Re: KASAN: slab-out-of-bounds Read in strcmp

[Tetsuo Handa]

On 2017/12/02 3:52, syzbot wrote:
> ==================================================================
> BUG: KASAN: slab-out-of-bounds in strcmp+0x96/0xb0 lib/string.c:328
> Read of size 1 at addr ffff8801cd99d2c1 by task syzkaller242593/3087
> 
> CPU: 0 PID: 3087 Comm: syzkaller242593 Not tainted 4.15.0-rc1-next-20171201+ #57
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> Call Trace:
>  __dump_stack lib/dump_stack.c:17 [inline]
>  dump_stack+0x194/0x257 lib/dump_stack.c:53
>  print_address_description+0x73/0x250 mm/kasan/report.c:252
>  kasan_report_error mm/kasan/report.c:351 [inline]
>  kasan_report+0x25b/0x340 mm/kasan/report.c:409
>  __asan_report_load1_noabort+0x14/0x20 mm/kasan/report.c:427
>  strcmp+0x96/0xb0 lib/string.c:328

This seems to be out of bound read for "scontext" at

	for (i = 1; i < SECINITSID_NUM; i++) {
		if (!strcmp(initial_sid_to_string[i], scontext)) {
			*sid = i;
			return 0;
		}
	}

because "initial_sid_to_string[i]" is "const char *".

>  security_context_to_sid_core+0x437/0x620 security/selinux/ss/services.c:1420
>  security_context_to_sid+0x32/0x40 security/selinux/ss/services.c:1479
>  selinux_setprocattr+0x51c/0xb50 security/selinux/hooks.c:5986
>  security_setprocattr+0x85/0xc0 security/security.c:1264

If "value" does not terminate with '\0' or '\n', "value" and
"size" are as-is passed to "scontext" and "scontext_len" above

	/* Obtain a SID for the context, if one was specified. */
	if (size && str[0] && str[0] != '\n') {
		if (str[size-1] == '\n') {
			str[size-1] = 0;
			size--;
		}
		error = security_context_to_sid(value, size, &sid, GFP_KERNEL);

which will allow strcmp() to trigger out of bound read when "size" is
larger than strlen(initial_sid_to_string[i]).

Thus, I guess the simplest fix is to use strncmp() instead of strcmp().

>  proc_pid_attr_write+0x1e6/0x280 fs/proc/base.c:2574
>  __vfs_write+0xef/0x970 fs/read_write.c:480
>  __kernel_write+0xfe/0x350 fs/read_write.c:501
>  write_pipe_buf+0x175/0x220 fs/splice.c:797
>  splice_from_pipe_feed fs/splice.c:502 [inline]
>  __splice_from_pipe+0x328/0x730 fs/splice.c:626
>  splice_from_pipe+0x1e9/0x330 fs/splice.c:661
>  default_file_splice_write+0x40/0x90 fs/splice.c:809
>  do_splice_from fs/splice.c:851 [inline]
>  direct_splice_actor+0x125/0x180 fs/splice.c:1018
>  splice_direct_to_actor+0x2c1/0x820 fs/splice.c:973
>  do_splice_direct+0x2a7/0x3d0 fs/splice.c:1061
>  do_sendfile+0x5d5/0xe90 fs/read_write.c:1413
>  SYSC_sendfile64 fs/read_write.c:1468 [inline]
>  SyS_sendfile64+0xbd/0x160 fs/read_write.c:1460
>  entry_SYSCALL_64_fastpath+0x1f/0x96