debug/gdbstub,cpu/idt: Report page faults as SIGSEGV in gdbstub

The gdbstub currently triggers a breakpoint on a panic, which allows
the debugger to directly probe the state of the kernel when a panic
occurs. However, some of the context is lost before the panic handler
is invoked. Therefore this patch introduces direct handling of unhandled
page faults within the gdbstub to provide that additional context.

If the gdbstub feature is not enabled then these changes have no effect.

Signed-off-by: Roy Hopkins <rhopkins@suse.de>

src/cpu/idt.rs

@@ -10,7 +10,7 @@ use super::vc::handle_vc_exception;
 use super::{X86GeneralRegs, X86InterruptFrame};
 use crate::address::{Address, VirtAddr};
 use crate::cpu::extable::handle_exception_table;
-use crate::debug::gdbstub::svsm_gdbstub::handle_bp_exception;
+use crate::debug::gdbstub::svsm_gdbstub::handle_debug_exception;
 use crate::types::SVSM_CS;
 use core::arch::{asm, global_asm};
 use core::mem;
@@ -186,6 +186,7 @@ fn generic_idt_handler(ctx: &mut X86ExceptionContext) {
         let err = ctx.error_code;
 
         if !handle_exception_table(ctx) {
+            handle_debug_exception(ctx, ctx.vector);
             panic!(
                 "Unhandled Page-Fault at RIP {:#018x} CR2: {:#018x} error code: {:#018x}",
                 rip, cr2, err
@@ -194,7 +195,7 @@ fn generic_idt_handler(ctx: &mut X86ExceptionContext) {
     } else if ctx.vector == VC_VECTOR {
         handle_vc_exception(ctx);
     } else if ctx.vector == BP_VECTOR {
-        handle_bp_exception(ctx);
+        handle_debug_exception(ctx, ctx.vector);
     } else {
         let err = ctx.error_code;
         let vec = ctx.vector;

src/debug/gdbstub.rs

@@ -15,7 +15,7 @@ pub mod svsm_gdbstub {
 
     use crate::address::{Address, VirtAddr};
     use crate::cpu::control_regs::read_cr3;
-    use crate::cpu::idt::X86ExceptionContext;
+    use crate::cpu::idt::{X86ExceptionContext, BP_VECTOR};
     use crate::cpu::percpu::{this_cpu, this_cpu_mut};
     use crate::cpu::X86GeneralRegs;
     use crate::error::SvsmError;
@@ -66,10 +66,15 @@ pub mod svsm_gdbstub {
     enum ExceptionType {
         Debug,
         SwBreakpoint,
+        PageFault,
     }
 
-    pub fn handle_bp_exception(ctx: &mut X86ExceptionContext) {
-        handle_exception(ctx, ExceptionType::SwBreakpoint);
+    pub fn handle_debug_exception(ctx: &mut X86ExceptionContext, exception: usize) {
+        let tp = match exception {
+            BP_VECTOR => ExceptionType::SwBreakpoint,
+            _ => ExceptionType::PageFault,
+        };
+        handle_exception(ctx, tp);
     }
 
     pub fn handle_db_exception(ctx: &mut X86ExceptionContext) {
@@ -260,6 +265,11 @@ pub mod svsm_gdbstub {
                         tid,
                         signal: Signal::SIGINT,
                     }
+                } else if exception_type == ExceptionType::PageFault {
+                    MultiThreadStopReason::SignalWithThread {
+                        tid,
+                        signal: Signal::SIGSEGV,
+                    }
                 } else {
                     MultiThreadStopReason::SwBreak(tid)
                 };
@@ -707,7 +717,7 @@ pub mod svsm_gdbstub {
         Ok(())
     }
 
-    pub fn handle_bp_exception(_ctx: &mut X86ExceptionContext) {}
+    pub fn handle_debug_exception(_ctx: &mut X86ExceptionContext, _exception: usize) {}
 
     pub fn handle_db_exception(_ctx: &mut X86ExceptionContext) {}