platform/kvm: refactor handleBluepillFault to reduce stack usage

PiperOrigin-RevId: 681217279

pkg/sentry/platform/kvm/BUILD

@@ -24,23 +24,6 @@ config_setting(
     },
 )
 
-# @unused
-glaze_ignore = [
-    "seccomp_mmap_dbg.go",
-    "seccomp_mmap_real.go",
-]
-
-# Use either seccomp_mmap_dbg.go or seccomp_mmap_real.go as seccomp_mmap.go.
-genrule(
-    name = "seccomp_mmap",
-    srcs = select({
-        ":debug_build": ["seccomp_mmap_dbg.go"],
-        "//conditions:default": ["seccomp_mmap_real.go"],
-    }),
-    outs = ["seccomp_mmap_unsafe.go"],
-    cmd = "cat < $(SRCS) > $(OUTS)",
-)
-
 go_library(
     name = "kvm",
     srcs = [

pkg/sentry/platform/kvm/address_space.go

@@ -121,7 +121,7 @@ func (as *addressSpace) mapLocked(addr hostarch.Addr, m hostMapEntry, at hostarc
 		// not have physical mappings, the KVM module may inject
 		// spurious exceptions when emulation fails (i.e. it tries to
 		// emulate because the RIP is pointed at those pages).
-		as.machine.mapPhysical(physical, length, physicalRegions)
+		as.machine.mapPhysical(physical, length)
 
 		// Install the page table mappings. Note that the ordering is
 		// important; if the pagetable mappings were installed before

pkg/sentry/platform/kvm/bluepill_allocator.go

@@ -73,7 +73,7 @@ func (a *allocator) PhysicalFor(ptes *pagetables.PTEs) uintptr {
 //
 //go:nosplit
 func (a *allocator) LookupPTEs(physical uintptr) *pagetables.PTEs {
-	virtualStart, physicalStart, _, pr := calculateBluepillFault(physical, physicalRegions)
+	virtualStart, physicalStart, _, pr := calculateBluepillFault(physical)
 	if pr == nil {
 		panic(fmt.Sprintf("LookupPTEs failed for 0x%x", physical)) // escapes: panic.
 	}

pkg/sentry/platform/kvm/bluepill_fault.go

@@ -47,9 +47,9 @@ func yield() {
 // calculateBluepillFault calculates the fault address range.
 //
 //go:nosplit
-func calculateBluepillFault(physical uintptr, phyRegions []physicalRegion) (virtualStart, physicalStart, length uintptr, pr *physicalRegion) {
+func calculateBluepillFault(physical uintptr) (virtualStart, physicalStart, length uintptr, pr *physicalRegion) {
 	alignedPhysical := physical &^ uintptr(hostarch.PageSize-1)
-	for i, pr := range phyRegions {
+	for i, pr := range physicalRegions {
 		end := pr.physical + pr.length
 		if physical < pr.physical || physical >= end {
 			continue
@@ -63,27 +63,14 @@ func calculateBluepillFault(physical uintptr, phyRegions []physicalRegion) (virt
 			physicalEnd = end
 		}
 		length = physicalEnd - physicalStart
-		return virtualStart, physicalStart, length, &phyRegions[i]
+		return virtualStart, physicalStart, length, &physicalRegions[i]
 	}
 
 	return 0, 0, 0, nil
 }
 
-// handleBluepillFault handles a physical fault.
-//
-// The corresponding virtual address is returned. This may throw on error.
-//
 //go:nosplit
-func handleBluepillFault(m *machine, physical uintptr, phyRegions []physicalRegion) (uintptr, bool) {
-	// Paging fault: we need to map the underlying physical pages for this
-	// fault. This all has to be done in this function because we're in a
-	// signal handler context. (We can't call any functions that might
-	// split the stack.)
-	virtualStart, physicalStart, length, pr := calculateBluepillFault(physical, phyRegions)
-	if pr == nil {
-		return 0, false
-	}
-
+func (m *machine) mapMemorySlot(virtualStart, physicalStart, length uintptr, readOnly bool) {
 	// Set the KVM slot.
 	//
 	// First, we need to acquire the exclusive right to set a slot.  See
@@ -94,7 +81,7 @@ func handleBluepillFault(m *machine, physical uintptr, phyRegions []physicalRegi
 		slot = m.nextSlot.Swap(^uint32(0))
 	}
 	flags := _KVM_MEM_FLAGS_NONE
-	if pr.readOnly {
+	if readOnly {
 		flags |= _KVM_MEM_READONLY
 	}
 	errno := m.setMemoryRegion(int(slot), physicalStart, length, virtualStart, flags)
@@ -106,7 +93,7 @@ func handleBluepillFault(m *machine, physical uintptr, phyRegions []physicalRegi
 		// Successfully added region; we can increment nextSlot and
 		// allow another set to proceed here.
 		m.nextSlot.Store(slot + 1)
-		return virtualStart + (physical - physicalStart), true
+		return
 	}
 
