tjh.dev / kernel
Linux kernel mirror (for testing) git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
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install_special_mapping skips security_file_mmap check.

The install_special_mapping routine (used, for example, to setup the
vdso) skips the security check before insert_vm_struct, allowing a local
attacker to bypass the mmap_min_addr security restriction by limiting
the available pages for special mappings.

bprm_mm_init() also skips the check, and although I don't think this can
be used to bypass any restrictions, I don't see any reason not to have
the security check.

$ uname -m
x86_64
$ cat /proc/sys/vm/mmap_min_addr
65536
$ cat install_special_mapping.s
section .bss
resb BSS_SIZE
section .text
global _start
_start:
mov eax, __NR_pause
int 0x80
$ nasm -D__NR_pause=29 -DBSS_SIZE=0xfffed000 -f elf -o install_special_mapping.o install_special_mapping.s
$ ld -m elf_i386 -Ttext=0x10000 -Tbss=0x11000 -o install_special_mapping install_special_mapping.o
$ ./install_special_mapping &
[1] 14303
$ cat /proc/14303/maps
0000f000-00010000 r-xp 00000000 00:00 0 [vdso]
00010000-00011000 r-xp 00001000 00:19 2453665 /home/taviso/install_special_mapping
00011000-ffffe000 rwxp 00000000 00:00 0 [stack]

It's worth noting that Red Hat are shipping with mmap_min_addr set to
4096.

Signed-off-by: Tavis Ormandy <taviso@google.com>
Acked-by: Kees Cook <kees@ubuntu.com>
Acked-by: Robert Swiecki <swiecki@google.com>
[ Changed to not drop the error code - akpm ]
Reviewed-by: James Morris <jmorris@namei.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

authored by

Tavis Ormandy and committed by
Linus Torvalds 462e635e 0fcdcfbb

+17 -4
+5
fs/exec.c
··· 275 275 vma->vm_flags = VM_STACK_FLAGS | VM_STACK_INCOMPLETE_SETUP; 276 276 vma->vm_page_prot = vm_get_page_prot(vma->vm_flags); 277 277 INIT_LIST_HEAD(&vma->anon_vma_chain); 278 + 279 + err = security_file_mmap(NULL, 0, 0, 0, vma->vm_start, 1); 280 + if (err) 281 + goto err; 282 + 278 283 err = insert_vm_struct(mm, vma); 279 284 if (err) 280 285 goto err;
+12 -4
mm/mmap.c
··· 2462 2462 unsigned long addr, unsigned long len, 2463 2463 unsigned long vm_flags, struct page **pages) 2464 2464 { 2465 + int ret; 2465 2466 struct vm_area_struct *vma; 2466 2467 2467 2468 vma = kmem_cache_zalloc(vm_area_cachep, GFP_KERNEL); ··· 2480 2479 vma->vm_ops = &special_mapping_vmops; 2481 2480 vma->vm_private_data = pages; 2482 2481 2483 - if (unlikely(insert_vm_struct(mm, vma))) { 2484 - kmem_cache_free(vm_area_cachep, vma); 2485 - return -ENOMEM; 2486 - } 2482 + ret = security_file_mmap(NULL, 0, 0, 0, vma->vm_start, 1); 2483 + if (ret) 2484 + goto out; 2485 + 2486 + ret = insert_vm_struct(mm, vma); 2487 + if (ret) 2488 + goto out; 2487 2489 2488 2490 mm->total_vm += len >> PAGE_SHIFT; 2489 2491 2490 2492 perf_event_mmap(vma); 2491 2493 2492 2494 return 0; 2495 + 2496 + out: 2497 + kmem_cache_free(vm_area_cachep, vma); 2498 + return ret; 2493 2499 } 2494 2500 2495 2501 static DEFINE_MUTEX(mm_all_locks_mutex);