Re: Hyper-Threading Vulnerability

[andrea@cpushare.com]

On Sat, May 14, 2005 at 04:23:10PM +0100, Alan Cox wrote:
> You cannot use rdtsc for anything but rough instruction timing. The
> timers for different processors run at different speeds on some SMP
> systems, the timer rates vary as processors change clock rate nowdays.
> Rdtsc may also jump dramatically on a suspend/resume.

x86-64 uses it for vgettimeofday very safely (i386 could do too but it
doesn't).

Anyway I believe at least for seccomp it's worth to turn off the tsc,
not just for HT but for the L2 cache too. So it's up to you, either you
turn it off completely (which isn't very nice IMHO) or I recommend to
apply this below patch. This has been tested successfully on x86-64
against current cogito repository (i686 compiles so I didn't bother
testing ;). People selling the cpu through cpushare may appreciate this
bit for a peace of mind. There's no way to get any timing info anymore
with this applied (gettimeofday is forbidden of course). The seccomp
environment is completely deterministic so it can't be allowed to get
timing info, it has to be deterministic so in the future I can enable a
computing mode that does a parallel computing for each task with server
side transparent checkpointing and verification that the output is the
same from all the 2/3 seller computers for each task, without the buyer
even noticing (for now the verification is left to the buyer client
side and there's no checkpointing, since that would require more kernel
changes to track the dirty bits but it'll be easy to extend once the
basic mode is finished).

Thanks.

Signed-off-by: Andrea Arcangeli <andrea@cpushare.com>

Index: arch/i386/kernel/process.c
===================================================================
--- eed337ef5e9ae7d62caa84b7974a11fddc7f06e0/arch/i386/kernel/process.c  (mode:100644)
+++ uncommitted/arch/i386/kernel/process.c  (mode:100644)
@@ -561,6 +561,25 @@
 }
 
 /*
+ * This function selects if the context switch from prev to next
+ * has to tweak the TSC disable bit in the cr4.
+ */
+static void disable_tsc(struct thread_info *prev,
+			struct thread_info *next)
+{
+	if (unlikely(has_secure_computing(prev) ||
+		     has_secure_computing(next))) {
+		/* slow path here */
+		if (has_secure_computing(prev) &&
+		    !has_secure_computing(next)) {
+			clear_in_cr4(X86_CR4_TSD);
+		} else if (!has_secure_computing(prev) &&
+			   has_secure_computing(next))
+			set_in_cr4(X86_CR4_TSD);
+	}
+}
+
+/*
  *	switch_to(x,yn) should switch tasks from x to y.
  *
  * We fsave/fwait so that an exception goes off at the right time
@@ -639,6 +658,8 @@
 	if (unlikely(prev->io_bitmap_ptr || next->io_bitmap_ptr))
 		handle_io_bitmap(next, tss);
 
+	disable_tsc(prev_p->thread_info, next_p->thread_info);
+
 	return prev_p;
 }
 
Index: arch/x86_64/kernel/process.c
===================================================================
--- eed337ef5e9ae7d62caa84b7974a11fddc7f06e0/arch/x86_64/kernel/process.c  (mode:100644)
+++ uncommitted/arch/x86_64/kernel/process.c  (mode:100644)
@@ -439,6 +439,25 @@
 }
 
 /*
+ * This function selects if the context switch from prev to next
+ * has to tweak the TSC disable bit in the cr4.
+ */
+static void disable_tsc(struct thread_info *prev,
+			struct thread_info *next)
+{
+	if (unlikely(has_secure_computing(prev) ||
+		     has_secure_computing(next))) {
+		/* slow path here */
+		if (has_secure_computing(prev) &&
+		    !has_secure_computing(next)) {
+			clear_in_cr4(X86_CR4_TSD);
+		} else if (!has_secure_computing(prev) &&
+			   has_secure_computing(next))
+			set_in_cr4(X86_CR4_TSD);
+	}
+}
+
+/*
  * This special macro can be used to load a debugging register
  */
 #define loaddebug(thread,r) set_debug(thread->debugreg ## r, r)
@@ -556,6 +575,8 @@
 		}
 	}
 
+	disable_tsc(prev_p->thread_info, next_p->thread_info);
+
 	return prev_p;
 }
 
Index: include/linux/seccomp.h
===================================================================
--- eed337ef5e9ae7d62caa84b7974a11fddc7f06e0/include/linux/seccomp.h  (mode:100644)
+++ uncommitted/include/linux/seccomp.h  (mode:100644)
@@ -19,6 +19,11 @@
 		__secure_computing(this_syscall);
 }
 
+static inline int has_secure_computing(struct thread_info *ti)
+{
+	return unlikely(test_ti_thread_flag(ti, TIF_SECCOMP));
+}
+
 #else /* CONFIG_SECCOMP */
 
 #if (__GNUC__ > 2)
@@ -28,6 +33,7 @@
 #endif
 
 #define secure_computing(x) do { } while (0)
+#define has_secure_computing(x) 0
 
 #endif /* CONFIG_SECCOMP */
 
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