linux-kernel - [RFC] kmemcheck: TODO for stack tracking

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Date:	Fri, 28 Nov 2008 12:16:48 +0100
From:	"Vegard Nossum" <vegard.nossum@...il.com>
To:	"Ingo Molnar" <mingo@...e.hu>,
	"Pekka Enberg" <penberg@...helsinki.fi>
Cc:	"Eugene Teo" <eugeneteo@...nel.sg>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: [RFC] kmemcheck: TODO for stack tracking

Hi,

Here's a plan for how to do stack tracking with kmemcheck. It is not
entirely trivial, but as far as I can see, it SHOULD be possible.
Please let me know if you can spot any fallacies or other problems.
I've probably missed something...

/*
 * TODO for stack tracking in kmemcheck:
 *
 * 1. Make kernel run at CPL = 1
 *
 * This includes (I guess) changing the various privilege levels in most
 * system descriptors and descriptor tables, and probably the IOPL. Are there
 * any CPU features which always require CPL = 0 to work? Paging requires no
 * change, as the U/S flag distinguishes between CPL = 0, 1, 2 and CPL = 3
 * only.
 *
 * 2. Modify TSS to use separate stacks for CPL = 0 and CPL = 1
 * 3. Install a Call Gate for Page Faults in the GDT with DPL = 0
 * 4. Change IDT entry for #PF to point to Call Gate in GDT
 *
 * Now when a #PF occurs in kernel mode, CPU will look up the IDT entry for
 * #PF. It points to our Call Gate in the GDT, which has a different privilege
 * level, so the CPU will look up the new stack to use in the TSS. In the new
 * stack, SS, ESP, CS, and EIP are saved. Note: page_fault() will have to take
 * care of handling the extra SS/ESP parameters. End of note. Observe that the
 * old stack has not been touched by the CPU at all (this would lead to a #DF,
 * Double Fault, which is irrecoverable). Observe also that none of the
 * interrupted task's registers have been modified. Now the CPU transfers
 * control to page_fault(), which must save all registers, etc. as usual.
 *
 * do_page_fault() must NOT be allowed to enable interrupts, otherwise we
 * could take interrupts that would use the new stack. If the interrupt
 * handler takes another page fault, the CPU will already be in CPL = 0 and no
 * stack switch will occur!
 *
 * I think we need to make the kernel switch stacks on ALL interrupts. When
 * the CPU is interrupted, it will attempt to push CS/EIP on the current
 * stack. If the PTE of the current stack is non-present, a Page Fault will be
 * generated (not a Double Fault!). However, we have no way to tell if the #PF
 * was generated by an interrupt.
 *
 * 5. Implement support for PUSHA/POPA instruction handling in kmemcheck. No
 *    extra support will be needed for IRET, as interrupts must not be allowed
 *    to occur when the stack is located in a non-present page.
 *
 * Note that it is possible to track POPF/IRET instructions (even though they
 * modify EFLAGS and the Trap Flag), because the CPU does the right thing and
 * raises the Debug Exception based on the previous setting of TF.
 *
 * 6. The kernel stack tracer would need to be modified to understand stack
 *    changes/boundaries.
 */

Vegard

-- 
"The animistic metaphor of the bug that maliciously sneaked in while
the programmer was not looking is intellectually dishonest as it
disguises that the error is the programmer's own creation."
	-- E. W. Dijkstra, EWD1036
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