[PATCH] powerpc/livepatch: Fix livepatch stack access

[Michael Ellerman]

Kamalesh Babulal <kamalesh at linux.vnet.ibm.com> writes:

> While running stress test with livepatch module loaded, kernel
> bug was triggered.
>
> cpu 0x5: Vector: 400 (Instruction Access) at [c0000000eb9d3b60]
>     pc: c0000000eb9d3e30
>     lr: c0000000eb9d3e30
>     sp: c0000000eb9d3de0
>    msr: 800000001280b033
>   current = 0xc0000000dbd38700
>   paca    = 0xc00000000fe01400   softe: 0        irq_happened: 0x01
>     pid   = 8618, comm = make
> Linux version 4.13.0+ (root at ubuntu) (gcc version 6.3.0 20170406 (Ubuntu 6.3.0-12ubuntu2)) #1 SMP Wed Sep 13 03:49:27 EDT 2017
>
> 5:mon> t
> [c0000000eb9d3de0] c0000000eb9d3e30 (unreliable)
> [c0000000eb9d3e30] c000000000008ab4 hardware_interrupt_common+0x114/0x120
>  --- Exception: 501 (Hardware Interrupt) at c000000000053040 livepatch_handler+0x4c/0x74
> [c0000000eb9d4120] 0000000057ac6e9d (unreliable)
> [d0000000089d9f78] 2e0965747962382e
> SP (965747962342e09) is in userspace
>
> When an interrupt is served in between the livepatch_handler execution,
> there are chances of the livepatch_stack/task task getting corrupted.

Ouch. That's pretty broken by me.

> Fix the corruption by using r11 register for livepatch stack manipulation,
> instead of shuffling task stack and livepatch stack into r1 register.
> Using r11 register also avoids disabling/enabling irq's while setting
> up the livepatch stack.

I'm trying to think if there's some reason I didn't use r11. But I can't
remember anything specific. I suspect I just didn't check the ABI
closely enough, and knew I could use r0 and r12 so stuck with them.

cheers