Processor Privilege-Levels How the x86 processor accomplishes transitions among its four distinct privilege-levels

Rationale The usefulness of protected-mode derives from its ability to enforce restrictions upon software’s ability to perform certain actions Four distinct privilege-levels are supported Organizing concept: concentric “rings” Innermost ring has greatest privileges, and privileges diminish as rings move outward

Four Privilege Rings Ring 3 Ring 2 Ring 1 Ring 0 Least-trusted level Most-trusted level

Suggested purposes Ring0: operating system kernel Ring1: operating system services Rin2: custom extensions Ring3: ordinary user applications

Unix/Linux and Windows Ring0: operating system Ring1: unused Ring2: unused Ring3: application programs

Legal Ring-Transitions A transition from an outer ring to an inner ring is only possible by using a special control-structure (known as a call gate) The ‘gate’ is defined by a data-structure located in a system memory-segment normally inaccessible to modifications A transition from an inner ring to an outer ring is not nearly so strictly controlled

Data-sharing Procedure-calls typically require that two separate routines share some data-values (e.g., parameter-values get passed from the calling routine to the called routine) To support reentrancy and recursion, the processor’s stack is frequently used as a shared-access storage-area But among routines with different levels of privilege, this could create “security hole”

An example senario Say a procedure that executes in ring 3 calls a procedure that executes in ring 2 The ring 2 procedure uses a portion of its stack-area to create ‘automatic’ variables that it uses for temporary workspace Upon return, the ring 3 procedure would be able to examine whatever values are left behind in this ring 2 workspace

Data Isolation To guard against unintentional sharing of privileged information, different stacks are provided at each distinct privilege-level Accordingly, any transition from one ring to another must necessarily be accompanied by a ‘stack-switch’ operation The CPU provides for automatic switching of stacks and copying of parameter-values

Call-Gate Descriptors offset[ ] code-selectoroffset[ ] gate type P0 DPLDPL parameter count Legend: P=present (1=yes, 0=no) DPL=Descriptor Prvilege Level (0,1,2,3) code-selector (specifies memory-segment containing procedure code) offset (specifies the procedure’s entry-point within its code-segment) parameter count (specifies how many parameter-values will be copied) gate-type (‘0x4’ means a 16-bit call-gate, ‘0xC’ means a 32-bit call-gate)

An Interprivilege Call When a lesser privileged routine wants to invoke a more privileged routine, it does so by using a ‘far call’ machine-instruction In as86 assembly language: callf#0, #callgate-selector 0x9A (ignored)callgate-selector opcode offset-field segment-field

What does the CPU do? When CPU fetches a far-call instruction, it looks up that instruction’s specified descriptor If it’s a call-gate descriptor, and if access is allowed (i.e., CPL  DPL), then the CPU will perform a complex series of actions to accomplish a requested ring-transition CPL (Current Privilege Level) is based on least significant 2-bits in CS (also in SS)

Series of CPU Actions - pushes the current SS:SP register-values onto a new stack-segment - copies the specified number of parameters from the old stack onto the new stack - pushes the updated CS:IP register-values onto the new stack - loads new values into registers CS:IP (from the callgate-descriptor) and into SS:SP

The missing info? Where do the new values for SS:SP come from? (They’re not found in the callgate) They’re from a special system-segment, called the TSS (Task State Segment) The CPU locates its TSS by referring to the value in register TR (Task Register)

Diagram of Relationships TASK STATE SEGMENT NEW STACK SEGMENT stack-pointer OLD STACK SEGMENT params SS:SP Descriptor-Table gate-descriptor call-instruction TSS-descriptor TR CS:IP GDTR old code-segment new code-segment called procedure

Return to an Outer Ring Use the far-return instruction: retf –Restores CS:IP from current stack –Restores SS:SP from current stack Or use the far-return instruction: retf n –Restores CS:IP from current stack –Discards n bytes from stack (parameters) –Restores SS:IP from current stack

Demo-program: ‘tryring1.s’ We have created a short program to show how this ring-transition mechanism works It enters protected-mode (at ring0) It ‘returns’ to a procedure in ring1 Procedure shows a confirmation-message The ring1 procedure then ‘calls’ to ring0 The ring0 procedure exits protected-mode

Data-structures needed Global Descriptor Table needs to contain the protected-mode segment-descriptors and the call-gate descriptor –Code-segments for Ring0 and Ring1 –Stack-segments for Ring0 and Ring1 –Data-segment (for Ring1 to write to VRAM) –Task-State Segment (for ring0 SS:SP) –Task-Gate and TSS Descriptors (for ‘callf’)

In-class Exercise #1 Modify the ‘tryring1.s’ demo so that it uses a 32-bit call-gate and a 32-bit TSS TSS for (16-bits) SP1 SP2 SS SP0 SS0 SS1 ESP0 ESP1 ESP2 SS0 SS1 SS2 TSS for (32-bits) ……

System Segment-Descriptors Base[ ]Limit[ ] Limit [19..16] Base[ ]Base[ ]type DPLDPL P0 0 = reserved 1 = 16-bit TSS (available) 2 = LDT 3 = 16-bit TSS (busy) type 8 = reserved 9 = 32-bit TSS (available) A = reserved B = 32-bit TSS (busy) S-bit is zero

In-class exercise #2 Modify the ‘tryring1.s’ demo so that it first enters ring2, then calls to ring1 from ring2 (but returns to ring2), and then finally calls to ring0 in order to exit protected-mode How many stack-segments do you need? How many code-segment descriptors? How many VRAM-segment descriptors?