Re: [PATCH v3] x86/asm/entry/32, selftests: Add test_syscall_vdso test

[Kees Cook]

On Mon, Jun 15, 2015 at 6:30 AM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
> The test checks that all registers are preserved across
> 32-bit syscalls. It tests syscalls through VDSO
> (if available) and through INT 0x80, normally
> and under ptrace.
>
> If kernel is a 64-bit one, high registers (r8..r15) are poisoned
> before call and checked afterwards. They must be either preserved,
> or cleared to zero (but r11 is special); r12..15 must be preserved
> for INT 0x80.
>
> EFLAGS is checked for changes too, but change there is not considered
> to be a bug (paravirt kernels do not preserve arithmetic flags).
>
> Run-tested on 64-bit kernel:
>
> $ ./test_syscall_vdso_32
> [RUN]   Executing 6-argument 32-bit syscall via VDSO
> [OK]    Arguments are preserved across syscall
> [NOTE]  R11 has changed:0000000000200ed7 - assuming clobbered by SYSRET insn
> [OK]    R8..R15 did not leak kernel data
> [RUN]   Executing 6-argument 32-bit syscall via INT 80
> [OK]    Arguments are preserved across syscall
> [OK]    R8..R15 did not leak kernel data
> [RUN]   Running tests under ptrace
> [RUN]   Executing 6-argument 32-bit syscall via VDSO
> [OK]    Arguments are preserved across syscall
> [OK]    R8..R15 did not leak kernel data
> [RUN]   Executing 6-argument 32-bit syscall via INT 80
> [OK]    Arguments are preserved across syscall
> [OK]    R8..R15 did not leak kernel data
>
> On 32-bit paravirt kernel:
>
> $ ./test_syscall_vdso_32
> [NOTE]  Not a 64-bit kernel, won't test R8..R15 leaks
> [RUN]   Executing 6-argument 32-bit syscall via VDSO
> [WARN]  Flags before=0000000000200ed7 id 0 00 o d i s z 0 a 0 p 1 c
> [WARN]  Flags  after=0000000000200246 id 0 00 i z 0 0 p 1
> [WARN]  Flags change=0000000000000c91 0 00 o d s 0 a 0 0 c
> [OK]    Arguments are preserved across syscall
> [RUN]   Executing 6-argument 32-bit syscall via INT 80
> [OK]    Arguments are preserved across syscall
> [RUN]   Running tests under ptrace
> [RUN]   Executing 6-argument 32-bit syscall via VDSO
> [OK]    Arguments are preserved across syscall
> [RUN]   Executing 6-argument 32-bit syscall via INT 80
> [OK]    Arguments are preserved across syscall
>
> Signed-off-by: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
> CC: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> CC: Steven Rostedt <rostedt@xxxxxxxxxxx>
> CC: Ingo Molnar <mingo@xxxxxxxxxx>
> CC: Borislav Petkov <bp@xxxxxxxxx>
> CC: "H. Peter Anvin" <hpa@xxxxxxxxx>
> CC: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> CC: Oleg Nesterov <oleg@xxxxxxxxxx>
> CC: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> CC: Alexei Starovoitov <ast@xxxxxxxxxxxx>
> CC: Will Drewry <wad@xxxxxxxxxxxx>
> CC: Kees Cook <keescook@xxxxxxxxxxxx>
> CC: x86@xxxxxxxxxx
> CC: linux-kernel@xxxxxxxxxxxxxxx

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

Thanks for adding this test! :)

-Kees

> ---
>
> Changes in v2:
>  does not fail if VDSO can't be found;
>  tests INT 80 syscall method;
>  tests syscalls under ptrace;
>  switched to /* */ comments
>
> Changes in v3:
>  added checking for r8..r15 info leaks
>
>  tools/testing/selftests/x86/test_syscall_vdso.c | 401 ++++++++++++++++++++++++
>  tools/testing/selftests/x86/thunks_32.S         |  55 ++++
>  2 files changed, 456 insertions(+)
>  create mode 100644 tools/testing/selftests/x86/test_syscall_vdso.c
>  create mode 100644 tools/testing/selftests/x86/thunks_32.S
>
> diff --git a/tools/testing/selftests/x86/test_syscall_vdso.c b/tools/testing/selftests/x86/test_syscall_vdso.c
> new file mode 100644
> index 0000000..0792aef
> --- /dev/null
> +++ b/tools/testing/selftests/x86/test_syscall_vdso.c
> @@ -0,0 +1,401 @@
> +/*
> + * 32-bit syscall ABI conformance test.