 	// Release our slot (still available).
@@ -117,7 +104,7 @@ func handleBluepillFault(m *machine, physical uintptr, phyRegions []physicalRegi
 		// The region already exists. It's possible that we raced with
 		// another vCPU here. We just revert nextSlot and return true,
 		// because this must have been satisfied by some other vCPU.
-		return virtualStart + (physical - physicalStart), true
+		return
 	case unix.EINVAL:
 		throw("set memory region failed; out of slots")
 	case unix.ENOMEM:
@@ -125,8 +112,9 @@ func handleBluepillFault(m *machine, physical uintptr, phyRegions []physicalRegi
 	case unix.EFAULT:
 		throw("set memory region failed: invalid physical range")
 	default:
+		printHex([]byte("set memory region failed:"), uint64(errno))
 		throw("set memory region failed: unknown reason")
 	}
 
-	panic("unreachable")
+	throw("unreachable")
 }

pkg/sentry/platform/kvm/machine.go

@@ -371,7 +371,7 @@ func newMachine(vm int) (*machine, error) {
 			}
 
 			// Ensure the physical range is mapped.
-			m.mapPhysical(physical, length, physicalRegions)
+			m.mapPhysical(physical, length)
 			virtual += length
 		}
 	}
@@ -396,7 +396,7 @@ func newMachine(vm int) (*machine, error) {
 	})
 	if mapEntireAddressSpace {
 		for _, r := range physicalRegions {
-			m.mapPhysical(r.physical, r.length, physicalRegions)
+			m.mapPhysical(r.physical, r.length)
 		}
 	}
 	enableAsyncPreemption()
@@ -438,9 +438,9 @@ func (m *machine) hasSlot(physical uintptr) bool {
 // This throws on error.
 //
 //go:nosplit
-func (m *machine) mapPhysical(physical, length uintptr, phyRegions []physicalRegion) {
+func (m *machine) mapPhysical(physical, length uintptr) {
 	for end := physical + length; physical < end; {
-		_, physicalStart, length, pr := calculateBluepillFault(physical, phyRegions)
+		virtualStart, physicalStart, length, pr := calculateBluepillFault(physical)
 		if pr == nil {
 			// Should never happen.
 			throw("mapPhysical on unknown physical address")
@@ -448,9 +448,7 @@ func (m *machine) mapPhysical(physical, length uintptr, phyRegions []physicalReg
 
 		// Is this already mapped? Check the usedSlots.
 		if !m.hasSlot(physicalStart) {
-			if _, ok := handleBluepillFault(m, physical, phyRegions); !ok {
-				throw("handleBluepillFault failed")
-			}
+			m.mapMemorySlot(virtualStart, physicalStart, length, pr.readOnly)
 		}
 
 		// Move to the next chunk.

pkg/sentry/platform/kvm/machine_amd64_unsafe.go

@@ -192,7 +192,11 @@ func seccompMmapSyscall(context unsafe.Pointer) (uintptr, uintptr, unix.Errno) {
 	// MAP_DENYWRITE is deprecated and ignored by kernel. We use it only for seccomp filters.
 	addr, e := hostsyscall.RawSyscall6(uintptr(ctx.Rax), uintptr(ctx.Rdi), uintptr(ctx.Rsi),
 		uintptr(ctx.Rdx), uintptr(ctx.R10)|unix.MAP_DENYWRITE, uintptr(ctx.R8), uintptr(ctx.R9))
-	ctx.Rax = uint64(addr)
+	if e != 0 {
+		ctx.Rax = uint64(-e)
+	} else {
+		ctx.Rax = uint64(addr)
+	}
 
 	return addr, uintptr(ctx.Rsi), unix.Errno(e)
 }

pkg/sentry/platform/kvm/seccomp_mmap_dbg.go

@@ -1,27 +0,0 @@
-// Copyright 2024 The gVisor Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//     http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-//go:build linux
-// +build linux
-
-package kvm
-
-import (
-	"unsafe"
-)
-
-//go:nosplit
-func seccompMmapHandler(context unsafe.Pointer) {
-	throw("seccompMmapHandler isn't implemented for debug builds")
-}

pkg/sentry/platform/kvm/seccomp_mmap_unsafe.go

@@ -61,7 +61,7 @@ func seccompMmapHandler(context unsafe.Pointer) {
 			}
 
 			// Ensure the physical range is mapped.
-			m.mapPhysical(physical, length, physicalRegions)
+			m.mapPhysical(physical, length)
 			virtual += length
 		}
 	}