On Mon, Oct 3, 2016 at 9:13 AM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> 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: > > [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. > > 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: > > https://lkml.org/lkml/2012/9/30/138 > > Lightly run-tested. > > Signed-off-by: Jason Gunthorpe <jgunthorpe@xxxxxxxxxxxxxxxxxxxx> > Signed-off-by: Denys Vlasenko <dvlasenk@xxxxxxxxxx> > Acked-by: Kees Cook <keescook@xxxxxxxxxxxx> > Acked-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx> > CC: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx> > CC: Paul Mackerras <paulus@xxxxxxxxx> > CC: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx> > CC: Kees Cook <keescook@xxxxxxxxxxxx> > CC: Oleg Nesterov <oleg@xxxxxxxxxx> > CC: Michael Ellerman <mpe@xxxxxxxxxxxxxx> > CC: Florian Weimer <fweimer@xxxxxxxxxx> > CC: linux-mm@xxxxxxxxx > CC: linuxppc-dev@xxxxxxxxxxxxxxxx > CC: linux-kernel@xxxxxxxxxxxxxxx > --- > Changes since v5: > * made do_brk_flags() error out if any bits other than VM_EXEC are set. > (Kees Cook: "With this, I'd be happy to Ack.") > See https://patchwork.ozlabs.org/patch/661595/ Excellent, thanks for the v6! Should this go via the ppc tree or the -mm tree? -Kees > > Changes since v4: > * if (current->personality & READ_IMPLIES_EXEC), still use VM_EXEC > for 32-bit executables. > > Changes since v3: > * typo fix in commit message > * rebased to current Linus tree > > Changes since v2: > * moved capability to map with VM_EXEC into vm_brk_flags() > > Changes since v1: > * wrapped lines to not exceed 79 chars > * improved comment > * expanded CC list > > arch/powerpc/include/asm/page.h | 4 +++- > fs/binfmt_elf.c | 30 ++++++++++++++++++++++-------- > include/linux/mm.h | 1 + > mm/mmap.c | 24 +++++++++++++++++++----- > 4 files changed, 45 insertions(+), 14 deletions(-) > > diff --git a/arch/powerpc/include/asm/page.h b/arch/powerpc/include/asm/page.h > index 56398e7..17d3d2c 100644 > --- a/arch/powerpc/include/asm/page.h > +++ b/arch/powerpc/include/asm/page.h > @@ -230,7 +230,9 @@ extern long long virt_phys_offset; > * and needs to be executable. This means the whole heap ends > * up being executable. > */ > -#define VM_DATA_DEFAULT_FLAGS32 (VM_READ | VM_WRITE | VM_EXEC | \ > +#define VM_DATA_DEFAULT_FLAGS32 \ > + (((current->personality & READ_IMPLIES_EXEC) ? VM_EXEC : 0) | \ > + VM_READ | VM_WRITE | \ > VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC) > > #define VM_DATA_DEFAULT_FLAGS64 (VM_READ | VM_WRITE | \ > diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c > index e5495f3..12b0d19 100644 > --- a/fs/binfmt_elf.c > +++ b/fs/binfmt_elf.c > @@ -91,12 +91,18 @@ static struct linux_binfmt elf_format = { > > #define BAD_ADDR(x) ((unsigned long)(x) >= TASK_SIZE) > > -static int set_brk(unsigned long start, unsigned long end) > +static int set_brk(unsigned long start, unsigned long end, int prot) > { > start = ELF_PAGEALIGN(start); > end = ELF_PAGEALIGN(end); > if (end > start) { > - int error = vm_brk(start, end - start); > + /* > + * Map the last of the bss segment. > + * If the header is requesting these pages to be > + * executable, honour that (ppc32 needs this). > + */ > + int error = vm_brk_flags(start, end - start, > + prot & PROT_EXEC ? VM_EXEC : 0); > if (error) > return error; > } > @@ -524,6 +530,7 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex, > unsigned long load_addr = 0; > int load_addr_set = 0; > unsigned long last_bss = 0, elf_bss = 0; > + int bss_prot = 0; > unsigned long error = ~0UL; > unsigned long total_size; > int i; > @@ -606,8 +613,10 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex, > * elf_bss and last_bss is the bss section. > */ > k = load_addr + eppnt->p_vaddr + eppnt->p_memsz; > - if (k > last_bss) > + if (k > last_bss) { > last_bss = k; > + bss_prot = elf_prot; > + } > } > } > > @@ -623,13 +632,14 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex, > /* > * Next, align both the file and mem bss up to the page size, > * since this is where elf_bss was just zeroed up to, and where > - * last_bss will end after the