Kernel: Add support for jails

Our implementation for Jails resembles much of how FreeBSD jails are
working - it's essentially only a matter of using a RefPtr in the
Process class to a Jail object. Then, when we iterate over all processes
in various cases, we could ensure if either the current process is in
jail and therefore should be restricted what is visible in terms of
PID isolation, and also to be able to expose metadata about Jails in
/sys/kernel/jails node (which does not reveal anything to a process
which is in jail).

A lifetime model for the Jail object is currently plain simple - there's
simpy no way to manually delete a Jail object once it was created. Such
feature should be carefully designed to allow safe destruction of a Jail
without the possibility of releasing a process which is in Jail from the
actual jail. Each process which is attached into a Jail cannot leave it
until the end of a Process (i.e. when finalizing a Process). All jails
are kept being referenced in the JailManagement. When a last attached
process is finalized, the Jail is automatically destroyed.

Kernel/FileSystem/SysFS/Subsystems/Kernel/Variables/BooleanVariable.cpp

@@ -5,6 +5,7 @@
  */
 
 #include <Kernel/FileSystem/SysFS/Subsystems/Kernel/Variables/BooleanVariable.h>
+#include <Kernel/Process.h>
 #include <Kernel/Sections.h>
 
 namespace Kernel {
@@ -16,18 +17,26 @@ ErrorOr<void> SysFSSystemBoolean::try_generate(KBufferBuilder& builder)
 
 ErrorOr<size_t> SysFSSystemBoolean::write_bytes(off_t, size_t count, UserOrKernelBuffer const& buffer, OpenFileDescription*)
 {
-    if (count != 1)
-        return EINVAL;
     MutexLocker locker(m_refresh_lock);
+    // Note: We do all of this code before taking the spinlock because then we disable
+    // interrupts so page faults will not work.
     char value = 0;
     TRY(buffer.read(&value, 1));
-    if (value == '0')
-        set_value(false);
-    else if (value == '1')
-        set_value(true);
-    else
-        return EINVAL;
-    return 1;
+
+    return Process::current().jail().with([&](auto& my_jail) -> ErrorOr<size_t> {
+        // Note: If we are in a jail, don't let the current process to change the variable.
+        if (my_jail)
+            return Error::from_errno(EPERM);
+        if (count != 1)
+            return Error::from_errno(EINVAL);
+        if (value == '0')
+            set_value(false);
+        else if (value == '1')
+            set_value(true);
+        else
+            return Error::from_errno(EINVAL);
+        return 1;
+    });
 }
 
 ErrorOr<void> SysFSSystemBoolean::truncate(u64 size)

Kernel/FileSystem/SysFS/Subsystems/Kernel/Variables/CapsLockRemap.cpp

@@ -22,12 +22,12 @@ UNMAP_AFTER_INIT NonnullLockRefPtr<SysFSCapsLockRemap> SysFSCapsLockRemap::must_
 
 bool SysFSCapsLockRemap::value() const
 {
-    MutexLocker locker(m_lock);
+    SpinlockLocker locker(m_lock);
     return g_caps_lock_remapped_to_ctrl.load();
 }
 void SysFSCapsLockRemap::set_value(bool new_value)
 {
-    MutexLocker locker(m_lock);
+    SpinlockLocker locker(m_lock);
     g_caps_lock_remapped_to_ctrl.exchange(new_value);
 }
 

Kernel/FileSystem/SysFS/Subsystems/Kernel/Variables/CapsLockRemap.h

@@ -9,6 +9,7 @@
 #include <AK/Types.h>
 #include <Kernel/FileSystem/SysFS/Subsystems/Kernel/Variables/BooleanVariable.h>
 #include <Kernel/Library/LockRefPtr.h>
+#include <Kernel/Locking/Spinlock.h>
 #include <Kernel/UserOrKernelBuffer.h>
 
 namespace Kernel {
@@ -24,7 +25,7 @@ private:
 
     explicit SysFSCapsLockRemap(SysFSDirectory const&);
 
-    mutable Mutex m_lock;
+    mutable Spinlock m_lock { LockRank::None };
 };
 
 }

Kernel/FileSystem/SysFS/Subsystems/Kernel/Variables/DumpKmallocStack.cpp

@@ -22,13 +22,13 @@ UNMAP_AFTER_INIT NonnullLockRefPtr<SysFSDumpKmallocStacks> SysFSDumpKmallocStack
 
 bool SysFSDumpKmallocStacks::value() const
 {
-    MutexLocker locker(m_lock);
+    SpinlockLocker locker(m_lock);
     return g_dump_kmalloc_stacks;
 }
 
 void SysFSDumpKmallocStacks::set_value(bool new_value)
 {
-    MutexLocker locker(m_lock);
+    SpinlockLocker locker(m_lock);
     g_dump_kmalloc_stacks = new_value;
 }
 

Kernel/FileSystem/SysFS/Subsystems/Kernel/Variables/DumpKmallocStack.h

@@ -9,6 +9,7 @@
 #include <AK/Types.h>
 #include <Kernel/FileSystem/SysFS/Subsystems/Kernel/Variables/BooleanVariable.h>
 #include <Kernel/Library/LockRefPtr.h>
+#include <Kernel/Locking/Spinlock.h>
 #include <Kernel/UserOrKernelBuffer.h>
 
 namespace Kernel {
@@ -24,7 +25,7 @@ private:
 
     explicit SysFSDumpKmallocStacks(SysFSDirectory const&);
 
-    mutable Mutex m_lock;
+    mutable Spinlock m_lock { LockRank::None };
 };
 
 }