On Thu, Nov 16, 2017 at 2:19 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote: > From: Michal Hocko <mhocko@xxxxxxxx> > > Both load_elf_interp and load_elf_binary rely on elf_map to map segments > on a controlled address and they use MAP_FIXED to enforce that. This is > however dangerous thing prone to silent data corruption which can be > even exploitable. Let's take CVE-2017-1000253 as an example. At the time > (before eab09532d400 ("binfmt_elf: use ELF_ET_DYN_BASE only for PIE")) > ELF_ET_DYN_BASE was at TASK_SIZE / 3 * 2 which is not that far away from > the stack top on 32b (legacy) memory layout (only 1GB away). Therefore > we could end up mapping over the existing stack with some luck. > > The issue has been fixed since then (a87938b2e246 ("fs/binfmt_elf.c: > fix bug in loading of PIE binaries")), ELF_ET_DYN_BASE moved moved much > further from the stack (eab09532d400 and later by c715b72c1ba4 ("mm: > revert x86_64 and arm64 ELF_ET_DYN_BASE base changes")) and excessive > stack consumption early during execve fully stopped by da029c11e6b1 > ("exec: Limit arg stack to at most 75% of _STK_LIM"). So we should be > safe and any attack should be impractical. On the other hand this is > just too subtle assumption so it can break quite easily and hard to > spot. > > I believe that the MAP_FIXED usage in load_elf_binary (et. al) is still > fundamentally dangerous. Moreover it shouldn't be even needed. We are > at the early process stage and so there shouldn't be unrelated mappings > (except for stack and loader) existing so mmap for a given address > should succeed even without MAP_FIXED. Something is terribly wrong if > this is not the case and we should rather fail than silently corrupt the > underlying mapping. > > Address this issue by changing MAP_FIXED to the newly added > MAP_FIXED_SAFE. This will mean that mmap will fail if there is an > existing mapping clashing with the requested one without clobbering it. > > Cc: Abdul Haleem <abdhalee@xxxxxxxxxxxxxxxxxx> > Cc: Joel Stanley <joel@xxxxxxxxx> > Cc: Kees Cook <keescook@xxxxxxxxxxxx> > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx> Once (if?) the name gets settled, this looks good to me: Acked-by: Kees Cook <keescook@xxxxxxxxxxxx> -Kees > --- > arch/metag/kernel/process.c | 6 +++++- > fs/binfmt_elf.c | 12 ++++++++---- > 2 files changed, 13 insertions(+), 5 deletions(-) > > diff --git a/arch/metag/kernel/process.c b/arch/metag/kernel/process.c > index c4606ce743d2..2286140e54e0 100644 > --- a/arch/metag/kernel/process.c > +++ b/arch/metag/kernel/process.c > @@ -398,7 +398,7 @@ unsigned long __metag_elf_map(struct file *filep, unsigned long addr, > tcm_tag = tcm_lookup_tag(addr); > > if (tcm_tag != TCM_INVALID_TAG) > - type &= ~MAP_FIXED; > + type &= ~(MAP_FIXED | MAP_FIXED_SAFE); > > /* > * total_size is the size of the ELF (interpreter) image. > @@ -416,6 +416,10 @@ unsigned long __metag_elf_map(struct file *filep, unsigned long addr, > } else > map_addr = vm_mmap(filep, addr, size, prot, type, off); > > + if ((type & MAP_FIXED_SAFE) && BAD_ADDR(map_addr)) > + pr_info("%d (%s): Uhuuh, elf segement at %p requested but the memory is mapped already\n", > + task_pid_nr(current), tsk->comm, (void*)addr); > + > if (!BAD_ADDR(map_addr) && tcm_tag != TCM_INVALID_TAG) { > struct tcm_allocation *tcm; > unsigned long tcm_addr; > diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c > index 6466153f2bf0..12b21942ccde 100644 > --- a/fs/binfmt_elf.c > +++ b/fs/binfmt_elf.c > @@ -372,6 +372,10 @@ static unsigned long elf_map(struct file *filep, unsigned long addr, > } else > map_addr = vm_mmap(filep, addr, size, prot, type, off); > > + if ((type & MAP_FIXED_SAFE) && BAD_ADDR(map_addr)) > + pr_info("%d (%s): Uhuuh, elf segement at %p requested but the memory is mapped already\n", > + task_pid_nr(current), current->comm, (void*)addr); > + > return(map_addr); > } > > @@ -569,7 +573,7 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex, > elf_prot |= PROT_EXEC; > vaddr = eppnt->p_vaddr; > if (interp_elf_ex->e_type == ET_EXEC || load_addr_set) > - elf_type |= MAP_FIXED; > + elf_type |= MAP_FIXED_SAFE; > else if (no_base && interp_elf_ex->e_type == ET_DYN) > load_addr = -vaddr; > > @@ -929,7 +933,7 @@ static int load_elf_binary(struct linux_binprm *bprm) > * the ET_DYN load_addr calculations, proceed normally. > */ > if (loc->elf_ex.e_type == ET_EXEC || load_addr_set) { > - elf_flags |= MAP_FIXED; > + elf_flags |= MAP_FIXED_SAFE; > } else if (loc->elf_ex.e_type == ET_DYN) { > /* > * This logic is run once for the first LOAD Program > @@ -965,7 +969,7 @@ 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(); > - elf_flags |= MAP_FIXED; > + elf_flags |= MAP_FIXED_SAFE; > } else > load_bias = 0; > > @@ -1220,7 +1224,7 @@ static int load_elf_library(struct file *file) > (eppnt->p_filesz + > ELF_PAGEOFFSET(eppnt->p_vaddr)), > PROT_READ | PROT_WRITE | PROT_EXEC, > - MAP_FIXED | MAP_PRIVATE | MAP_DENYWRITE, > + MAP_FIXED_SAFE | MAP_PRIVATE | MAP_DENYWRITE, > (eppnt->p_offset - > ELF_PAGEOFFSET(eppnt->p_vaddr))); > if (error != ELF_PAGESTART(eppnt->p_vaddr)) > -- > 2.15.0 > -- Kees Cook Pixel Security