Re: [PATCH 2/2] powerpc/64s/radix: Fix RWX mapping with relocated kernel

[Sachin Sant <sachinp@linux.ibm.com>]

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> On 10-Jan-2023, at 6:17 PM, Michael Ellerman <mpe@ellerman.id.au> wrote:
> 
> If a relocatable kernel is loaded at a non-zero address and told not to
> relocate to zero (kdump or RELOCATABLE_TEST), the mapping of the
> interrupt code at zero is left with RWX permissions.
> 
> That is a security weakness, and leads to a warning at boot if
> CONFIG_DEBUG_WX is enabled:
> 
>  powerpc/mm: Found insecure W+X mapping at address 00000000056435bc/0xc000000000000000
>  WARNING: CPU: 1 PID: 1 at arch/powerpc/mm/ptdump/ptdump.c:193 note_page+0x484/0x4c0
>  CPU: 1 PID: 1 Comm: swapper/0 Not tainted 6.2.0-rc1-00001-g8ae8e98aea82-dirty #175
>  Hardware name: IBM pSeries (emulated by qemu) POWER9 (raw) 0x4e1202 0xf000005 of:SLOF,git-dd0dca hv:linux,kvm pSeries
>  NIP:  c0000000004a1c34 LR: c0000000004a1c30 CTR: 0000000000000000
>  REGS: c000000003503770 TRAP: 0700   Not tainted  (6.2.0-rc1-00001-g8ae8e98aea82-dirty)
>  MSR:  8000000002029033 <SF,VEC,EE,ME,IR,DR,RI,LE>  CR: 24000220  XER: 00000000
>  CFAR: c000000000545a58 IRQMASK: 0
>  ...
>  NIP note_page+0x484/0x4c0
>  LR  note_page+0x480/0x4c0
>  Call Trace:
>    note_page+0x480/0x4c0 (unreliable)
>    ptdump_pmd_entry+0xc8/0x100
>    walk_pgd_range+0x618/0xab0
>    walk_page_range_novma+0x74/0xc0
>    ptdump_walk_pgd+0x98/0x170
>    ptdump_check_wx+0x94/0x100
>    mark_rodata_ro+0x30/0x70
>    kernel_init+0x78/0x1a0
>    ret_from_kernel_thread+0x5c/0x64
> 
> The fix has two parts. Firstly the pages from zero up to the end of
> interrupts need to be marked read-only, so that they are left with R-X
> permissions. Secondly the mapping logic needs to be taught to ensure
> there is a page boundary at the end of the interrupt region, so that the
> permission change only applies to the interrupt text, and not the region
> following it.
> 
> Fixes: c55d7b5e6426 ("powerpc: Remove STRICT_KERNEL_RWX incompatibility with RELOCATABLE")
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> ---

Thanks Michael. This fixes the problem reported earlier

https://lore.kernel.org/linuxppc-dev/48206911-FD3D-401A-A69D-1A79403E79E2@linux.ibm.com/

Reported-by: Sachin Sant <sachinp@linux.ibm.com>
Tested-by: Sachin Sant <sachinp@linux.ibm.com>

- Sachin

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