Magic SysRq Keys

Magic SysRq Key

The Magic SysRq key is a tool for Linux administrators, providing a backdoor to issue direct commands to the kernel in cases where the system becomes unresponsive.

This feature is not just a single key but a combination that includes the Alt and SysRq keys, followed by a command character.

Interfaces

To leverage the Magic SysRq key, users must navigate varying key combinations depending on their system’s architecture

• x86 Systems: Employ the ALT-SysRq plus a specific command key. Note that the SysRq functionality might be combined with the Print Screen key. For keyboards that struggle with simultaneous key presses, sequentially pressing and releasing keys may improve reliability.

• SPARC Systems: The combination is ALT-STOP plus the command key.

• Serial Console: Send a BREAK, followed by a command key within 5 seconds. A double BREAK signals a normal BREAK.

• PowerPC: Use ALT-Print Screen (or F13) followed by the command key. Sometimes, just Print Screen or F13 with the command key suffices.

• General Method: For systems where key combinations are not applicable, writing directly to /proc/sysrq-trigger is a universal approach, e.g.: echo t > /proc/sysrq-trigger. This method ensures that regardless of physical or remote access limitations, system administrators can invoke SysRq commands to manage or recover systems effectively.

Commands

• b: Immediately reboots the system without syncing or unmounting disks.
• c: Triggers a system crash through a NULL pointer dereference, capturing a crashdump if configured.
• d: Displays all held locks.
• e: Sends SIGTERM to all processes except init.
• f: Calls the OOM killer on a memory-intensive process without causing a panic.
• g: Utilized by the kernel debugger (kgdb).
• h: Displays help; any unrecognized key also triggers help.
• i: Sends SIGKILL to all processes except init.
• j: Thaws filesystems frozen by the FIFREEZE ioctl.
• k: Activates the Secure Access Key (SAK), killing all programs on the current virtual console.
• l: Shows a stack backtrace for all active CPUs.
• m: Dumps current memory information to the console.
• n: Makes RT tasks nice-able.
• o: Powers off the system if configured and supported.
• p: Dumps current registers and flags to the console.
• q: Dumps per CPU lists of all armed hrtimers and detailed clockevent device information.
• r: Switches the keyboard from raw mode to XLATE.
• s: Attempts to sync all mounted filesystems.
• t: Dumps a list of current tasks and their information to the console.
• u: Attempts to remount all mounted filesystems read-only.
• v: Forcefully restores the framebuffer console.
• w: Dumps tasks in an uninterruptable (blocked) state.
• x: Interface for xmon on ppc/powerpc platforms or shows global PMU Registers on sparc64 and dumps all TLB entries on MIPS.
• y: Shows global CPU Registers (SPARC-64 specific).
• z: Dumps the ftrace buffer.
• 0-9: Sets the console log level, controlling which kernel messages are printed to the console.

Activation and Configuration

Before using Magic SysRq commands, verify that the feature is enabled on your system. This can often be done by checking or modifying kernel parameters. For example, to enable all functions, you can echo a value to /proc/sys/kernel/sysrq:

echo 1 > /proc/sys/kernel/sysrq

Here’s the list of possible values in /proc/sys/kernel/sysrq:

• 0 - disable sysrq completely
• 1 - enable all functions of sysrq
• >1 - bitmask of allowed sysrq functions (see below for detailed function description):
  • 2 - 0x2 - enable control of console logging level
  • 4 - 0x4 - enable control of keyboard (SAK, unraw)
  • 8 - 0x8 - enable debugging dumps of processes etc.
  • 16 - 0x10 - enable sync command
  • 32 - 0x20 - enable remount read-only
  • 64 - 0x40 - enable signalling of processes (term, kill, oom-kill)
  • 128 - 0x80 - allow reboot/power-off
  • 256 - 0x100 - allow nicing of all RT tasks