Re: [PATCH v4] powerpc: Do not make the entire heap executable

[Denys Vlasenko]

On 08/21/2016 05:47 PM, Aneesh Kumar K.V wrote:
> Denys Vlasenko <dvlasenk@xxxxxxxxxx> 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:
> >
> >   [17] .sbss             NOBITS          0002aff8 01aff8 000014 00  WA  0   0  4
> >   [18] .plt              NOBITS          0002b00c 01aff8 000084 00 WAX  0   0  4
> >   [19] .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.