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

[Stephen Smalley]

On Sun, 2017-12-03 at 20:33 +0900, Tetsuo Handa wrote:
> 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:
> > Â__dump_stack lib/dump_stack.c:17 [inline]
> > Âdump_stack+0x194/0x257 lib/dump_stack.c:53
> > Âprint_address_description+0x73/0x250 mm/kasan/report.c:252
> > Âkasan_report_error mm/kasan/report.c:351 [inline]
> > Âkasan_report+0x25b/0x340 mm/kasan/report.c:409
> > Â__asan_report_load1_noabort+0x14/0x20 mm/kasan/report.c:427
> > Âstrcmp+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 *".
> 
> > Âsecurity_context_to_sid_core+0x437/0x620
> > security/selinux/ss/services.c:1420
> > Âsecurity_context_to_sid+0x32/0x40
> > security/selinux/ss/services.c:1479
> > Âselinux_setprocattr+0x51c/0xb50 security/selinux/hooks.c:5986
> > Âsecurity_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().

Already fixed by
https://www.spinics.net/lists/selinux/msg23589.html

> 
> > Âproc_pid_attr_write+0x1e6/0x280 fs/proc/base.c:2574
> > Â__vfs_write+0xef/0x970 fs/read_write.c:480
> > Â__kernel_write+0xfe/0x350 fs/read_write.c:501
> > Âwrite_pipe_buf+0x175/0x220 fs/splice.c:797
> > Âsplice_from_pipe_feed fs/splice.c:502 [inline]
> > Â__splice_from_pipe+0x328/0x730 fs/splice.c:626
> > Âsplice_from_pipe+0x1e9/0x330 fs/splice.c:661
> > Âdefault_file_splice_write+0x40/0x90 fs/splice.c:809
> > Âdo_splice_from fs/splice.c:851 [inline]
> > Âdirect_splice_actor+0x125/0x180 fs/splice.c:1018
> > Âsplice_direct_to_actor+0x2c1/0x820 fs/splice.c:973
> > Âdo_splice_direct+0x2a7/0x3d0 fs/splice.c:1061
> > Âdo_sendfile+0x5d5/0xe90 fs/read_write.c:1413
> > ÂSYSC_sendfile64 fs/read_write.c:1468 [inline]
> > ÂSyS_sendfile64+0xbd/0x160 fs/read_write.c:1460
> > Âentry_SYSCALL_64_fastpath+0x1f/0x96