amd64/138318: [patch] select(2) in i386 emulation can overwrite user data

[Peter Jeremy]

>Number:         138318
>Category:       amd64
>Synopsis:       [patch] select(2) in i386 emulation can overwrite user data
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    freebsd-amd64
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Sat Aug 29 23:10:01 UTC 2009
>Closed-Date:
>Last-Modified:
>Originator:     Peter Jeremy
>Release:        FreeBSD 8.0-BETA2 amd64
>Organization:
n/a
>Environment:
System: FreeBSD server.vk2pj.dyndns.org 8.0-BETA2 FreeBSD 8.0-BETA2 #8: Sat Aug 8 21:54:17 EST 2009 root at server.vk2pj.dyndns.org:/var/obj/usr/src/sys/server amd64

Code inspection shows that this bug still exists in 9-current.

>Description:
	The select() wrapper for freebsd32 and linux32 emulation does not
	wrap the fd_set arguments.  fd_set is an array of fd_mask - which
	is 'long' on all architectures.  This means that kern_select() on
	64-bit kernels expects that the fd_set arguments are arrays of
	8-byte objects whilst 32-bit code passes arrays of 4-byte objects.
	As a result, the kernel can overwrite 4-bytes more than userland
	expects.

	This obviously breaks 32-bit sshd with PrivilegeSeparation enabled
	but may have other less-obvious breakage.

>How-To-Repeat:

Run a FreeBSD/i386 sshd on FreeBSD/amd64:

server# file /tank/aspire/usr/sbin/sshd 
/tank/aspire/usr/sbin/sshd: ELF 32-bit LSB executable, Intel 80386, version 1 (FreeBSD), dynamically linked (uses shared libs), for FreeBSD 8.0 (800096), stripped
server# /tank/aspire/usr/sbin/sshd -p 8022 -d -o UsePrivilegeSeparation=yes
debug1: sshd version OpenSSH_5.1p1 FreeBSD-20080801
...
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST received
debug1: SSH2_MSG_KEX_DH_GEX_GROUP sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_INIT
buffer_put_bignum2_ret: BN too small
buffer_put_bignum2: buffer error
debug1: do_cleanup
debug1: do_cleanup
server# 

As a more contrived (but more obvious) example, compile the following
code on i386 and run it on amd64:

---- 8-< ---- 8-< ---- 8-< ---- 8-< ---- 8-< ---- 8-< ---- 8-< ----
#include <sys/select.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void)
{
    fd_set *fd, *rd, *wr, *ex;
    int r;
    fd = malloc(sizeof(fd_mask) * 3 * 4);
    memset(fd, 0xa5, sizeof(fd_mask) * 3 * 4);
    rd = (fd_set *)&fd->fds_bits[1];
    wr = (fd_set *)&fd->fds_bits[5];
    ex = (fd_set *)&fd->fds_bits[9];
    rd->fds_bits[0] = wr->fds_bits[0] = ex->fds_bits[0] = 0;
    FD_SET(0, rd);
    FD_SET(1, wr);
    FD_SET(2, wr);
    FD_SET(0, ex);
    FD_SET(1, ex);
    FD_SET(2, ex);
    printf("read:   %08lx %08lx %08lx %08lx\n",
	   fd->fds_bits[0], fd->fds_bits[1], fd->fds_bits[2], fd->fds_bits[3]);
    printf("write:  %08lx %08lx %08lx %08lx\n",
	   fd->fds_bits[4], fd->fds_bits[5], fd->fds_bits[6], fd->fds_bits[7]);
    printf("except: %08lx %08lx %08lx %08lx\n",
	 fd->fds_bits[8], fd->fds_bits[9], fd->fds_bits[10], fd->fds_bits[11]);
    r = select(3, rd, wr, ex, NULL);
    printf("select returns %d:\n", r);
    printf("read:   %08lx %08lx %08lx %08lx\n",
	   fd->fds_bits[0], fd->fds_bits[1], fd->fds_bits[2], fd->fds_bits[3]);
    printf("write:  %08lx %08lx %08lx %08lx\n",
	   fd->fds_bits[4], fd->fds_bits[5], fd->fds_bits[6], fd->fds_bits[7]);
    printf("except: %08lx %08lx %08lx %08lx\n",
	 fd->fds_bits[8], fd->fds_bits[9], fd->fds_bits[10], fd->fds_bits[11]);
    return 0;
}
---- 8-< ---- 8-< ---- 8-< ---- 8-< ---- 8-< ---- 8-< ---- 8-< ----
server# /tank/aspire/root/seltest 
read:   a5a5a5a5 00000001 a5a5a5a5 a5a5a5a5
write:  a5a5a5a5 00000006 a5a5a5a5 a5a5a5a5
except: a5a5a5a5 00000007 a5a5a5a5 a5a5a5a5
read:   a5a5a5a5 00000000 00000000 a5a5a5a5
write:  a5a5a5a5 00000006 00000000 a5a5a5a5
except: a5a5a5a5 00000000 00000000 a5a5a5a5
server# 

>Fix:
Either:
1) Change the definition of fd_mask from ulong to uint32 (at least within
   the kernel)
2) Wrap the fd_set arguments on freebsd32 and linux for 64-bit kernels.

The latter may appear stylistically cleaner but requires significantly
more effort because the fd_set copyin()s are all currently done within
kern_select() and are non-trivial blocks of code to optimise performance
whilst minimising kvm usage.  The attached patch therefore implements
the former behaviour:
Index: select.h
===================================================================
RCS file: /usr/ncvs/src/sys/sys/select.h,v
retrieving revision 1.20
diff -u -r1.20 select.h
--- select.h	6 Jan 2006 22:12:46 -0000	1.20
+++ select.h	29 Aug 2009 23:00:08 -0000
@@ -39,7 +39,7 @@
 #include <sys/_timeval.h>
 #include <sys/timespec.h>
 
-typedef	unsigned long	__fd_mask;
+typedef	__uint32_t	__fd_mask;
 #if __BSD_VISIBLE
 typedef	__fd_mask	fd_mask;
 #endif



>Release-Note:
>Audit-Trail:
>Unformatted: