| Message ID | 20200820170541.1132271-2-ckennelly@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Selecting Load Addresses According to p_align | expand |
On Thu, 20 Aug 2020 13:05:40 -0400 Chris Kennelly <ckennelly@google.com> wrote: > 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. > > @@ -421,6 +422,24 @@ 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) { > + /* skip non-power of two alignments */ Comment isn't terribly helpful. It explains "what" (which is utterly obvious from the code anyway) but it fails to explain "why". > + if (!is_power_of_2(cmds[i].p_align)) > + continue; > + alignment = max(alignment, cmds[i].p_align); generates a max() warning: fs/binfmt_elf.c:435:16: note: in expansion of macro `max' alignment = max(alignment, cmds[i].p_align); p_align may be Elf64_Xword, may be Elf32_Word, may be something else. That's quite unwieldy and I don't like max_t. How about this? --- a/fs/binfmt_elf.c~fs-binfmt_elf-use-pt_load-p_align-values-for-suitable-start-address-fix +++ a/fs/binfmt_elf.c @@ -429,10 +429,12 @@ static unsigned long maximum_alignment(s 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 */ - if (!is_power_of_2(cmds[i].p_align)) + if (!is_power_of_2(p_align)) continue; - alignment = max(alignment, cmds[i].p_align); + alignment = max(alignment, p_align); } }
diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c index 13d053982dd73..a980dc3704639 100644 --- a/fs/binfmt_elf.c +++ b/fs/binfmt_elf.c @@ -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 +422,24 @@ 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) { + /* skip non-power of two alignments */ + if (!is_power_of_2(cmds[i].p_align)) + continue; + alignment = max(alignment, cmds[i].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 +1027,7 @@ static int load_elf_binary(struct linux_binprm *bprm) 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 +1106,9 @@ static int load_elf_binary(struct linux_binprm *bprm) 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;
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: verified program with -Wl,-z,max-page-size=0x200000 loading Signed-off-by: Chris Kennelly <ckennelly@google.com> --- fs/binfmt_elf.c | 23 +++++++++++++++++++++++ 1 file changed, 23 insertions(+)