Date:   Sun, 28 Jul 2019 17:27:10 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Paul Bolle <pebolle@...cali.nl>
Subject: Re: [5.2 REGRESSION] Generic vDSO breaks seccomp-enabled userspace on i386

On Sun, Jul 28, 2019 at 12:30 PM Thomas Gleixner <tglx@...utronix.de> wrote:
> On Sun, 28 Jul 2019, Arnd Bergmann wrote:
> > On Sat, Jul 27, 2019 at 7:53 PM Andy Lutomirski <luto@...nel.org> wrote:

> Which is totally irrelevant because res is NULL and that NULL pointer check
> should simply return -EFAULT, which is what the syscall fallback returns
> because the pointer is NULL.
>
> But that NULL pointer check is inconsistent anyway:
>
>  - 64 bit does not have it and never had
>
>  - the vdso is not capable of handling faults properly anyway. If the
>    pointer is not valid, then it will segfault. So just preventing the
>    segfault for NULL is silly.
>
> I'm going to just remove it.

Ah, you are right, I misread.

Anyway, if we want to keep the traditional behavior and get fewer surprises
for users of seccomp and anything else that might observe clock_gettime
behavior, below is how I'd do it. (not even build tested, x86-only. I'll
send a proper patch if this is where we want to take it).

        Arnd

diff --git a/arch/x86/include/asm/vdso/gettimeofday.h
b/arch/x86/include/asm/vdso/gettimeofday.h
index ae91429129a6..f7b6a50d4811 100644
--- a/arch/x86/include/asm/vdso/gettimeofday.h
+++ b/arch/x86/include/asm/vdso/gettimeofday.h
@@ -70,6 +70,13 @@ long clock_gettime_fallback(clockid_t _clkid,
struct __kernel_timespec *_ts)
        return ret;
 }

+
+static __always_inline
+long clock_gettime32_fallback(clockid_t _clkid, struct old_timespec32 *_ts)
+{
+       return -ENOSYS;
+}
+
 static __always_inline
 long gettimeofday_fallback(struct __kernel_old_timeval *_tv,
                           struct timezone *_tz)
@@ -97,7 +104,7 @@ long clock_getres_fallback(clockid_t _clkid, struct
__kernel_timespec *_ts)
 #else

 static __always_inline
-long clock_gettime_fallback(clockid_t _clkid, struct __kernel_timespec *_ts)
+long clock_gettime_fallback(clockid_t _clkid, struct old_timespec32 *_ts)
 {
        long ret;

@@ -113,6 +120,23 @@ long clock_gettime_fallback(clockid_t _clkid,
struct __kernel_timespec *_ts)
        return ret;
 }

+static __always_inline
+long clock_gettime32_fallback(clockid_t _clkid, struct __kernel_timespec *_ts)
+{
+       long ret;
+
+       asm (
+               "mov %%ebx, %%edx \n"
+               "mov %[clock], %%ebx \n"
+               "call __kernel_vsyscall \n"
+               "mov %%edx, %%ebx \n"
+               : "=a" (ret), "=m" (*_ts)
+               : "0" (__NR_clock_gettime), [clock] "g" (_clkid), "c" (_ts)
+               : "edx");
+
+       return ret;
+}
+
 static __always_inline
 long gettimeofday_fallback(struct __kernel_old_timeval *_tv,
                           struct timezone *_tz)
diff --git a/lib/vdso/gettimeofday.c b/lib/vdso/gettimeofday.c
index 2d1c1f241fd9..3b3dab53d611 100644
--- a/lib/vdso/gettimeofday.c
+++ b/lib/vdso/gettimeofday.c
@@ -18,6 +18,7 @@
  *   clock_mode.
  * - gettimeofday_fallback(): fallback for gettimeofday.
  * - clock_gettime_fallback(): fallback for clock_gettime.
+ * - clock_gettime_fallback(): fallback for clock_gettime32.
  * - clock_getres_fallback(): fallback for clock_getres.
  */
 #ifdef ENABLE_COMPAT_VDSO
@@ -51,7 +52,7 @@ static int do_hres(const struct vdso_data *vd, clockid_t clk,
                ns = vdso_ts->nsec;
                last = vd->cycle_last;
                if (unlikely((s64)cycles < 0))
-                       return clock_gettime_fallback(clk, ts);
+                       return -ENOSYS;

                ns += vdso_calc_delta(cycles, last, vd->mask, vd->mult);
                ns >>= vd->shift;
@@ -81,15 +82,15 @@ static void do_coarse(const struct vdso_data *vd,
clockid_t clk,
        } while (unlikely(vdso_read_retry(vd, seq)));
 }

-static __maybe_unused int
-__cvdso_clock_gettime(clockid_t clock, struct __kernel_timespec *ts)
+static int
+__cvdso_clock_gettime_common(clockid_t clock, struct __kernel_timespec *ts)
 {
        const struct vdso_data *vd = __arch_get_vdso_data();
        u32 msk;

        /* Check for negative values or invalid clocks */
        if (unlikely((u32) clock >= MAX_CLOCKS))
-               goto fallback;
+               return -EINVAL;

        /*
         * Convert the clockid to a bitmask and use it to check which
@@ -105,7 +106,15 @@ __cvdso_clock_gettime(clockid_t clock, struct
__kernel_timespec *ts)
                return do_hres(&vd[CS_RAW], clock, ts);
        }

-fallback:
+       return -EINVAL;
+}
+
+static __maybe_unused int
+__cvdso_clock_gettime(clockid_t clock, struct __kernel_timespec *ts)
+{
+       if (!__cvdso_clock_gettime_common(clock, ts))
+               return 0;
+
        return clock_gettime_fallback(clock, ts);
 }

@@ -113,22 +122,14 @@ static __maybe_unused int
 __cvdso_clock_gettime32(clockid_t clock, struct old_timespec32 *res)
 {
        struct __kernel_timespec ts;
-       int ret;
-
-       if (res == NULL)
-               goto fallback;
-
-       ret = __cvdso_clock_gettime(clock, &ts);

-       if (ret == 0) {
+       if (!__cvdso_clock_gettime_common(clock, &ts)) {
                res->tv_sec = ts.tv_sec;
                res->tv_nsec = ts.tv_nsec;
+               return 0;
        }

-       return ret;
-
-fallback:
-       return clock_gettime_fallback(clock, (struct __kernel_timespec *)res);
+       return clock_gettime32_fallback(clock, res);
 }

 static __maybe_unused int

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