Can anyone else reproduce building ARCH=um with the attached .config, and then 
going:

$ ./vmlinux rw init=/bin/sh rootfstype=hostfs
Core dump limits :
	soft - 0
	hard - NONE
Checking that ptrace can change system call numbers...OK
Checking syscall emulation patch for ptrace...OK
Checking advanced syscall emulation patch for ptrace...OK
Checking for tmpfs mount on /dev/shm...OK
Checking PROT_EXEC mmap in /dev/shm/...OK
Checking for the skas3 patch in the host:
  - /proc/mm...not found: No such file or directory
  - PTRACE_FAULTINFO...not found
  - PTRACE_LDT...not found
UML running in SKAS0 mode
Adding 11874304 bytes to physical memory to account for exec-shield gap
Linux version 2.6.28 (landley@driftwood) (gcc version 4.3.2 (Ubuntu 
4.3.2-1ubuntu11) ) #6 Tue Jan 13 01:54:34 CST 2009
Built 1 zonelists in Zone order, mobility grouping on.  Total pages: 10939
Kernel command line: rw init=/bin/sh rootfstype=hostfs root=98:0
PID hash table entries: 256 (order: 8, 2048 bytes)
Dentry cache hash table entries: 8192 (order: 4, 65536 bytes)
Inode-cache hash table entries: 4096 (order: 3, 32768 bytes)
Memory: 29688k available
SLUB: Genslabs=12, HWalign=64, Order=0-3, MinObjects=0, CPUs=1, Nodes=1
Calibrating delay loop... 369.45 BogoMIPS (lpj=1847296)
Mount-cache hash table entries: 256
Checking that host ptys support output SIGIO...Yes
Checking that host ptys support SIGIO on close...No, enabling workaround
Using 2.6 host AIO
io scheduler noop registered (default)
loop: module loaded
Initialized stdio console driver
Using a channel type which is configured out of UML
parse_chan_pair failed for device 1 : Configuration failed
Using a channel type which is configured out of UML
parse_chan_pair failed for device 2 : Configuration failed
Using a channel type which is configured out of UML
parse_chan_pair failed for device 3 : Configuration failed
Using a channel type which is configured out of UML
parse_chan_pair failed for device 4 : Configuration failed
Using a channel type which is configured out of UML
parse_chan_pair failed for device 5 : Configuration failed
Using a channel type which is configured out of UML
parse_chan_pair failed for device 6 : Configuration failed
Using a channel type which is configured out of UML
parse_chan_pair failed for device 7 : Configuration failed
Using a channel type which is configured out of UML
parse_chan_pair failed for device 8 : Configuration failed
Using a channel type which is configured out of UML
parse_chan_pair failed for device 9 : Configuration failed
Using a channel type which is configured out of UML
parse_chan_pair failed for device 10 : Configuration failed
Using a channel type which is configured out of UML
parse_chan_pair failed for device 11 : Configuration failed
Using a channel type which is configured out of UML
parse_chan_pair failed for device 12 : Configuration failed
Using a channel type which is configured out of UML
parse_chan_pair failed for device 13 : Configuration failed
Using a channel type which is configured out of UML
parse_chan_pair failed for device 14 : Configuration failed
Using a channel type which is configured out of UML
parse_chan_pair failed for device 15 : Configuration failed
Console initialized on /dev/tty0
console [tty0] enabled
VFS: Mounted root (hostfs filesystem).

Pid: 1, comm: swapper Not tainted 2.6.28
RIP: 0033:[<000000006001e112>]
RSP: 0000000062425de0  EFLAGS: 00010216
RAX: 00000000626a4800 RBX: 00000000625a5000 RCX: 0000000003ffc09f
RDX: fffffffffff02800 RSI: 0000000060280c20 RDI: 00000000626a4800
RBP: 0000000061537048 R08: 0000000000000000 R09: 0000000000100000
R10: 0000000000000000 R11: 00007f74c6795143 R12: 0000000062522300
R13: 0000000062522300 R14: 000000006014e440 R15: 0000000062420330
Call Trace: 
6014d908:  [<60011108>] segv+0x2b8/0x300
6014d948:  [<6003cb19>] hrtimer_run_pending+0x9/0xe0
6014d968:  [<6002e8bd>] run_timer_softirq+0x1cd/0x210
6014d9e8:  [<600111a0>] segv_handler+0x50/0xe0
6014da08:  [<60040f10>] tick_handle_periodic+0x10/0x60
6014da48:  [<6002954d>] do_softirq+0x4d/0x70
6014da68:  [<600296d5>] irq_exit+0x45/0x60
6014da88:  [<6000daff>] do_IRQ+0x2f/0x50
6014daa8:  [<60018154>] sig_handler_common+0x64/0xe0
6014db30:  [<6001e112>] __memcpy+0xe/0xac
6014db50:  [<600096a4>] calibrate_delay+0xb1/0x19d
6014dbd8:  [<6001831a>] sig_handler+0x1a/0x40
6014dbe8:  [<600185f3>] handle_signal+0x73/0xb0
6014dcd8:  [<6001e112>] __memcpy+0xe/0xac

Kernel panic - not syncing: Segfault with no mm

Pid: 1, comm: swapper Not tainted 2.6.28
RIP: 0033:[<00007f74c67442c7>]
RSP: 00007fffceea2fe8  EFLAGS: 00000206
RAX: 0000000000000000 RBX: 0000000000004f7c RCX: ffffffffffffffff
RDX: 0000000000000000 RSI: 0000000000000013 RDI: 0000000000004f7c
RBP: 0000000000004f78 R08: 00007fffceea2f30 R09: 0000000000000000
R10: 00007fffceea2d70 R11: 0000000000000206 R12: 00007fffceea30f8
R13: 0000000000000004 R14: 00007fffceea32c0 R15: 00007fffceea3154
Call Trace: 
6014d7c8:  [<600113ed>] panic_exit+0x2d/0x50
6014d7e8:  [<6003d967>] notifier_call_chain+0x37/0x70
6014d818:  [<600fcfe7>] panic+0xd0/0x164
6014d8b8:  [<6001e112>] __memcpy+0xe/0xac
6014d8c8:  [<6000fca0>] show_trace+0x60/0xc0
6014d8e8:  [<6001df18>] show_regs+0x28/0x30
6014d908:  [<60011114>] segv+0x2c4/0x300
6014d948:  [<6003cb19>] hrtimer_run_pending+0x9/0xe0
6014d968:  [<6002e8bd>] run_timer_softirq+0x1cd/0x210
6014d9e8:  [<600111a0>] segv_handler+0x50/0xe0
6014da08:  [<60040f10>] tick_handle_periodic+0x10/0x60
6014da48:  [<6002954d>] do_softirq+0x4d/0x70
6014da68:  [<600296d5>] irq_exit+0x45/0x60
6014da88:  [<6000daff>] do_IRQ+0x2f/0x50
6014daa8:  [<60018154>] sig_handler_common+0x64/0xe0
6014db30:  [<6001e112>] __memcpy+0xe/0xac
6014db50:  [<600096a4>] calibrate_delay+0xb1/0x19d
6014dbd8:  [<6001831a>] sig_handler+0x1a/0x40
6014dbe8:  [<600185f3>] handle_signal+0x73/0xb0
6014dcd8:  [<6001e112>] __memcpy+0xe/0xac

Segmentation fault