[PATCH v4] powerpc: Do not make the entire heap executable
dvlasenk at redhat.com
Fri Aug 19 22:33:06 AEST 2016
On 08/10/2016 03:00 PM, Denys Vlasenko wrote:
> 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.
> Stop doing that.
> Teach the ELF loader to check the X bit in the relevant load header
> and create 0 filled anonymous mappings that are executable
> if the load header requests that.
> The patch was originally posted in 2012 by Jason Gunthorpe
> and apparently ignored:
> Lightly run-tested.
Ping powerpc/mm people.
How does this patch look? Are you taking it?
> -static int do_brk(unsigned long addr, unsigned long request)
> +static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long flags)
> struct mm_struct *mm = current->mm;
> struct vm_area_struct *vma, *prev;
> - unsigned long flags, len;
> + unsigned long len;
> struct rb_node **rb_link, *rb_parent;
> pgoff_t pgoff = addr >> PAGE_SHIFT;
> int error;
> @@ -2668,7 +2668,7 @@ static int do_brk(unsigned long addr, unsigned long request)
> if (!len)
> return 0;
> - flags = VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags;
> + flags |= VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags;
> error = get_unmapped_area(NULL, addr, len, 0, MAP_FIXED);
Regarding "maybe VM_LOCKED needs to be masked out of flags?"
in the fragment above.
I agree. In a sense that "Yes, maybe. I don't really know
whether mm people feel it is worth the cost."
I'd be happy to send a new version if someone will express
a definite request to add that.
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