> + *
> + * Copyright (c) 2015 Denys Vlasenko
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms and conditions of the GNU General Public License,
> + * version 2, as published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope it will be useful, but
> + * WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> + * General Public License for more details.
> + */
> +/*
> + * Can be built statically:
> + * gcc -Os -Wall -static -m32 test_syscall_vdso.c thunks_32.S
> + */
> +#undef _GNU_SOURCE
> +#define _GNU_SOURCE 1
> +#undef __USE_GNU
> +#define __USE_GNU 1
> +#include <unistd.h>
> +#include <stdlib.h>
> +#include <string.h>
> +#include <stdio.h>
> +#include <signal.h>
> +#include <sys/types.h>
> +#include <sys/select.h>
> +#include <sys/time.h>
> +#include <elf.h>
> +#include <sys/ptrace.h>
> +#include <sys/wait.h>
> +
> +#if !defined(__i386__)
> +int main(int argc, char **argv, char **envp)
> +{
> +       printf("[SKIP]\tNot a 32-bit x86 userspace\n");
> +       return 0;
> +}
> +#else
> +
> +long syscall_addr;
> +long get_syscall(char **envp)
> +{
> +       Elf32_auxv_t *auxv;
> +       while (*envp++ != NULL)
> +               continue;
> +       for (auxv = (void *)envp; auxv->a_type != AT_NULL; auxv++)
> +               if (auxv->a_type == AT_SYSINFO)
> +                       return auxv->a_un.a_val;
> +       printf("[WARN]\tAT_SYSINFO not supplied\n");
> +       return 0;
> +}
> +
> +asm (
> +       "       .pushsection .text\n"
> +       "       .global int80\n"
> +       "int80:\n"
> +       "       int     $0x80\n"
> +       "       ret\n"
> +       "       .popsection\n"
> +);
> +extern char int80;
> +
> +struct regs64 {
> +       uint64_t rax, rbx, rcx, rdx;
> +       uint64_t rsi, rdi, rbp, rsp;
> +       uint64_t r8,  r9,  r10, r11;
> +       uint64_t r12, r13, r14, r15;
> +};
> +struct regs64 regs64;
> +int kernel_is_64bit;
> +
> +asm (
> +       "       .pushsection .text\n"
> +       "       .code64\n"
> +       "get_regs64:\n"
> +       "       push    %rax\n"
> +       "       mov     $regs64, %eax\n"
> +       "       pop     0*8(%rax)\n"
> +       "       movq    %rbx, 1*8(%rax)\n"
> +       "       movq    %rcx, 2*8(%rax)\n"
> +       "       movq    %rdx, 3*8(%rax)\n"
> +       "       movq    %rsi, 4*8(%rax)\n"
> +       "       movq    %rdi, 5*8(%rax)\n"
> +       "       movq    %rbp, 6*8(%rax)\n"
> +       "       movq    %rsp, 7*8(%rax)\n"
> +       "       movq    %r8,  8*8(%rax)\n"
> +       "       movq    %r9,  9*8(%rax)\n"
> +       "       movq    %r10, 10*8(%rax)\n"
> +       "       movq    %r11, 11*8(%rax)\n"
> +       "       movq    %r12, 12*8(%rax)\n"
> +       "       movq    %r13, 13*8(%rax)\n"
> +       "       movq    %r14, 14*8(%rax)\n"
> +       "       movq    %r15, 15*8(%rax)\n"
> +       "       ret\n"
> +       "poison_regs64:\n"
> +       "       movq    $0x7f7f7f7f, %r8\n"
> +       "       shl     $32, %r8\n"
> +       "       orq     $0x7f7f7f7f, %r8\n"
> +       "       movq    %r8, %r9\n"
> +       "       movq    %r8, %r10\n"
> +       "       movq    %r8, %r11\n"
> +       "       movq    %r8, %r12\n"
> +       "       movq    %r8, %r13\n"
> +       "       movq    %r8, %r14\n"
> +       "       movq    %r8, %r15\n"
> +       "       ret\n"
> +       "       .code32\n"
> +       "       .popsection\n"
> +);
> +extern void get_regs64(void);
> +extern void poison_regs64(void);
> +extern unsigned long call64_from_32(void (*function)(void));
> +void print_regs64(void)
> +{
> +       if (!kernel_is_64bit)
> +               return;
> +       printf("ax:%016llx bx:%016llx cx:%016llx dx:%016llx\n", regs64.rax,  regs64.rbx,  regs64.rcx,  regs64.rdx);
> +       printf("si:%016llx di:%016llx bp:%016llx sp:%016llx\n", regs64.rsi,  regs64.rdi,  regs64.rbp,  regs64.rsp);
> +       printf(" 8:%016llx  9:%016llx 10:%016llx 11:%016llx\n", regs64.r8 ,  regs64.r9 ,  regs64.r10,  regs64.r11);
> +       printf("12:%016llx 13:%016llx 14:%016llx 15:%016llx\n", regs64.r12,  regs64.r13,  regs64.r14,  regs64.r15);
> +}
> +
> +int check_regs64(void)
> +{
> +       int err = 0;
> +       int num = 8;
> +       uint64_t *r64 = &regs64.r8;
> +
> +       if (!kernel_is_64bit)
> +               return 0;
> +
> +       do {
> +               if (*r64 == 0x7f7f7f7f7f7f7f7fULL)
> +                       continue; /* register did not change */
> +               if (syscall_addr != (long)&int80) {
> +                       /*
> +                        * Non-INT80 syscall entrypoints are allowed to clobber R8+ regs:
> +                        * either clear them to 0, or for R11, load EFLAGS.
