Re: [PATCH v2 2/2] amr64: map FDT as RW for early_init_dt_scan()

[Ard Biesheuvel]

On Wed, 15 May 2019 at 12:24, Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx> wrote:
>
> On Tue, May 14, 2019 at 11:42 PM Mike Rapoport <rppt@xxxxxxxxxxxxx> wrote:
>
> > I'm not sure if early console is available at the time kaslr_early_init()
> > is called, but if yes, running with memblock=debug may shed some light.
> >
> > > I didn't trace the real reason causing this. But in this case, maybe
> > > don't call memblock_reserve() in kaslr?
> >
> > My concern that this uncovered a real bug which might hit us later.
> >
> Hi Mike,
> Thanks for the hint. I tried on my device but seems that earlycon
> happens after the warning call trace, so can't more information.
>
> Since on my device kaslr will be runned, I tried call
> memblock_reserve() in kaslr and not in
> setup_machine_fdt()#fixmap_remap_fdt, but got following warning
>

I realize this is not documented sufficiently in the commit log, but
the reason I introduced the separate __fixmap_remap_fdt() [which does
not call memblock_reserve()] was that the KASLR init code should set
as little global state as possible, given that it is called with the
kernel mapped at the wrong virtual address.

The KASLR boot sequence is something like
- map kernel at default [unrandomized] address
- apply relocations and clear BSS
- run KASLR init to map and parse the FDT [*]
- if KASLR is enabled, unmap the kernel and remap it at the randomized address
- apply relocations and clear BSS
- proceed with start_kernel()

The issue you are seeing is caused by the fact that the memblock
bookkeeping gets into an inconsistent state due to the 2nd clearing of
BSS.

[*] The reason we need to map the FDT this early is to obtain the
random seed, and to check whether 'nokaslr' was passed on the kernel
command line. The reason arm64 deviates from other architectures in
this regard is that we don't have a decompressor, and so there is no
other execution context available where we can run C code to parse the
FDT etc before we enter the kernel proper.




> [    0.000000] memblock_remove:
> [0x0001000000000000-0x0000fffffffffffe] arm64_memblock_init+0x28/0x224
> [    0.000000] memblock_remove:
> [0x0000004040000000-0x000000403ffffffe] arm64_memblock_init+0x64/0x224
> [    0.000000] memblock_reserve:
> [0x0000000040080000-0x00000000413c3fff]
> arm64_memblock_init+0x188/0x224
> [    0.000000] WARNING: CPU: 0 PID: 0 at
> /mnt/host/source/src/third_party/kernel/v4.19/mm/memblock.c:583
> memblock_add_range+0x1bc/0x1c8
> [    0.000000] Modules linked in:
> [    0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 4.19.38 #222
> [    0.000000] Hardware name: MediaTek kukui rev2 board (DT)
> [    0.000000] pstate: 60000085 (nZCv daIf -PAN -UAO)
> [    0.000000] pc : memblock_add_range+0x1bc/0x1c8
> [    0.000000] lr : memblock_add_range+0x30/0x1c8
> [    0.000000] sp : ffffffab68603ea0
> [    0.000000] x29: ffffffab68603ef0 x28: 0000000040954324
> [    0.000000] x27: 0000000040080000 x26: 0000000000080000
> [    0.000000] x25: 0000000080127e4b x24: ffffffab68716000
> [    0.000000] x23: ffffffab680b5000 x22: 0000000001344000
> [    0.000000] x21: 0000000040080000 x20: 0000000000000000
> [    0.000000] x19: ffffffab6864bf00 x18: 00000000fffffc94
> [    0.000000] x17: 000000000000003c x16: ffffffab67d49064
> [    0.000000] x15: 0000000000000006 x14: 626d656d5f34366d
> [    0.000000] x13: 7261205d66666633 x12: 0000000000000000
> [    0.000000] x11: 0000000000000000 x10: ffffffffffffffff
> [    0.000000] x9 : 0000000000011547 x8 : ffffffab68765690
> [    0.000000] x7 : 696e695f6b636f6c x6 : ffffffab6875dd41
> [    0.000000] x5 : 0000000000000000 x4 : 0000000000000000
> [    0.000000] x3 : ffffffab678a24a0 x2 : 0000000001344000
> [    0.000000] x1 : 0000000040080000 x0 : ffffffab6864bf00
> [    0.000000] Call trace:
> [    0.000000]  memblock_add_range+0x1bc/0x1c8
> [    0.000000]  memblock_reserve+0x60/0xac
> [    0.000000]  arm64_memblock_init+0x188/0x224
> [    0.000000]  setup_arch+0x138/0x19c
> [    0.000000]  start_kernel+0x68/0x380
> [    0.000000] random: get_random_bytes called from
> print_oops_end_marker+0x3c/0x58 with crng_init=0
> [    0.000000] ---[ end trace ea99802b425f7adf ]---
> [    0.000000] memblock_reserve:
> [0x000000005f800000-0x000000005f811536]
> early_init_dt_reserve_memory_arch+0x38/0x48
> [    0.000000] memblock_reserve:
> [0x00000000ffe00000-0x00000000ffffffff]
> early_init_dt_reserve_memory_arch+0x38/0x48
>
> So I guess we just can't call memblock_reserve() in kaslr?