Re: [PATCH v2 6/9] s390/module: Use s390_kernel_write() for late relocations

Christian Borntraeger
2020-04-24T07:20:42Z

On 24.04.20 01:26, Josh Poimboeuf wrote:
> On Thu, Apr 23, 2020 at 01:10:30PM -0500, Josh Poimboeuf wrote:
>> On Thu, Apr 23, 2020 at 09:12:28AM -0500, Josh Poimboeuf wrote:
>>>>> this is strange. While I would have expected an exception similar to
>>>>> this, it really should have happened on the "sturg" instruction which
>>>>> does the DAT-off store in s390_kernel_write(), and certainly not with
>>>>> an ID of 0004 (protection). However, in your case, it happens on a
>>>>> normal store instruction, with 0004 indicating a protection exception.
>>>>>
>>>>> This is more like what I would expect e.g. in the case where you do
>>>>> _not_ use the s390_kernel_write() function for RO module text patching,
>>>>> but rather normal memory access. So I am pretty sure that this is not
>>>>> related to the s390_kernel_write(), but some other issue, maybe some
>>>>> place left where you still use normal memory access?
>>>>
>>>> The call trace above also suggests that it is not a late relocation, no?
>>>> The path is from KLP module init function through klp_enable_patch. It should
>>>> mean that the to-be-patched object is loaded (it must be a module thanks
>>>> to a check klp_init_object_loaded(), vmlinux relocations were processed
>>>> earlier in apply_relocations()).
>>>>
>>>> However, the KLP module state here must be COMING, so s390_kernel_write()
>>>> should be used. What are we missing?
>>>
>>> I'm also scratching my head. It _should_ be using s390_kernel_write()
>>> based on the module state, but I don't see that on the stack trace.
>>>
>>> This trace (and Gerald's comment) seem to imply it's using
>>> __builtin_memcpy(), which might expected for UNFORMED state.
>>>
>>> Weird...
>>
>> Mystery solved:
>>
>> $ CROSS_COMPILE=s390x-linux-gnu- scripts/faddr2line vmlinux apply_rela+0x16a/0x520
>> apply_rela+0x16a/0x520:
>> apply_rela at arch/s390/kernel/module.c:336
>>
>> which corresponds to the following code in apply_rela():
>>
>>
>> case R_390_PLTOFF64: /* 16 bit offset from GOT to PLT. */
>> if (info->plt_initialized == 0) {
>> unsigned int *ip;
>> ip = me->core_layout.base + me->arch.plt_offset +
>> info->plt_offset;
>> ip[0] = 0x0d10e310; /* basr 1,0 */
>> ip[1] = 0x100a0004; /* lg 1,10(1) */
>>
>>
>> Notice how it's writing directly to text... oops.
>
> Here's a fix, using write() for the PLT and the GOT.

Are you going to provide a proper patch?

>
> diff --git a/arch/s390/kernel/module.c b/arch/s390/kernel/module.c
> index 2798329ebb74..fe446f42818f 100644
> --- a/arch/s390/kernel/module.c
> +++ b/arch/s390/kernel/module.c
> @@ -297,7 +297,7 @@ static int apply_rela(Elf_Rela *rela, Elf_Addr base, Elf_Sym *symtab,
>
> gotent = me->core_layout.base + me->arch.got_offset +
> info->got_offset;
> - *gotent = val;
> + write(gotent, &val, sizeof(*gotent));
> info->got_initialized = 1;
> }
> val = info->got_offset + rela->r_addend;
> @@ -330,25 +330,29 @@ static int apply_rela(Elf_Rela *rela, Elf_Addr base, Elf_Sym *symtab,
> case R_390_PLTOFF32: /* 32 bit offset from GOT to PLT. */
> case R_390_PLTOFF64: /* 16 bit offset from GOT to PLT. */
> if (info->plt_initialized == 0) {
> - unsigned int *ip;
> + unsigned int *ip, insn[5];
> +
> ip = me->core_layout.base + me->arch.plt_offset +
> info->plt_offset;
> - ip[0] = 0x0d10e310; /* basr 1,0 */
> - ip[1] = 0x100a0004; /* lg 1,10(1) */
> +
> + insn[0] = 0x0d10e310; /* basr 1,0 */
> + insn[1] = 0x100a0004; /* lg 1,10(1) */
> if (IS_ENABLED(CONFIG_EXPOLINE) && !nospec_disable) {
> unsigned int *ij;
> ij = me->core_layout.base +
> me->arch.plt_offset +
> me->arch.plt_size - PLT_ENTRY_SIZE;
> - ip[2] = 0xa7f40000 + /* j __jump_r1 */
> + insn[2] = 0xa7f40000 + /* j __jump_r1 */
> (unsigned int)(u16)
> (((unsigned long) ij - 8 -
> (unsigned long) ip) / 2);
> } else {
> - ip[2] = 0x07f10000; /* br %r1 */
> + insn[2] = 0x07f10000; /* br %r1 */
> }
> - ip[3] = (unsigned int) (val >> 32);
> - ip[4] = (unsigned int) val;
> + insn[3] = (unsigned int) (val >> 32);
> + insn[4] = (unsigned int) val;
> +
> + write(ip, insn, sizeof(insn));
> info->plt_initialized = 1;
> }
> if (r_type == R_390_PLTOFF16 ||
>