> +                        */
> +                       if (*r64 == 0)
> +                               continue;
> +                       if (num == 11) {
> +                               printf("[NOTE]\tR11 has changed:%016llx - assuming clobbered by SYSRET insn\n", *r64);
> +                               continue;
> +                       }
> +               } else {
> +                       /* INT80 syscall entrypoint can be used by
> +                        * 64-bit programs too, unlike SYSCALL/SYSENTER.
> +                        * Therefore it must preserve R12+
> +                        * (they are callee-saved registers in 64-bit C ABI).
> +                        *
> +                        * This was probably historically not intended,
> +                        * but R8..11 are clobbered (cleared to 0).
> +                        * IOW: they are the only registers which aren't
> +                        * preserved across INT80 syscall.
> +                        */
> +                       if (*r64 == 0 && num <= 11)
> +                               continue;
> +               }
> +               printf("[FAIL]\tR%d has changed:%016llx\n", num, *r64);
> +               err++;
> +       } while (r64++, ++num < 16);
> +
> +       if (!err)
> +               printf("[OK]\tR8..R15 did not leak kernel data\n");
> +       return err;
> +}
> +
> +int nfds;
> +fd_set rfds;
> +fd_set wfds;
> +fd_set efds;
> +struct timespec timeout;
> +sigset_t sigmask;
> +struct {
> +       sigset_t *sp;
> +       int sz;
> +} sigmask_desc;
> +
> +void prep_args()
> +{
> +       nfds = 42;
> +       FD_ZERO(&rfds);
> +       FD_ZERO(&wfds);
> +       FD_ZERO(&efds);
> +       FD_SET(0, &rfds);
> +       FD_SET(1, &wfds);
> +       FD_SET(2, &efds);
> +       timeout.tv_sec = 0;
> +       timeout.tv_nsec = 123;
> +       sigemptyset(&sigmask);
> +       sigaddset(&sigmask, SIGINT);
> +       sigaddset(&sigmask, SIGUSR2);
> +       sigaddset(&sigmask, SIGRTMAX);
> +       sigmask_desc.sp = &sigmask;
> +       sigmask_desc.sz = 8; /* bytes */
> +}
> +
> +static void print_flags(const char *name, unsigned long r)
> +{
> +       static const char *bitarray[] = {
> +       "\n" ,"c\n" ,/* Carry Flag */
> +       "0 " ,"1 "  ,/* Bit 1 - always on */
> +       ""   ,"p "  ,/* Parity Flag */
> +       "0 " ,"3? " ,
> +       ""   ,"a "  ,/* Auxiliary carry Flag */
> +       "0 " ,"5? " ,
> +       ""   ,"z "  ,/* Zero Flag */
> +       ""   ,"s "  ,/* Sign Flag */
> +       ""   ,"t "  ,/* Trap Flag */
> +       ""   ,"i "  ,/* Interrupt Flag */
> +       ""   ,"d "  ,/* Direction Flag */
> +       ""   ,"o "  ,/* Overflow Flag */
> +       "0 " ,"1 "  ,/* I/O Privilege Level (2 bits) */
> +       "0"  ,"1"   ,/* I/O Privilege Level (2 bits) */
> +       ""   ,"n "  ,/* Nested Task */
> +       "0 " ,"15? ",
> +       ""   ,"r "  ,/* Resume Flag */
> +       ""   ,"v "  ,/* Virtual Mode */
> +       ""   ,"ac " ,/* Alignment Check/Access Control */
> +       ""   ,"vif ",/* Virtual Interrupt Flag */
