From 239af7c7132a617f9dcd05da1dc92b96bc6d0645 Mon Sep 17 00:00:00 2001 From: Juergen Gross Date: Tue, 14 Oct 2014 11:00:18 +0200 Subject: [PATCH] x86/xen: avoid writing to freed memory after race in p2m handling In case a race was detected during allocation of a new p2m tree element in alloc_p2m() the new allocated mid_mfn page is freed without updating the pointer to the found value in the tree. This will result in overwriting the just freed page with the mfn of the p2m leaf. Signed-off-by: Juergen Gross Signed-off-by: David Vrabel --- arch/x86/xen/p2m.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c index 9f5983b01ed9..4534320e66e4 100644 --- a/arch/x86/xen/p2m.c +++ b/arch/x86/xen/p2m.c @@ -566,6 +566,7 @@ static bool alloc_p2m(unsigned long pfn) /* Separately check the mid mfn level */ unsigned long missing_mfn; unsigned long mid_mfn_mfn; + unsigned long old_mfn; mid_mfn = alloc_p2m_page(); if (!mid_mfn) @@ -575,10 +576,13 @@ static bool alloc_p2m(unsigned long pfn) missing_mfn = virt_to_mfn(p2m_mid_missing_mfn); mid_mfn_mfn = virt_to_mfn(mid_mfn); - if (cmpxchg(top_mfn_p, missing_mfn, mid_mfn_mfn) != missing_mfn) + old_mfn = cmpxchg(top_mfn_p, missing_mfn, mid_mfn_mfn); + if (old_mfn != missing_mfn) { free_p2m_page(mid_mfn); - else + mid_mfn = mfn_to_virt(old_mfn); + } else { p2m_top_mfn_p[topidx] = mid_mfn; + } } if (p2m_top[topidx][mididx] == p2m_identity || -- 2.20.1