[PATCH] x86/traps: Re-enable IRQs after reading cr2 in the #PF handler

[Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>]

Moves sti directly after the cr2 read and immediately after the #PF
handler.

While in the area, remove redundant q suffix to a movq in entry.S

Signed-off-by: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>
---
I don't think this is a bug as much as an accident about to happen. Even if
there's no cases at the moment in which the IRQ handler may page fault, that
might change in the future.

Note: I haven't tested it extensively beyond running it on GitLab.

pipeline:
    https://gitlab.com/xen-project/people/agvallejo/xen/-/pipelines/1449182525

---
 xen/arch/x86/traps.c        |  2 ++
 xen/arch/x86/x86_64/entry.S | 11 +++++++++--
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index 708136f625..1c04c03d9f 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -1600,6 +1600,8 @@ void asmlinkage do_page_fault(struct cpu_user_regs *regs)
 
     addr = read_cr2();
 
+    local_irq_enable();
+
     /* fixup_page_fault() might change regs->error_code, so cache it here. */
     error_code = regs->error_code;
 
diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
index b8482de8ee..ef803f6288 100644
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -844,8 +844,7 @@ handle_exception_saved:
 #elif !defined(CONFIG_PV)
         ASSERT_CONTEXT_IS_XEN
 #endif /* CONFIG_PV */
-        sti
-1:      movq  %rsp,%rdi
+1:      mov   %rsp,%rdi
         movzbl UREGS_entry_vector(%rsp),%eax
 #ifdef CONFIG_PERF_COUNTERS
         lea   per_cpu__perfcounters(%rip), %rcx
@@ -866,7 +865,15 @@ handle_exception_saved:
         jmp   .L_exn_dispatch_done;    \
 .L_ ## vec ## _done:
 
+        /*
+         * IRQs kept off to derisk being hit by a nested interrupt before
+         * reading %cr2. Otherwise a page fault in the nested interrupt hadnler
+         * would corrupt %cr2.
+         */
         DISPATCH(X86_EXC_PF, do_page_fault)
+
+        sti
+
         DISPATCH(X86_EXC_GP, do_general_protection)
         DISPATCH(X86_EXC_UD, do_invalid_op)
         DISPATCH(X86_EXC_NM, do_device_not_available)
-- 
2.46.0