> +       ""   ,"vip ",/* Virtual Interrupt Pending */
> +       ""   ,"id " ,/* CPUID detection */
> +       NULL
> +       };
> +       const char **bitstr;
> +       int bit;
> +
> +       printf("%s=%016lx ", name, r);
> +       bitstr = bitarray + 42;
> +       bit = 21;
> +       if ((r >> 22) != 0)
> +               printf("(extra bits are set) ");
> +       do {
> +               if (bitstr[(r >> bit) & 1][0])
> +                       printf(bitstr[(r >> bit) & 1]);
> +               bitstr -= 2;
> +               bit--;
> +       } while (bit >= 0);
> +}
> +
> +int run_syscall(void)
> +{
> +       long flags, bad_arg;
> +
> +       prep_args();
> +
> +       if (kernel_is_64bit)
> +               call64_from_32(poison_regs64);
> +       /*print_regs64();*/
> +
> +       asm("\n"
> +       /* Try 6-arg syscall: pselect. It should return quickly */
> +       "       push    %%ebp\n"
> +       "       mov     $308, %%eax\n"     /* PSELECT */
> +       "       mov     nfds, %%ebx\n"     /* ebx  arg1 */
> +       "       mov     $rfds, %%ecx\n"    /* ecx  arg2 */
> +       "       mov     $wfds, %%edx\n"    /* edx  arg3 */
> +       "       mov     $efds, %%esi\n"    /* esi  arg4 */
> +       "       mov     $timeout, %%edi\n" /* edi  arg5 */
> +       "       mov     $sigmask_desc, %%ebp\n" /* %ebp arg6 */
> +       "       push    $0x200ed7\n"      /* set almost all flags */
> +       "       popf\n"         /* except TF, IOPL, NT, RF, VM, AC, VIF, VIP */
> +       "       call    *syscall_addr\n"
> +       /* Check that registers are not clobbered */
> +       "       pushf\n"
> +       "       pop     %%eax\n"
> +       "       cld\n"
> +       "       cmp     nfds, %%ebx\n"     /* ebx  arg1 */
> +       "       mov     $1, %%ebx\n"
> +       "       jne     1f\n"
> +       "       cmp     $rfds, %%ecx\n"    /* ecx  arg2 */
> +       "       mov     $2, %%ebx\n"
> +       "       jne     1f\n"
> +       "       cmp     $wfds, %%edx\n"    /* edx  arg3 */
> +       "       mov     $3, %%ebx\n"
> +       "       jne     1f\n"
> +       "       cmp     $efds, %%esi\n"    /* esi  arg4 */
> +       "       mov     $4, %%ebx\n"
> +       "       jne     1f\n"
> +       "       cmp     $timeout, %%edi\n" /* edi  arg5 */
> +       "       mov     $5, %%ebx\n"
> +       "       jne     1f\n"
> +       "       cmpl    $sigmask_desc, %%ebp\n" /* %ebp arg6 */
> +       "       mov     $6, %%ebx\n"
> +       "       jne     1f\n"
> +       "       mov     $0, %%ebx\n"
> +       "1:\n"
> +       "       pop     %%ebp\n"
> +       : "=a" (flags), "=b" (bad_arg)
> +       :
> +       : "cx", "dx", "si", "di"
> +       );
> +
> +       if (kernel_is_64bit) {
> +               memset(&regs64, 0x77, sizeof(regs64));
> +               call64_from_32(get_regs64);
> +               /*print_regs64();*/
> +       }
> +
> +       /*
> +        * On paravirt kernels, flags are not preserved across syscalls.
> +        * Thus, we do not consider it a bug if some are changed.
> +        * We just show ones which do.
