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SubjectRe: [patch] let CONFIG_SECCOMP default to n
In-Reply-To: <p73wtain80h.fsf@verdi.suse.de>

On 12 Jul 2006 17:43:42 +0200, Andi Kleen wrote:

Alan Cox <alan@lxorguk.ukuu.org.uk> writes:
> >
> > I really don't care about cpushare and patents for some users of the
> > code in question. On the other hand turning on performance harming code
> > for a tiny number of users is dumb. If it were a loadable module it
> > would be different.
>
> Actually there are some promising applications of seccomp outside
> cpushare.
>
> e.g. Andrea at some point proposed to run codecs which often
> have security issues in a simple cpusec jail. That's ok for
> them because they normally don't need to do any system calls.
>
> I liked the idea. While this can be done with LSM (e.g. apparmor) too
> seccomp is definitely much easier and simpler and more "obviously safe"
> than anything LSM based.
>
> If the TSC disabling code is taken out the runtime overhead
> of seccomp is also very small because it's only tested in slow
> paths.

We can just fold the TSC disable stuff into the new thread_flags test
at context-switch time:

[compile tested only; requires just-sent fix to i386 system.h]

arch/i386/kernel/process.c | 61 ++++++++++++++---------------------------
include/asm-i386/thread_info.h | 3 +-
2 files changed, 24 insertions(+), 40 deletions(-)
--- 2.6.18-rc1-nb.orig/arch/i386/kernel/process.c
+++ 2.6.18-rc1-nb/arch/i386/kernel/process.c
@@ -535,12 +535,11 @@ int dump_task_regs(struct task_struct *t
return 1;
}

-static noinline void __switch_to_xtra(struct task_struct *next_p,
- struct tss_struct *tss)
+static noinline void
+__switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
+ struct tss_struct *tss)
{
- struct thread_struct *next;
-
- next = &next_p->thread;
+ struct thread_struct *next = &next_p->thread;

if (test_tsk_thread_flag(next_p, TIF_DEBUG)) {
set_debugreg(next->debugreg[0], 0);
@@ -552,6 +551,19 @@ static noinline void __switch_to_xtra(st
set_debugreg(next->debugreg[7], 7);
}

+ /*
+ * Context switch may need to tweak the TSC disable bit in CR4.
+ * The optimizer should remove this code when !CONFIG_SECCOMP.
+ */
+ if (has_secure_computing(task_thread_info(prev_p)) ^
+ has_secure_computing(task_thread_info(next_p))) {
+ /* prev and next are different */
+ if (has_secure_computing(task_thread_info(next_p)))
+ write_cr4(read_cr4() | X86_CR4_TSD);
+ else
+ write_cr4(read_cr4() & ~X86_CR4_TSD);
+ }
+
if (!test_tsk_thread_flag(next_p, TIF_IO_BITMAP)) {
/*
* Disable the bitmap via an invalid offset. We still cache
@@ -561,7 +573,7 @@ static noinline void __switch_to_xtra(st
return;
}

- if (likely(next == tss->io_bitmap_owner)) {
+ if (likely(tss->io_bitmap_owner == next)) {
/*
* Previous owner of the bitmap (hence the bitmap content)
* matches the next task, we dont have to do anything but
@@ -583,33 +595,6 @@ static noinline void __switch_to_xtra(st
}

/*
- * 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 = task_thread_info(prev_p);
- next = task_thread_info(next_p);
-
- 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
@@ -688,13 +673,11 @@ struct task_struct fastcall * __switch_t
set_iopl_mask(next->iopl);

/*
- * Now maybe handle debug registers and/or IO bitmaps
+ * Now maybe handle debug registers, IO bitmaps, TSC disable
*/
- if (unlikely((task_thread_info(next_p)->flags & _TIF_WORK_CTXSW))
- || test_tsk_thread_flag(prev_p, TIF_IO_BITMAP))
- __switch_to_xtra(next_p, tss);
-
- disable_tsc(prev_p, next_p);
+ if (unlikely(task_thread_info(prev_p)->flags & _TIF_WORK_CTXSW_PREV ||
+ task_thread_info(next_p)->flags & _TIF_WORK_CTXSW_NEXT))
+ __switch_to_xtra(prev_p, next_p, tss);

return prev_p;
}
--- 2.6.18-rc1-nb.orig/include/asm-i386/thread_info.h
+++ 2.6.18-rc1-nb/include/asm-i386/thread_info.h
@@ -164,7 +164,8 @@ static inline struct thread_info *curren
#define _TIF_ALLWORK_MASK (0x0000FFFF & ~_TIF_SECCOMP)

/* flags to check in __switch_to() */
-#define _TIF_WORK_CTXSW (_TIF_DEBUG|_TIF_IO_BITMAP)
+#define _TIF_WORK_CTXSW_NEXT (_TIF_IO_BITMAP | _TIF_SECCOMP | _TIF_DEBUG)
+#define _TIF_WORK_CTXSW_PREV (_TIF_IO_BITMAP | _TIF_SECCOMP)

/*
* Thread-synchronous status.
--
Chuck
"You can't read a newspaper if you can't read." --George W. Bush
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