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SubjectRe: [5.2 REGRESSION] Generic vDSO breaks seccomp-enabled userspace on i386
On Sat, 27 Jul 2019, Thomas Gleixner wrote:
> On Sat, 27 Jul 2019, Andy Lutomirski wrote:
> >
> > I think it's getting quite late to start inventing new seccomp
> > features to fix this. I think the right solution for 5.3 is to change
> > the 32-bit vdso fallback to use the old clock_gettime, i.e.
> > clock_gettime32. This is obviously not an acceptable long-term
> > solution.
>
> Sigh. I'll have a look....

Completely untested patch below.

For the record: I have to say that I hate it.

Just to be clear. Once a clever seccomp admin decides to enforce Y2038
compliance by filtering out the legacy syscalls this will force glibc into
the syscall slowpath directly because __vdso_clock_gettime64() is gone.

So this needs a proper secccomp solution soener than later.

The fallback change to the legacy syscall is on purpose conditional on
CONFIG_SECCOMP so those people who care can get access to
__vdso_clock_gettime64() nevertheless.

Thanks,

tglx

8<-----------------
--- a/arch/x86/entry/vdso/vdso32/vdso32.lds.S
+++ b/arch/x86/entry/vdso/vdso32/vdso32.lds.S
@@ -27,7 +27,9 @@ VERSION
__vdso_gettimeofday;
__vdso_time;
__vdso_clock_getres;
+#ifndef CONFIG_SECCOMP
__vdso_clock_gettime64;
+#endif
};

LINUX_2.5 {
--- a/arch/x86/include/asm/vdso/gettimeofday.h
+++ b/arch/x86/include/asm/vdso/gettimeofday.h
@@ -101,6 +101,7 @@ long clock_gettime_fallback(clockid_t _c
{
long ret;

+#ifndef CONFIG_SECCOMP
asm (
"mov %%ebx, %%edx \n"
"mov %[clock], %%ebx \n"
@@ -110,6 +111,36 @@ long clock_gettime_fallback(clockid_t _c
: "0" (__NR_clock_gettime64), [clock] "g" (_clkid), "c" (_ts)
: "edx");

+#else
+ struct old_timespec32 tmpts;
+
+ /*
+ * Using clock_gettime and not clock_gettime64 here is a
+ * temporary workaround to pacify seccomp filters which are
+ * unaware of the Y2038 safe variant.
+ */
+
+ asm (
+ "mov %%ebx, %%edx \n"
+ "mov %[clock], %%ebx \n"
+ "call __kernel_vsyscall \n"
+ "mov %%edx, %%ebx \n"
+ : "=a" (ret), "=m" (tmpts)
+ : "0" (__NR_clock_gettime), [clock] "g" (_clkid), "c" (&tmpts)
+ : "edx");
+
+ /*
+ * The core code will have to convert that back. A smart compiler
+ * should notice and avoid the double conversion. If not, bad luck;
+ * we we are not going to change the core code just to make seccomp
+ * happy.
+ */
+
+ if (!ret) {
+ _ts->tv_sec = tmpts.tv_sec;
+ _ts->tv_nsec = tmpts.tv_nsec;
+ }
+#endif
return ret;
}

@@ -136,6 +167,7 @@ clock_getres_fallback(clockid_t _clkid,
{
long ret;

+#ifndef CONFIG_SECCOMP
asm (
"mov %%ebx, %%edx \n"
"mov %[clock], %%ebx \n"
@@ -144,7 +176,32 @@ clock_getres_fallback(clockid_t _clkid,
: "=a" (ret), "=m" (*_ts)
: "0" (__NR_clock_getres_time64), [clock] "g" (_clkid), "c" (_ts)
: "edx");
+#else
+ struct old_timespec32 tmpts;
+
+ /*
+ * Using clock_getres and not clock_getres_time64 here is a
+ * temporary workaround to pacify seccomp filters which are unaware
+ * of the time64 variants. Technically there is no requirement to
+ * use the 64bit variant here as the resolution is definitely not
+ * affected by Y2038, but the end goal of Y2038 is to utilize the
+ * new 64bit timespec variants for everything.
+ */
+
+ asm (
+ "mov %%ebx, %%edx \n"
+ "mov %[clock], %%ebx \n"
+ "call __kernel_vsyscall \n"
+ "mov %%edx, %%ebx \n"
+ : "=a" (ret), "=m" (tmpts)
+ : "0" (__NR_clock_getres), [clock] "g" (_clkid), "c" (&tmpts)
+ : "edx");

+ if (!ret) {
+ _ts->tv_sec = tmpts.tv_sec;
+ _ts->tv_nsec = tmpts.tv_nsec;
+ }
+#endif
return ret;
}

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