[PATCH v4] powerpc: Do not make the entire heap executable
dvlasenk at redhat.com
Tue Aug 23 04:37:05 AEST 2016
On 08/21/2016 05:47 PM, Aneesh Kumar K.V wrote:
> Denys Vlasenko <dvlasenk at redhat.com> writes:
>> On 32-bit powerpc the ELF PLT sections of binaries (built with --bss-plt,
>> or with a toolchain which defaults to it) look like this:
>>  .sbss NOBITS 0002aff8 01aff8 000014 00 WA 0 0 4
>>  .plt NOBITS 0002b00c 01aff8 000084 00 WAX 0 0 4
>>  .bss NOBITS 0002b090 01aff8 0000a4 00 WA 0 0 4
>> Which results in an ELF load header:
>> Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
>> LOAD 0x019c70 0x00029c70 0x00029c70 0x01388 0x014c4 RWE 0x10000
>> This is all correct, the load region containing the PLT is marked as
>> executable. Note that the PLT starts at 0002b00c but the file mapping ends at
>> 0002aff8, so the PLT falls in the 0 fill section described by the load header,
>> and after a page boundary.
>> Unfortunately the generic ELF loader ignores the X bit in the load headers
>> when it creates the 0 filled non-file backed mappings. It assumes all of these
>> mappings are RW BSS sections, which is not the case for PPC.
>> gcc/ld has an option (--secure-plt) to not do this, this is said to incur
>> a small performance penalty.
>> Currently, to support 32-bit binaries with PLT in BSS kernel maps *entire
>> brk area* with executable rights for all binaries, even --secure-plt ones.
> Is this going to break any application ? I am asking because you
> mentioned the patch is lightly tested.
I booted powerpc64 machine with RHEL7 installation,
it did not catch fire.
> x86 do have a
> #define VM_DATA_DEFAULT_FLAGS \
> (((current->personality & READ_IMPLIES_EXEC) ? VM_EXEC : 0 ) | \
> VM_READ | VM_WRITE | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC)
> ie, it can force a read implies exec mode. Do we need that ?
powerpc64 never had that. 32-bit mode may need it, since before
this patch all 32-bit tasks were unconditionally getting
VM_DATA_DEFAULT_FLAGS with VM_EXEC bit.
I'll send an updated patch.
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