Jan Beulich
2010-Nov-29 11:22 UTC
[Xen-devel] [PATCH] x86: tighten filter on ptwr_do_page_fault()
Even not-so-recent Linux may, due to post-2.6.18 changes to the
process creation code, cause quite a number (depending on environment
and argument size) of faulting accesses to user space originating from
kernel mode. Generally those happen for non-present pages and would
lead to a nested page fault from guest_get_eff_l1e(). They can be
avoided by checking for PFEC_page_present as long as the guest isn''t
running on shadow page tables.
Signed-off-by: Jan Beulich <jbeulich@novell.com>
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -1244,13 +1244,19 @@ static int fixup_page_fault(unsigned lon
}
if ( VM_ASSIST(d, VMASST_TYPE_writable_pagetables) &&
- guest_kernel_mode(v, regs) &&
- /* Do not check if access-protection fault since the page may
- legitimately be not present in shadow page tables */
- ((regs->error_code & (PFEC_write_access|PFEC_reserved_bit)) =-
PFEC_write_access) &&
- ptwr_do_page_fault(v, addr, regs) )
- return EXCRET_fault_fixed;
+ guest_kernel_mode(v, regs) )
+ {
+ unsigned int mbs = PFEC_write_access;
+
+ /* Do not check if access-protection fault since the page may
+ legitimately be not present in shadow page tables */
+ if ( !paging_mode_enabled(d) )
+ mbs |= PFEC_page_present;
+ if ( (regs->error_code & (mbs | PFEC_reserved_bit |
+ PFEC_insn_fetch)) == mbs &&
+ ptwr_do_page_fault(v, addr, regs) )
+ return EXCRET_fault_fixed;
+ }
/* For non-external shadowed guests, we fix up both their own
* pagefaults and Xen''s, since they share the pagetables. */
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