x86/xen: avoid race in p2m handling (3a0e94f8) · Commits · 戴 / test

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 <jgross@suse.com> Signed-off-by:

David Vrabel <david.vrabel@citrix.com>

arch/x86/xen/p2m.c

+5 −5

Original line number	Diff line number	Diff line
		@@ -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)

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