The reserved_bit_page_fault() paths effectively turn reserved bit faults into
a warning, but in the light of L1TF, the real impact is far more serious.
Make #PF[Rsvd] a hard error, irrespective of mode. Any new panic() caused by
this constitutes pagetable corruption, and probably an L1TF gadget needing
fixing.
Drop the PFEC_reserved_bit check in __page_fault_type() which has been made
dead by the rearrangement in do_page_fault().
Additionally, drop the comment for do_page_fault(). It is inaccurate (bit 0
being set isn't always a protection violation) and stale (missing bits
5,6,15,31).
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Wei Liu <wl@xen.org>
CC: Roger Pau Monné <roger.pau@citrix.com>
v2:
* Reword commit message and comment in do_page_fault().
---
xen/arch/x86/traps.c | 42 ++++++++++++++++--------------------------
1 file changed, 16 insertions(+), 26 deletions(-)
diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index 1f6f1dde76..e8a0877344 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -1137,15 +1137,6 @@ void do_int3(struct cpu_user_regs *regs)
pv_inject_hw_exception(TRAP_int3, X86_EVENT_NO_EC);
}
-static void reserved_bit_page_fault(unsigned long addr,
- struct cpu_user_regs *regs)
-{
- printk("%pv: reserved bit in page table (ec=%04X)\n",
- current, regs->error_code);
- show_page_walk(addr);
- show_execution_state(regs);
-}
-
#ifdef CONFIG_PV
static int handle_ldt_mapping_fault(unsigned int offset,
struct cpu_user_regs *regs)
@@ -1248,10 +1239,6 @@ static enum pf_type __page_fault_type(unsigned long addr,
if ( in_irq() )
return real_fault;
- /* Reserved bit violations are never spurious faults. */
- if ( error_code & PFEC_reserved_bit )
- return real_fault;
-
required_flags = _PAGE_PRESENT;
if ( error_code & PFEC_write_access )
required_flags |= _PAGE_RW;
@@ -1413,14 +1400,6 @@ static int fixup_page_fault(unsigned long addr, struct cpu_user_regs *regs)
return 0;
}
-/*
- * #PF error code:
- * Bit 0: Protection violation (=1) ; Page not present (=0)
- * Bit 1: Write access
- * Bit 2: User mode (=1) ; Supervisor mode (=0)
- * Bit 3: Reserved bit violation
- * Bit 4: Instruction fetch
- */
void do_page_fault(struct cpu_user_regs *regs)
{
unsigned long addr, fixup;
@@ -1439,6 +1418,21 @@ void do_page_fault(struct cpu_user_regs *regs)
if ( unlikely(fixup_page_fault(addr, regs) != 0) )
return;
+ /*
+ * Xen doesn't have reserved bits set in its pagetables, nor do we permit
+ * PV guests to write any. Such entries would generally be vulnerable to
+ * the L1TF sidechannel.
+ *
+ * The shadow pagetable logic may use reserved bits as part of
+ * SHOPT_FAST_FAULT_PATH. Pagefaults arising from these will be resolved
+ * via the fixup_page_fault() path.
+ *
+ * Anything remaining is an error, constituting corruption of the
+ * pagetables and probably an L1TF vulnerable gadget.
+ */
+ if ( error_code & PFEC_reserved_bit )
+ goto fatal;
+
if ( unlikely(!guest_mode(regs)) )
{
enum pf_type pf_type = spurious_page_fault(addr, regs);
@@ -1457,13 +1451,12 @@ void do_page_fault(struct cpu_user_regs *regs)
if ( likely((fixup = search_exception_table(regs)) != 0) )
{
perfc_incr(copy_user_faults);
- if ( unlikely(regs->error_code & PFEC_reserved_bit) )
- reserved_bit_page_fault(addr, regs);
this_cpu(last_extable_addr) = regs->rip;
regs->rip = fixup;
return;
}
+ fatal:
if ( debugger_trap_fatal(TRAP_page_fault, regs) )
return;
@@ -1475,9 +1468,6 @@ void do_page_fault(struct cpu_user_regs *regs)
error_code, _p(addr));
}
- if ( unlikely(regs->error_code & PFEC_reserved_bit) )
- reserved_bit_page_fault(addr, regs);
-
pv_inject_page_fault(regs->error_code, addr);
}
--
2.11.0
On 21.05.2020 17:43, Andrew Cooper wrote: > @@ -1439,6 +1418,21 @@ void do_page_fault(struct cpu_user_regs *regs) > if ( unlikely(fixup_page_fault(addr, regs) != 0) ) > return; > > + /* > + * Xen doesn't have reserved bits set in its pagetables, nor do we permit > + * PV guests to write any. Such entries would generally be vulnerable to > + * the L1TF sidechannel. > + * > + * The shadow pagetable logic may use reserved bits as part of > + * SHOPT_FAST_FAULT_PATH. Pagefaults arising from these will be resolved > + * via the fixup_page_fault() path. > + * > + * Anything remaining is an error, constituting corruption of the > + * pagetables and probably an L1TF vulnerable gadget. > + */ > + if ( error_code & PFEC_reserved_bit ) > + goto fatal; > + > if ( unlikely(!guest_mode(regs)) ) > { > enum pf_type pf_type = spurious_page_fault(addr, regs); > @@ -1457,13 +1451,12 @@ void do_page_fault(struct cpu_user_regs *regs) > if ( likely((fixup = search_exception_table(regs)) != 0) ) While I continue to not fully agree with not trying to fix up such faults if the fault location has recovery code attached, I realize that we're not going to reach agreement here, so somewhat hesitantly Acked-by: Jan Beulich <jbeulich@suse.com> Jan
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