[Xen-devel] [PATCH v2 1/2] x86/traps: guard top-of-stack reads

["Jan Beulich" <JBeulich@xxxxxxxx>]

Nothing (afaics) guarantees that the original frame's stack pointer
points at readable memory. Avoid a (likely nested) crash by attaching
exception recovery to the read (making it a single read at the same
time). Don't even invoke _show_trace() in case of a non-readable top
slot.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v2: Name asm() arguments. Use explicit "fault" variable.

--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -484,17 +484,31 @@ static void _show_trace(unsigned long sp
 
 static void show_trace(const struct cpu_user_regs *regs)
 {
-    unsigned long *sp = ESP_BEFORE_EXCEPTION(regs);
+    unsigned long *sp = ESP_BEFORE_EXCEPTION(regs), tos = 0;
+    bool fault = false;
 
     printk("Xen call trace:\n");
 
+    /* Guarded read of the stack top. */
+    asm ( "1: mov %[data], %[tos]; 2:\n"
+          ".pushsection .fixup,\"ax\"\n"
+          "3: movb $1, %[fault]; jmp 2b\n"
+          ".popsection\n"
+          _ASM_EXTABLE(1b, 3b)
+          : [tos] "+r" (tos), [fault] "+qm" (fault) : [data] "m" (*sp) );
+
     /*
      * If RIP looks sensible, or the top of the stack doesn't, print RIP at
      * the top of the stack trace.
      */
     if ( is_active_kernel_text(regs->rip) ||
-         !is_active_kernel_text(*sp) )
+         !is_active_kernel_text(tos) )
         printk("   [<%p>] %pS\n", _p(regs->rip), _p(regs->rip));
+    else if ( fault )
+    {
+        printk("   [Fault on access]\n");
+        return;
+    }
     /*
      * Else RIP looks bad but the top of the stack looks good.  Perhaps we
      * followed a wild function pointer? Lets assume the top of the stack is a
@@ -503,7 +517,7 @@ static void show_trace(const struct cpu_
      */
     else
     {
-        printk("   [<%p>] %pS\n", _p(*sp), _p(*sp));
+        printk("   [<%p>] %pS\n", _p(tos), _p(tos));
         sp++;
     }
 





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