Re: [patch] let CONFIG_SECCOMP default to n

[Ingo Molnar <mingo@elte.hu>]

* Andi Kleen <ak@suse.de> wrote:

> 
> > > I can put in a patch into my tree for the next merge to disable the 
> > > TSC disable code on i386 too like I did earlier for x86-64.
> > 
> > please do.
> 
> Hmm, with the new thread test as it was pointed out it can be indeed 
> made zero cost for the common case. Perhaps that's not needed then.

putting aside the fundamental fallacy of disabling TSC based timing 
attacks while not even considering network-based timing attacks (which 
are still very much possible), Chuck's approach of pushing the seccomp 
TSC cr4 twiddling into the context-switch slowpath is the right 
solution, given the circumstances. Will Chuck's patch be in 2.6.18? If 
not then my months-old patch below should be applied.

	Ingo

----

remove TSC-disabling logic from the context-switch hotpath. It has
marginal security relevance. Truly paranoid users can boot with the
TSC disabled anyway.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
----

 arch/i386/kernel/process.c |   29 -----------------------------
 1 files changed, 29 deletions(-)

Index: linux/arch/i386/kernel/process.c
===================================================================
--- linux.orig/arch/i386/kernel/process.c
+++ linux/arch/i386/kernel/process.c
@@ -589,33 +589,6 @@ handle_io_bitmap(struct thread_struct *n
 }
 
 /*
- * This function selects if the context switch from prev to next
- * has to tweak the TSC disable bit in the cr4.
- */
-static inline void disable_tsc(struct task_struct *prev_p,
-			       struct task_struct *next_p)
-{
-	struct thread_info *prev, *next;
-
-	/*
-	 * gcc should eliminate the ->thread_info dereference if
-	 * has_secure_computing returns 0 at compile time (SECCOMP=n).
-	 */
-	prev = prev_p->thread_info;
-	next = next_p->thread_info;
-
-	if (has_secure_computing(prev) || has_secure_computing(next)) {
-		/* slow path here */
-		if (has_secure_computing(prev) &&
-		    !has_secure_computing(next)) {
-			write_cr4(read_cr4() & ~X86_CR4_TSD);
-		} else if (!has_secure_computing(prev) &&
-			   has_secure_computing(next))
-			write_cr4(read_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
@@ -709,8 +682,6 @@ struct task_struct fastcall * __switch_t
 	if (unlikely(prev->io_bitmap_ptr || next->io_bitmap_ptr))
 		handle_io_bitmap(next, tss);
 
-	disable_tsc(prev_p, next_p);
-
 	return prev_p;
 }
 
-
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