[PATCH] powerpc: Save AMR/IAMR when switching tasks

[Samuel Holland]

On 9/17/22 03:16, Christophe Leroy wrote:
> Le 16/09/2022 à 07:05, Samuel Holland a écrit :
>> With CONFIG_PREEMPT=y (involuntary preemption enabled), it is possible
>> to switch away from a task inside copy_{from,to}_user. This left the CPU
>> with userspace access enabled until after the next IRQ or privilege
>> level switch, when AMR/IAMR got reset to AMR_KU[AE]P_BLOCKED. Then, when
>> switching back to the original task, the userspace access would fault:
> 
> This is not supposed to happen. You never switch away from a task 
> magically. Task switch will always happen in an interrupt, that means 
> copy_{from,to}_user() get interrupted.

That makes sense, the interrupt handler is responsible for saving the
KUAP status. It looks like neither DEFINE_INTERRUPT_HANDLER_RAW nor any
of its users (performance_monitor_exception(), do_slb_fault()) do that.
Yet they still call one of the interrupt_return variants, which restores
the status from the stack.

> Whenever an interrupt is taken, kuap_save_amr_and_lock() macro is used 
> to save KUAP status into the stack then lock KUAP access. At interrupt 
> exit, kuap_kernel_restore() macro or function is used to restore KUAP 
> access from the stack. At the time the task switch happens, KUAP access 
> is expected to be locked. During task switch, the stack is switched so 
> the KUAP status is taken back from the new task's stack.

What if another task calls schedule() from kernel process context, and
the scheduler switches to a task that had been preempted inside
copy_{from,to}_user()? Then there is no interrupt involved, and I don't
see where kuap_kernel_restore() would get called.

> Your fix suggests that there is some path where the KUAP status is not 
> properly saved and/or restored. Did you try running with 
> CONFIG_PPC_KUAP_DEBUG ? It should warn whenever a KUAP access is left 
> unlocked.
> 
>>
>>    Kernel attempted to write user page (3fff7ab68190) - exploit attempt? (uid: 65536)
>>    ------------[ cut here ]------------
>>    Bug: Write fault blocked by KUAP!
>>    WARNING: CPU: 56 PID: 4939 at arch/powerpc/mm/fault.c:228 ___do_page_fault+0x7b4/0xaa0
>>    CPU: 56 PID: 4939 Comm: git Tainted: G        W         5.19.8-00005-gba424747260d #1
>>    NIP:  c0000000000555e4 LR: c0000000000555e0 CTR: c00000000079d9d0
>>    REGS: c00000008f507370 TRAP: 0700   Tainted: G        W          (5.19.8-00005-gba424747260d)
>>    MSR:  9000000000021033 <SF,HV,ME,IR,DR,RI,LE>  CR: 28042222  XER: 20040000
>>    CFAR: c000000000123780 IRQMASK: 3
>>    NIP [c0000000000555e4] ___do_page_fault+0x7b4/0xaa0
>>    LR [c0000000000555e0] ___do_page_fault+0x7b0/0xaa0
>>    Call Trace:
>>    [c00000008f507610] [c0000000000555e0] ___do_page_fault+0x7b0/0xaa0 (unreliable)
>>    [c00000008f5076c0] [c000000000055938] do_page_fault+0x68/0x130
>>    [c00000008f5076f0] [c000000000008914] data_access_common_virt+0x194/0x1f0
>>    --- interrupt: 300 at __copy_tofrom_user_base+0x9c/0x5a4
> 
> ...
> 
>>
>> Fix this by saving and restoring the kernel-side AMR/IAMR values when
>> switching tasks.
> 
> As explained above, KUAP access should be locked at that time, so saving 
> and restoring it should not have any effect. If it does, it means 
> something goes wrong somewhere else.
> 
>>
>> Fixes: 890274c2dc4c ("powerpc/64s: Implement KUAP for Radix MMU")
>> Signed-off-by: Samuel Holland <samuel at sholland.org>
>> ---
>> I have no idea if this is the right change to make, and it could be
>> optimized, but my system has been stable with this patch for 5 days now.
>>
>> Without the patch, I hit the bug every few minutes when my load average
>> is <1, and I hit it immediately if I try to do a parallel kernel build.
> 
> Great, then can you make a try with CONFIG_PPC_KUAP_DEBUG ?

Yes, I will try this out in the next few days.

Regards,
Samuel