From 3a0e94f8ead4a58b9719db0f78e13d02d059604f Mon Sep 17 00:00:00 2001 From: Juergen Gross Date: Fri, 17 Oct 2014 13:16:06 +0200 Subject: [PATCH] x86/xen: avoid race in p2m handling When a new p2m leaf is allocated this leaf is linked into the p2m tree via cmpxchg. Unfortunately the compare value for checking the success of the update is read after checking for the need of a new leaf. It is possible that a new leaf has been linked into the tree concurrently in between. This could lead to a leaked memory page and to the loss of some p2m entries. Avoid the race by using the read compare value for checking the need of a new p2m leaf and use ACCESS_ONCE() to get it. There are other places which seem to need ACCESS_ONCE() to ensure proper operation. Change them accordingly. Signed-off-by: Juergen Gross Signed-off-by: David Vrabel --- arch/x86/xen/p2m.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c index d1b3da2960ab..b456b048eca9 100644 --- a/arch/x86/xen/p2m.c +++ b/arch/x86/xen/p2m.c @@ -532,12 +532,13 @@ static bool alloc_p2m(unsigned long pfn) unsigned topidx, mididx; unsigned long ***top_p, **mid; unsigned long *top_mfn_p, *mid_mfn; + unsigned long *p2m_orig; topidx = p2m_top_index(pfn); mididx = p2m_mid_index(pfn); top_p = &p2m_top[topidx]; - mid = *top_p; + mid = ACCESS_ONCE(*top_p); if (mid == p2m_mid_missing) { /* Mid level is missing, allocate a new one */ @@ -552,7 +553,7 @@ static bool alloc_p2m(unsigned long pfn) } top_mfn_p = &p2m_top_mfn[topidx]; - mid_mfn = p2m_top_mfn_p[topidx]; + mid_mfn = ACCESS_ONCE(p2m_top_mfn_p[topidx]); BUG_ON(virt_to_mfn(mid_mfn) != *top_mfn_p); @@ -579,11 +580,10 @@ static bool alloc_p2m(unsigned long pfn) } } - if (p2m_top[topidx][mididx] == p2m_identity || - p2m_top[topidx][mididx] == p2m_missing) { + p2m_orig = ACCESS_ONCE(p2m_top[topidx][mididx]); + if (p2m_orig == p2m_identity || p2m_orig == p2m_missing) { /* p2m leaf page is missing */ unsigned long *p2m; - unsigned long *p2m_orig = p2m_top[topidx][mididx]; p2m = alloc_p2m_page(); if (!p2m) -- 2.30.2