vm_brk() below. > + * last_bss will end after the vm_brk_flags() below. > */ > elf_bss = ELF_PAGEALIGN(elf_bss); > last_bss = ELF_PAGEALIGN(last_bss); > /* Finally, if there is still more bss to allocate, do it. */ > if (last_bss > elf_bss) { > - error = vm_brk(elf_bss, last_bss - elf_bss); > + error = vm_brk_flags(elf_bss, last_bss - elf_bss, > + bss_prot & PROT_EXEC ? VM_EXEC : 0); > if (error) > goto out; > } > @@ -674,6 +684,7 @@ static int load_elf_binary(struct linux_binprm *bprm) > unsigned long error; > struct elf_phdr *elf_ppnt, *elf_phdata, *interp_elf_phdata = NULL; > unsigned long elf_bss, elf_brk; > + int bss_prot = 0; > int retval, i; > unsigned long elf_entry; > unsigned long interp_load_addr = 0; > @@ -882,7 +893,8 @@ static int load_elf_binary(struct linux_binprm *bprm) > before this one. Map anonymous pages, if needed, > and clear the area. */ > retval = set_brk(elf_bss + load_bias, > - elf_brk + load_bias); > + elf_brk + load_bias, > + bss_prot); > if (retval) > goto out_free_dentry; > nbyte = ELF_PAGEOFFSET(elf_bss); > @@ -976,8 +988,10 @@ static int load_elf_binary(struct linux_binprm *bprm) > if (end_data < k) > end_data = k; > k = elf_ppnt->p_vaddr + elf_ppnt->p_memsz; > - if (k > elf_brk) > + if (k > elf_brk) { > + bss_prot = elf_prot; > elf_brk = k; > + } > } > > loc->elf_ex.e_entry += load_bias; > @@ -993,7 +1007,7 @@ static int load_elf_binary(struct linux_binprm *bprm) > * mapping in the interpreter, to make sure it doesn't wind > * up getting placed where the bss needs to go. > */ > - retval = set_brk(elf_bss, elf_brk); > + retval = set_brk(elf_bss, elf_brk, bss_prot); > if (retval) > goto out_free_dentry; > if (likely(elf_bss != elf_brk) && unlikely(padzero(elf_bss))) { > diff --git a/include/linux/mm.h b/include/linux/mm.h > index ef815b9..09c9133 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -2059,6 +2059,7 @@ static inline void mm_populate(unsigned long addr, unsigned long len) {} > > /* These take the mm semaphore themselves */ > extern int __must_check vm_brk(unsigned long, unsigned long); > +extern int __must_check vm_brk_flags(unsigned long, unsigned long, unsigned long); > extern int vm_munmap(unsigned long, size_t); > extern unsigned long __must_check vm_mmap(struct file *, unsigned long, > unsigned long, unsigned long, > diff --git a/mm/mmap.c b/mm/mmap.c > index ca9d91b..307c154 100644 > --- a/mm/mmap.c > +++ b/mm/mmap.c > @@ -2653,11 +2653,11 @@ static inline void verify_mm_writelocked(struct mm_struct *mm) > * anonymous maps. eventually we may be able to do some > * brk-specific accounting here. > */ > -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 +2668,10 @@ static int do_brk(unsigned long addr, unsigned long request) > if (!len) > return 0; > > - flags = VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags; > + /* Until we need other flags, refuse anything except VM_EXEC. */ > + if ((flags & (~VM_EXEC)) != 0) > + return -EINVAL; > + flags |= VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags; > > error = get_unmapped_area(NULL, addr, len, 0, MAP_FIXED); > if (offset_in_page(error)) > @@ -2736,7 +2739,12 @@ out: > return 0; > } > > -int vm_brk(unsigned long addr, unsigned long len) > +static int do_brk(unsigned long addr, unsigned long len) > +{ > + return do_brk_flags(addr, len, 0); > +} > + > +int vm_brk_flags(unsigned long addr, unsigned long len, unsigned long flags) > { > struct mm_struct *mm = current->mm; > int ret; > @@ -2745,13 +2753,19 @@ int vm_brk(unsigned long addr, unsigned long len) > if (down_write_killable(&mm->mmap_sem)) > return -EINTR; > > - ret = do_brk(addr, len); > + ret = do_brk_flags(addr, len, flags); > populate = ((mm->def_flags & VM_LOCKED) != 0); > up_write(&mm->mmap_sem); > if (populate && !ret) > mm_populate(addr, len); > return ret; > } > +EXPORT_SYMBOL(vm_brk_flags); > + > +int vm_brk(unsigned long addr, unsigned long len) > +{ > + return vm_brk_flags(addr, len, 0); > +} > EXPORT_SYMBOL(vm_brk); > > /* Release all mmaps. */ > -- > 2.9.2 > -- Kees Cook Nexus Security -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. 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