Commit ce81bb25 authored Oct 15, 2020 by Chris Kennelly Committed by Linus Torvalds Oct 16, 2020

fs/binfmt_elf: use PT_LOAD p_align values for suitable start address

Patch series "Selecting Load Addresses According to p_align", v3.

The current ELF loading mechancism provides page-aligned mappings.  This
can lead to the program being loaded in a way unsuitable for file-backed,
transparent huge pages when handling PIE executables.

While specifying -z,max-page-size=0x200000 to the linker will generate
suitably aligned segments for huge pages on x86_64, the executable needs
to be loaded at a suitably aligned address as well.  This alignment
requires the binary's cooperation, as distinct segments need to be
appropriately paddded to be eligible for THP.

For binaries built with increased alignment, this limits the number of
bits usable for ASLR, but provides some randomization over using fixed
load addresses/non-PIE binaries.

This patch (of 2):

The current ELF loading mechancism provides page-aligned mappings.  This
can lead to the program being loaded in a way unsuitable for file-backed,
transparent huge pages when handling PIE executables.

For binaries built with increased alignment, this limits the number of
bits usable for ASLR, but provides some randomization over using fixed
load addresses/non-PIE binaries.

Tested by verifying program with -Wl,-z,max-page-size=0x200000 loading.

[akpm@linux-foundation.org: fix max() warning]
[ckennelly@google.com: augment comment]
  Link: https://lkml.kernel.org/r/20200821233848.3904680-2-ckennelly@google.com



Signed-off-by: Chris Kennelly <ckennelly@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Hugh Dickens <hughd@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Sandeep Patil <sspatil@google.com>
Cc: Fangrui Song <maskray@google.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Shuah Khan <shuah@kernel.org>
Link: https://lkml.kernel.org/r/20200820170541.1132271-1-ckennelly@google.com
Link: https://lkml.kernel.org/r/20200820170541.1132271-2-ckennelly@google.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 48ca2d8a

fs/binfmt_elf.c

+25 −0

Original line number	Diff line number	Diff line
		@@ -13,6 +13,7 @@
		#include <linux/module.h>
		#include <linux/kernel.h>
		#include <linux/fs.h>
		#include <linux/log2.h>
		#include <linux/mm.h>
		#include <linux/mman.h>
		#include <linux/errno.h>
		@@ -421,6 +422,26 @@ static int elf_read(struct file file, void buf, size_t len, loff_t pos)
		return 0;
		}

		static unsigned long maximum_alignment(struct elf_phdr *cmds, int nr)
		{
		unsigned long alignment = 0;
		int i;

		for (i = 0; i < nr; i++) {
		if (cmds[i].p_type == PT_LOAD) {
		unsigned long p_align = cmds[i].p_align;

		/* skip non-power of two alignments as invalid */
		if (!is_power_of_2(p_align))
		continue;
		alignment = max(alignment, p_align);
		}
		}

		/* ensure we align to at least one page */
		return ELF_PAGEALIGN(alignment);
		}

		/**
		* load_elf_phdrs() - load ELF program headers
		* @elf_ex: ELF header of the binary whose program headers should be loaded
		@@ -1008,6 +1029,7 @@ out_free_interp:
		int elf_prot, elf_flags;
		unsigned long k, vaddr;
		unsigned long total_size = 0;
		unsigned long alignment;

		if (elf_ppnt->p_type != PT_LOAD)
		continue;
		@@ -1086,6 +1108,9 @@ out_free_interp:
		load_bias = ELF_ET_DYN_BASE;
		if (current->flags & PF_RANDOMIZE)
		load_bias += arch_mmap_rnd();
		alignment = maximum_alignment(elf_phdata, elf_ex->e_phnum);
		if (alignment)
		load_bias &= ~(alignment - 1);
		elf_flags \|= MAP_FIXED;
		} else
		load_bias = 0;

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