at least calling check_regs_result() twice to assert.
+ */
+ if (regs_ok_state == REGS_UNDEFINED) {
+ /*
+ * First time calling check_regs_result().
+ */
+ regs_ok_state = ret;
+ } else {
+ assert(regs_ok_state == ret);
+ }
+}
+
+static long do_syscall(long nr_syscall, unsigned long arg1, unsigned long arg2,
+ unsigned long arg3, unsigned long arg4,
+ unsigned long arg5, unsigned long arg6)
+{
+ register unsigned long r11 asm("%r11");
+ register unsigned long r10 asm("%r10");
+ register unsigned long r8 asm("%r8");
+ register unsigned long r9 asm("%r9");
+ register void *rsp asm("%rsp");
+ unsigned long rcx, rbx;
+
+ r11 = r11_sentinel;
+ rcx = rcx_sentinel;
+ r10 = arg4;
+ r8 = arg5;
+ r9 = arg6;
+
+ asm volatile (
+ "pushq %[rflags_sentinel]\n\t"
+ "popf\n\t"
+ "leaq 1f(%%rip), %[rbx]\n\t"
+ "syscall\n"
+ "1:"
+
+ : "+a" (nr_syscall),
+ "+r" (r11),
+ "+c" (rcx),
+ [rbx] "=b" (rbx),
+ "+r" (rsp) /* Clobber the redzone */
+
+ : [rflags_sentinel] "g" (rflags_sentinel),
+ "D" (arg1), /* %rdi */
+ "S" (arg2), /* %rsi */
+ "d" (arg3), /* %rdx */
+ "r" (r10),
+ "r" (r8),
+ "r" (r9)
+
+ : "memory"
+ );
+
+ check_regs_result(r11, rcx, rbx);
+ return nr_syscall;
+}
+
static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
int flags)
{
@@ -88,24 +192,26 @@ static void sigusr1(int sig, siginfo_t *info, void *ctx_void)

memcpy(&initial_regs, &ctx->uc_mcontext.gregs, sizeof(gregset_t));

+ check_regs_result(ctx->uc_mcontext.gregs[REG_R11],
+ ctx->uc_mcontext.gregs[REG_RCX],
+ ctx->uc_mcontext.gregs[REG_RBX]);
+
/* Set IP and CX to match so that SYSRET can happen. */
ctx->uc_mcontext.gregs[REG_RIP] = rip;
ctx->uc_mcontext.gregs[REG_RCX] = rip;
-
- /* R11 and EFLAGS should already match. */
- assert(ctx->uc_mcontext.gregs[REG_EFL] ==
- ctx->uc_mcontext.gregs[REG_R11]);
-
sethandler(SIGSEGV, sigsegv_for_sigreturn_test, SA_RESETHAND);
+}

- return;
+static void __raise(int sig)
+{
+ do_syscall(__NR_kill, getpid(), sig, 0, 0, 0, 0);
}

static void test_sigreturn_to(unsigned long ip)
{
rip = ip;
printf("[RUN]\tsigreturn to 0x%lx\n", ip);
- raise(SIGUSR1);
+ __raise(SIGUSR1);
}

static jmp_buf jmpbuf;
--
Ammar Faizi