[PATCH] mips: vdso: conditionalize 32-bit time functions on COMPAT_32BIT_TIME

["Jason A. Donenfeld" <Jason@zx2c4.com>]

When the VDSO falls back to 32-bit time functions on kernels with
COMPAT_32BIT_TIME=n, userspace becomes corrupted and appears to crash
shortly after, with something like:

[    0.359617] do_page_fault(): sending SIGSEGV to init for invalid read access from 000000007ff790d0
[    0.359843] epc = 0000000077e45df4 in libc.so[77da6000+de000]
[    0.360319] ra  = 0000000010000c50 in init[10000000+2000]
[    0.364456] Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b

This can be reproduced with simply calling `clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &ts)`,
since `CLOCK_PROCESS_CPUTIME_ID` is not exported to the VDSO, invoking
the syscall callback branch. This crash was observed with musl 1.20's
clock_gettime implementation:

int __clock_gettime(clockid_t clk, struct timespec *ts)
{
  int r;

  int (*f)(clockid_t, struct timespec *) =
    (int (*)(clockid_t, struct timespec *))vdso_func;
  if (f) {
    r = f(clk, ts);
    if (!r) {
      return r;
    }
    if (r == -EINVAL)
      return __syscall_ret(r);
  }
  r = __syscall(SYS_clock_gettime, clk, ts); // <-- CRASH
  if (r == -ENOSYS) {
    if (clk == CLOCK_REALTIME) {
      __syscall(SYS_gettimeofday, ts, 0);
      ts->tv_nsec = (int)ts->tv_nsec * 1000;
      return 0;
    }
    r = -EINVAL;
  }
  return __syscall_ret(r);
}

The particular kernel and libc are built as part of the MIPS64 CI on
build.wireguard.com which generally uses as minimal configurations as
possible.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Paul Burton <paulburton@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Christian Brauner <christian.brauner@canonical.com>
Fixes: 942437c97fd9 ("y2038: allow disabling time32 system calls")
---
 arch/mips/include/asm/vdso/gettimeofday.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/mips/include/asm/vdso/gettimeofday.h b/arch/mips/include/asm/vdso/gettimeofday.h
index b08825531e9f..7f1aa610e68e 100644
--- a/arch/mips/include/asm/vdso/gettimeofday.h
+++ b/arch/mips/include/asm/vdso/gettimeofday.h
@@ -107,7 +107,7 @@ static __always_inline int clock_getres_fallback(
 	return error ? -ret : ret;
 }
 
-#if _MIPS_SIM != _MIPS_SIM_ABI64
+#if _MIPS_SIM != _MIPS_SIM_ABI64 && defined(CONFIG_COMPAT_32BIT_TIME)
 
 #define VDSO_HAS_32BIT_FALLBACK	1
 
-- 
2.24.1