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Subject[PATCH 7/7] x86 mmiotrace: fix race with release_kmmio_fault_page()
From c7bc3125d414938b3340a87516ecf47cd27efea4 Mon Sep 17 00:00:00 2001
From: Pekka Paalanen <pq@iki.fi>
Date: Tue, 24 Feb 2009 21:44:15 +0200
Subject: [PATCH] x86 mmiotrace: fix race with release_kmmio_fault_page()

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>
---
arch/x86/mm/kmmio.c | 9 +++++++--
1 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/kmmio.c b/arch/x86/mm/kmmio.c
index 4c66bd3..9f20503 100644
--- a/arch/x86/mm/kmmio.c
+++ b/arch/x86/mm/kmmio.c
@@ -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;
--
1.6.0.6


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