The current selftest asserts %r11 == %rflags after the 'syscall' returns to userspace. However, such an assertion doesn't apply to the Intel FRED system because, in that system, the 'syscall' instruction does not set %r11=%rflags and %rcx=%rip.

Handle the FRED system case. Now, test that:

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

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

Note for the '__raise()' helper: Those tests must manipulate registers before the 'syscall' instruction is invoked. However, the current 'raise()' function from libc cannot accomplish it. Therefore, create a syscall wrapper in inline Assembly to 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 | 122 +++++++++++++++++++++-- 1 file changed, 115 insertions(+), 7 deletions(-)

diff --git a/tools/testing/selftests/x86/sysret_rip.c b/tools/testing/selftests/x86/sysret_rip.c index 84d74be1d90207ab..300104900192d396 100644 --- a/tools/testing/selftests/x86/sysret_rip.c +++ b/tools/testing/selftests/x86/sysret_rip.c @@ -39,6 +39,112 @@ 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, + REGS_SAVED = 0, /* Registers properly preserved (Intel FRED). */ + REGS_SYSRET = 1 /* Registers match syscall/sysret. */ +}; + +/* + * @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; + + 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); + } +} + +/* + * 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, + * check_regs_result() will memorize the behavior, either (A) or (B). + * Then, the next do_syscall() call will verify that the 'syscall' + * behavior is the same. + * + * This function needs to be called at least twice to assert. + */ +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) +{ + unsigned long rbx; + unsigned long rcx = rcx_sentinel; + register unsigned long r11 __asm__("%r11") = r11_sentinel; + register unsigned long r10 __asm__("%r10") = arg4; + register unsigned long r8 __asm__("%r8") = arg5; + register unsigned long r9 __asm__("%r9") = arg6; + + __asm__ volatile ( + "movq -8(%%rsp), %%r12\n\t" // Do not clobber the red zone. + "pushq %[rflags_sentinel]\n\t" + "popfq\n\t" + "movq %%r12, -8(%%rsp)\n\t" + "leaq 1f(%%rip), %[rbx]\n\t" + "syscall\n" + "1:" + + : "+a" (nr_syscall), + "+r" (r11), + "+c" (rcx), + [rbx] "=b" (rbx) + + : [rflags_sentinel] "g" (rflags_sentinel), + "D" (arg1), /* %rdi */ + "S" (arg2), /* %rsi */ + "d" (arg3), /* %rdx */ + "r" (r10), + "r" (r8), + "r" (r9) + + : "r12", "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 +194,26 @@ 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;

The current test_sigreturn_to() goes to the slow-path syscall with IRET due to non-canonical addresses. It uses the SIGUSR1 signal to perform the test.

Add a similar test that goes to the SYSRET path instead of IRET using the SIGUSR2 signal. 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.

The __raise(SIGUSR2) call verifies the 'syscall' behavior consistency when dealing with a signal handler. It must always be (A) or always be (B). Not a mix of them.

Cc: Xin Li xin3.li@intel.com Link: https://lore.kernel.org/lkml/8770815f-0f23-d0c5-e56a-d401827842c9@zytor.com Suggested-by: H. Peter Anvin (Intel) hpa@zytor.com Signed-off-by: Ammar Faizi ammarfaizi2@gnuweeb.org --- tools/testing/selftests/x86/sysret_rip.c | 30 ++++++++++++++++++++++++ 1 file changed, 30 insertions(+)

diff --git a/tools/testing/selftests/x86/sysret_rip.c b/tools/testing/selftests/x86/sysret_rip.c index 1531593b50d02150..746801675fe77e9c 100644 --- a/tools/testing/selftests/x86/sysret_rip.c +++ b/tools/testing/selftests/x86/sysret_rip.c @@ -274,6 +274,28 @@ static void test_syscall_rcx_r11_consistent(void) do_syscall(__NR_getppid, 0, 0, 0, 0, 0, 0); }

+static unsigned long usr2_rcx; +static unsigned long usr2_r11; + +static void sigusr2(int sig, siginfo_t *info, void *ctx_void) +{ + ucontext_t *ctx = (ucontext_t*)ctx_void; + + usr2_r11 = ctx->uc_mcontext.gregs[REG_R11]; + usr2_rcx = ctx->uc_mcontext.gregs[REG_RCX]; + + check_regs_result(ctx->uc_mcontext.gregs[REG_R11], + ctx->uc_mcontext.gregs[REG_RCX], + ctx->uc_mcontext.gregs[REG_RBX]); +} + +static void test_sysret_consistent(void) +{ + printf("[RUN]\ttest_sysret_consistent\n"); + __raise(SIGUSR2); + printf("[OK]\tRCX = %#lx; R11 = %#lx\n", usr2_rcx, usr2_r11); +} + int main() { int i; @@ -291,6 +313,14 @@ int main() for (i = 47; i < 64; i++) test_sigreturn_to(1UL<<i);

+ /* + * test_sigreturn_to() above will test the IRET path. Now test + * the SYSRET path. + */ + sethandler(SIGUSR2, sigusr2, 0); + for (i = 0; i < 32; i++) + test_sysret_consistent(); + clearhandler(SIGUSR1);

sethandler(SIGSEGV, sigsegv_for_fallthrough, 0);