> +        */
> +       if ((0x200ed7 ^ flags) != 0) {
> +               print_flags("[WARN]\tFlags before", 0x200ed7);
> +               print_flags("[WARN]\tFlags  after", flags);
> +               print_flags("[WARN]\tFlags change", (0x200ed7 ^ flags));
> +       }
> +
> +       if (bad_arg) {
> +               printf("[FAIL]\targ#%ld clobbered\n", bad_arg);
> +               return 1;
> +       }
> +       printf("[OK]\tArguments are preserved across syscall\n");
> +
> +       return check_regs64();
> +}
> +
> +int run_syscall_twice()
> +{
> +       int exitcode = 0;
> +       long sv;
> +
> +       if (syscall_addr) {
> +               printf("[RUN]\tExecuting 6-argument 32-bit syscall via VDSO\n");
> +               exitcode = run_syscall();
> +       }
> +       sv = syscall_addr;
> +       syscall_addr = (long)&int80;
> +       printf("[RUN]\tExecuting 6-argument 32-bit syscall via INT 80\n");
> +       exitcode += run_syscall();
> +       syscall_addr = sv;
> +       return exitcode;
> +}
> +
> +void ptrace_me()
> +{
> +       pid_t pid;
> +
> +       fflush(NULL);
> +       pid = fork();
> +       if (pid < 0)
> +               exit(1);
> +       if (pid == 0) {
> +               /* child */
> +               if (ptrace(PTRACE_TRACEME, 0L, 0L, 0L) != 0)
> +                       exit(0);
> +               raise(SIGSTOP);
> +               return;
> +       }
> +       /* parent */
> +       printf("[RUN]\tRunning tests under ptrace\n");
> +       while (1) {
> +               int status;
> +               pid = waitpid(-1, &status, __WALL);
> +               if (WIFEXITED(status))
> +                       exit(WEXITSTATUS(status));
> +               if (WIFSIGNALED(status))
> +                       exit(WTERMSIG(status));
> +               if (pid <= 0 || !WIFSTOPPED(status)) /* paranoia */
> +                       exit(255);
> +               /*
> +                * Note: we do not inject sig = WSTOPSIG(status).
> +                * We probably should, but careful: do not inject SIGTRAP
> +                * generated by syscall entry/exit stops.
> +                * That kills the child.
> +                */
> +               ptrace(PTRACE_SYSCALL, pid, 0L, 0L /*sig*/);
> +       }
> +}
> +
> +int main(int argc, char **argv, char **envp)
> +{
> +       int exitcode = 0;
> +       int cs;
> +
> +       asm("\n"
> +       "       movl    %%cs, %%eax\n"
> +       : "=a" (cs)
> +       );
> +       kernel_is_64bit = (cs == 0x23);
> +       if (!kernel_is_64bit)
> +               printf("[NOTE]\tNot a 64-bit kernel, won't test R8..R15 leaks\n");
> +
> +       /* This only works for non-static builds:
> +        * syscall_addr = dlsym(dlopen("linux-gate.so.1", RTLD_NOW), "__kernel_vsyscall");
> +        */
> +       syscall_addr = get_syscall(envp);
> +
> +       exitcode += run_syscall_twice();
> +       ptrace_me();
> +       exitcode += run_syscall_twice();
> +
> +       return exitcode;
> +}
> +#endif
> diff --git a/tools/testing/selftests/x86/thunks_32.S b/tools/testing/selftests/x86/thunks_32.S
> new file mode 100644
> index 0000000..77e9159
> --- /dev/null
> +++ b/tools/testing/selftests/x86/thunks_32.S
> @@ -0,0 +1,55 @@
> +/*
> + * thunks_32.S - assembly helpers for mixed-bitness code
> + * Copyright (c) 2015 2015 Denys Vlasenko
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms and conditions of the GNU General Public License,
> + * version 2, as published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope it will be useful, but
> + * WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> + * General Public License for more details.
> + *
> + * These are little helpers that make it easier to switch bitness on
> + * the fly.
> + */
> +
> +       .text
> +       .code32
> +
> +       .global call64_from_32
> +       .type call32_from_64, @function
> +
> +       // 4(%esp): function to call
> +call64_from_32:
> +       // Fetch function address
> +       mov     4(%esp), %eax
> +
> +       // Save registers which are callee-clobbered by 64-bit ABI
> +       push    %ecx
> +       push    %edx
> +       push    %esi
> +       push    %edi
> +
> +       // Switch to long mode
> +       jmp     $0x33,$1f
> +1:     .code64
> +
> +       // Call the function
> +       call    *%rax
> +
> +       // Switch to compatibility mode
> +       push    $0x23  /* USER32_CS */
> +       .code32; push $1f; .code64 /* hack: can't have X86_64_32S relocation in 32-bit ELF */
> +       lretq
> +1:     .code32
> +
> +       pop     %edi
> +       pop     %esi
> +       pop     %edx
> +       pop     %ecx
> +
> +       ret
> +
> +.size call64_from_32, .-call64_from_32
> --
> 1.8.1.4
>



-- 
Kees Cook
Chrome OS Security
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