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

On 08/21/2016 05:47 PM, Aneesh Kumar K.V wrote:

Denys Vlasenko [off-list ref] writes:

quoted

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.