Re: [PATCH] powerpc/highmem: Properly handle fragmented memory

[Pali Rohár]

On Tuesday 20 September 2022 19:36:42 Christophe Leroy wrote:
> In addition to checking whether a page is reserved before allocating
> it to highmem, verify that it is valid memory.
> 
> Otherwise the kernel Oopses as below:
> 
> [    0.000000] mem auto-init: stack:off, heap alloc:off, heap free:off
> [    0.000000] Kernel attempted to read user page (7df58) - exploit attempt? (uid: 0)
> [    0.000000] BUG: Unable to handle kernel data access on read at 0x0007df58
> [    0.000000] Faulting instruction address: 0xc01c8348
> [    0.000000] Oops: Kernel access of bad area, sig: 11 [#1]
> [    0.000000] BE PAGE_SIZE=4K SMP NR_CPUS=2 P2020RDB-PC
> [    0.000000] Modules linked in:
> [    0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 6.0.0-rc2-0caacb197b677410bdac81bc34f05235+ #121
> [    0.000000] NIP:  c01c8348 LR: c01cb2bc CTR: 0000000a
> [    0.000000] REGS: c10d7e20 TRAP: 0300   Not tainted  (6.0.0-rc2-0caacb197b677410bdac81bc34f05235+)
> [    0.000000] MSR:  00021000 <CE,ME>  CR: 48044224  XER: 00000000
> [    0.000000] DEAR: 0007df58 ESR: 00000000
> [    0.000000] GPR00: c01cb294 c10d7f10 c1045340 00000001 00000004 c112bcc0 00000015 eedf1000
> [    0.000000] GPR08: 00000003 0007df58 00000000 f0000000 28044228 00000200 00000000 00000000
> [    0.000000] GPR16: 00000000 00000000 00000000 0275cb7a c0000000 00000001 0000075f 00000000
> [    0.000000] GPR24: c1031004 00000000 00000000 00000001 c10f0000 eedf1000 00080000 00080000
> [    0.000000] NIP [c01c8348] free_unref_page_prepare.part.93+0x48/0x60
> [    0.000000] LR [c01cb2bc] free_unref_page+0x84/0x4b8
> [    0.000000] Call Trace:
> [    0.000000] [c10d7f10] [eedf1000] 0xeedf1000 (unreliable)
> [    0.000000] [c10d7f20] [c01cb294] free_unref_page+0x5c/0x4b8
> [    0.000000] [c10d7f70] [c1007644] mem_init+0xd0/0x194
> [    0.000000] [c10d7fa0] [c1000e4c] start_kernel+0x4c0/0x6d0
> [    0.000000] [c10d7ff0] [c00003e0] set_ivor+0x13c/0x178
> [    0.000000] Instruction dump:
> [    0.000000] 552817be 5509103a 7d294214 55293830 7d4a4a14 812a003c 814a0038 5529002a
> [    0.000000] 7c892050 5484c23a 5489eafa 548406fe <7d2a482e> 7d242430 5484077e 90870010
> [    0.000000] ---[ end trace 0000000000000000 ]---
> 
> Reported-by: Pali Rohár <pali@xxxxxxxxxx>
> Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>

Hello and thank you for care about this issue!

I have already tested this change, it fixes crash, so add my:

Tested-by: Pali Rohár <pali@xxxxxxxxxx>

Anyway, should there be some Fixes: tag? E.g.

Fixes: b0e0d68b1c52 ("powerpc/32: Allow fragmented physical memory")

(which enabled support for fragmented memory and therefore kernel
started crashing?)

Link: https://lore.kernel.org/linuxppc-dev/20220908201701.sd3zqn5hfixmjvhh@pali/

> ---
>  arch/powerpc/mm/mem.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
> index 01772e79fd93..6ddbd6cb3a2a 100644
> --- a/arch/powerpc/mm/mem.c
> +++ b/arch/powerpc/mm/mem.c
> @@ -302,7 +302,7 @@ void __init mem_init(void)
>  		for (pfn = highmem_mapnr; pfn < max_mapnr; ++pfn) {
>  			phys_addr_t paddr = (phys_addr_t)pfn << PAGE_SHIFT;
>  			struct page *page = pfn_to_page(pfn);
> -			if (!memblock_is_reserved(paddr))
> +			if (memblock_is_memory(paddr) && !memblock_is_reserved(paddr))
>  				free_highmem_page(page);
>  		}
>  	}
> -- 
> 2.37.1
>