[PATCH] KVM: fix bogus pagefault on writable pages

If a page is marked as dirty in the guest pte, set_pte_common() can set the
writable bit on newly-instantiated shadow pte.  This optimization avoids
a write fault after the initial read fault.

However, if a write fault instantiates the pte, fix_write_pf() incorrectly
reports the fault as a guest page fault, and the guest oopses on what appears
to be a correctly-mapped page.

Fix is to detect the condition and only report a guest page fault on a user
access to a kernel page.

With the fix, a kvm guest can survive a whole night of running the kernel
hacker's screensaver (make -j9 in a loop).
Signed-off-by: Avi Kivity <avi@qumranet.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/kvm/paging_tmpl.h

@@ -274,7 +274,7 @@ static int FNAME(fix_write_pf)(struct kvm_vcpu *vcpu,
 	struct kvm_mmu_page *page;
 
 	if (is_writeble_pte(*shadow_ent))
-		return 0;
+		return !user || (*shadow_ent & PT_USER_MASK);
 
 	writable_shadow = *shadow_ent & PT_SHADOW_WRITABLE_MASK;
 	if (user) {