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x86 mmiotrace: fix race with release_kmmio_fault_page()

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There was a theoretical possibility to a race between arming a page in
post_kmmio_handler() and disarming the page in
release_kmmio_fault_page():

cpu0                             cpu1
------------------------------------------------------------------
mmiotrace shutdown
enter release_kmmio_fault_page
                                 fault on the page
                                 disarm the page
disarm the page
                                 handle the MMIO access
                                 re-arm the page
put the page on release list
remove_kmmio_fault_pages()
                                 fault on the page
                                 page not known to mmiotrace
                                 fall back to do_page_fault()
                                 *KABOOM*

(This scenario also shows the double disarm case which is allowed.)

Fixed by acquiring kmmio_lock in post_kmmio_handler() and checking
if the page is being released from mmiotrace.

Signed-off-by: Pekka Paalanen <pq@iki.fi>
Cc: Stuart Bennett <stuart@freedesktop.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Pekka Paalanen 2009-02-24 21:44:15 +02:00 committed by Ingo Molnar
parent 3e39aa156a
commit 340430c572
1 changed files with 7 additions and 2 deletions
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9
arch/x86/mm/kmmio.c
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@ -38,7 +38,8 @@ struct kmmio_fault_page {
/*
* Number of times this page has been registered as a part
* of a probe. If zero, page is disarmed and this may be freed.
* Used only by writers (RCU).
* Used only by writers (RCU) and post_kmmio_handler().
* Protected by kmmio_lock, when linked into kmmio_page_table.
*/
int count;
};
@ -317,7 +318,11 @@ static int post_kmmio_handler(unsigned long condition, struct pt_regs *regs)
if (ctx->probe && ctx->probe->post_handler)
ctx->probe->post_handler(ctx->probe, condition, regs);
arm_kmmio_fault_page(ctx->fpage);
/* Prevent racing against release_kmmio_fault_page(). */
spin_lock(&kmmio_lock);
if (ctx->fpage->count)
arm_kmmio_fault_page(ctx->fpage);
spin_unlock(&kmmio_lock);
regs->flags &= ~X86_EFLAGS_TF;
regs->flags |= ctx->saved_flags;