[PATCH] powerpc: booke: fix boot crash due to null hugepd

[Michael Ellerman]

Scott Wood <oss at buserror.net> writes:

> On Tue, 2017-02-28 at 14:55 +0000, Laurentiu Tudor wrote:
>> On 02/17/2017 02:18 PM, Aneesh Kumar K.V wrote:
>> > laurentiu.tudor at nxp.com writes:
>> > > From: Laurentiu Tudor <laurentiu.tudor at nxp.com>
>> > > 
>> > > On 32-bit book-e machines, hugepd_ok() does not take
>> > > into account null hugepd values, causing this crash at boot:
>> > > 
>> > > Unable to handle kernel paging request for data at address 0x80000000
>> > > Faulting instruction address: 0xc00182a8
>> > > Oops: Kernel access of bad area, sig: 11 [#1]
>> > > SMP NR_CPUS=24
>> > > CoreNet Generic
>> > > Modules linked in:
>> > > CPU: 1 PID: 1 Comm: swapper/0 Tainted: G        W       4.10.0-rc8-
>> > > 00016-g69b1f87 #11
>> > > task: e5050000 task.stack: e5058000
>> > > NIP: c00182a8 LR: c001829c CTR: 00007ffe
>> > > REGS: e5059c50 TRAP: 0300   Tainted: G        W        (4.10.0-rc8-
>> > > 00016-g69b1f87)
>> > > MSR: 00021002 <CE,ME>
>> > >    CR: 88428e82  XER: 00000000
>> > > DEAR: 80000000 ESR: 00000000
>> > > GPR00: c0107510 e5059d00 e5050000 80000000 bffffff1 e5059d0c e5059d08
>> > > 00002017
>> > > GPR08: 00000000 00000000 00000000 00000000 28428e82 00000000 c00027d0
>> > > 00000000
>> > > GPR16: 00000000 00000000 88a28e82 20000000 48422e82 00000000 88a28e84
>> > > dd004000
>> > > GPR24: e5059e38 00000000 00000000 bffffff1 dd004000 00000001 00029002
>> > > bffffff1
>> > > NIP [c00182a8] follow_huge_addr+0x38/0xf0
>> > > LR [c001829c] follow_huge_addr+0x2c/0xf0
>> > > Call Trace:
>> > > [e5059d00] [e5059d00] 0xe5059d00 (unreliable)
>> > > [e5059d20] [c0107510] follow_page_mask+0x40/0x3c0
>> > > [e5059d80] [c0107958] __get_user_pages+0xc8/0x420
>> > > [e5059de0] [c010817c] get_user_pages_remote+0x8c/0x230
>> > > [e5059e30] [c013f170] copy_strings+0x110/0x3a0
>> > > [e5059ea0] [c013f42c] copy_strings_kernel+0x2c/0x50
>> > > [e5059ec0] [c0141324] do_execveat_common+0x474/0x620
>> > > [e5059f10] [c01414fc] do_execve+0x2c/0x40
>> > > [e5059f20] [c0001f68] try_to_run_init_process+0x18/0x60
>> > > [e5059f30] [c000289c] kernel_init+0xcc/0x120
>> > > [e5059f40] [c000f1e8] ret_from_kernel_thread+0x5c/0x64
>> > > Instruction dump:
>> > > bfc10018 7c9f2378 90010024 7fc000a6 7c000146 80630020 38a1000c 38c10008
>> > > 4bfff869 2c030000 41c20090 81210008 <81430000> 81630004 3860ffea
>> > > 2f890000
>> > > ---[ end trace 4bf94e15fd9fa824 ]---
>> > 
>> > Which code path is that. That null should be filtered by the if
>> > (pmd_none(pmd)) check in find_linux_pte_or_hugepte right ?
>> The crash happens when __find_linux_pte_or_hugepte() calls hugepd_ok(),
>> on this line [1]. It's triggered when __find_linux_pte_or_hugepte() is
>> first called, when the kernel tries to spawn the init process. The input
>> effective address (ea arg) is bffffff1. This is the call stack:
>
> What is the pmd value?  There's a pmd_none() check before that line.

It's a pgd, so a pgd_none() check.

But that does nothing because this is 32-bit, 4K PAGE_SIZE, which uses
pgtable-nopmd.h and pgtable-nopud.h, so pgd_none() is just:

  int pgd_none(pgd_t pgd) { return 0; }

> That said, regardless of what's going wrong here, it would be simpler and more
> robust if is_hugepd() returned false for empty ptes rather than assuming the
> caller explicitly checked pmd_none().

Yeah, in fact it has to, because of the above.

So Laurentiu's patch is pretty much the correct fix.

cheers