Re: [PATCH 2/2] powerpc/rtas: Fix RTAS MSR[HV] handling for Cell

[Jordan Niethe <jniethe5@gmail.com>]

On Tue, 2022-08-23 at 21:59 +1000, Michael Ellerman wrote:
> The semi-recent changes to MSR handling when entering RTAS (firmware)
> cause crashes on IBM Cell machines. An example trace:
> 
>   kernel tried to execute user page (2fff01a8) - exploit attempt? (uid: 0)
>   BUG: Unable to handle kernel instruction fetch
>   Faulting instruction address: 0x2fff01a8
>   Oops: Kernel access of bad area, sig: 11 [#1]
>   BE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=4 NUMA Cell
>   Modules linked in:
>   CPU: 0 PID: 0 Comm: swapper/0 Tainted: G        W          6.0.0-rc2-00433-gede0a8d3307a #207
>   NIP:  000000002fff01a8 LR: 0000000000032608 CTR: 0000000000000000
>   REGS: c0000000015236b0 TRAP: 0400   Tainted: G        W           (6.0.0-rc2-00433-gede0a8d3307a)
>   MSR:  0000000008001002 <ME,RI>  CR: 00000000  XER: 20000000
>   ...
>   NIP 0x2fff01a8
>   LR  0x32608
>   Call Trace:
>     0xc00000000143c5f8 (unreliable)
>     .rtas_call+0x224/0x320
>     .rtas_get_boot_time+0x70/0x150
>     .read_persistent_clock64+0x114/0x140
>     .read_persistent_wall_and_boot_offset+0x24/0x80
>     .timekeeping_init+0x40/0x29c
>     .start_kernel+0x674/0x8f0
>     start_here_common+0x1c/0x50
> 
> Unlike PAPR platforms where RTAS is only used in guests, on the IBM Cell
> machines Linux runs with MSR[HV] set but also uses RTAS, provided by
> SLOF.
> 
> Fix it by copying the MSR[HV] bit from the MSR value we've just read
> using mfmsr into the value used for RTAS.
> 
> It seems like we could also fix it using an #ifdef CELL to set MSR[HV],
> but that doesn't work because it's possible to build a single kernel
> image that runs on both Cell native and pseries.
> 
> Fixes: b6b1c3ce06ca ("powerpc/rtas: Keep MSR[RI] set when calling RTAS")
> Cc: stable@vger.kernel.org # v5.19+
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> ---
>  arch/powerpc/kernel/rtas_entry.S | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/rtas_entry.S b/arch/powerpc/kernel/rtas_entry.S
> index 9a434d42e660..6ce95ddadbcd 100644
> --- a/arch/powerpc/kernel/rtas_entry.S
> +++ b/arch/powerpc/kernel/rtas_entry.S
> @@ -109,8 +109,12 @@ _GLOBAL(enter_rtas)
>  	 * its critical regions (as specified in PAPR+ section 7.2.1). MSR[S]
>  	 * is not impacted by RFI_TO_KERNEL (only urfid can unset it). So if
>  	 * MSR[S] is set, it will remain when entering RTAS.
> +	 * If we're in HV mode, RTAS must also run in HV mode, so extract MSR_HV
> +	 * from the saved MSR value and insert into the value RTAS will use.
>  	 */

Interestingly it looks like these are the first uses of these extended
mnemonics in the kernel?

> +	extrdi	r0, r6, 1, 63 - MSR_HV_LG

Or in non-mnemonic form...
rldicl  r0, r6, 64 - MSR_HV_LG, 63

>  	LOAD_REG_IMMEDIATE(r6, MSR_ME | MSR_RI)
> +	insrdi	r6, r0, 1, 63 - MSR_HV_LG

Or in non-mnemonic form...
rldimi	r6, r0, MSR_HV_LG, 63 - MSR_HV_LG

It is ok to use r0 as a scratch register as it is loaded with 0 afterwards anyway.

>  
>  	li      r0,0
>  	mtmsrd  r0,1                    /* disable RI before using SRR0/1 */

Reviewed-by: Jordan Niethe <jniethe5@gmail.com>