um: Fix out-of-bounds read in LDT setup

syscall_stub_data() expects the data_count parameter to be the number of
longs, not bytes.

==================================================================
BUG: KASAN: stack-out-of-bounds in syscall_stub_data+0x70/0xe0
Read of size 128 at addr 000000006411f6f0 by task swapper/1

CPU: 0 PID: 1 Comm: swapper Not tainted 5.18.0+ #18
Call Trace:
show_stack.cold+0x166/0x2a7
__dump_stack+0x3a/0x43
dump_stack_lvl+0x1f/0x27
print_report.cold+0xdb/0xf81
kasan_report+0x119/0x1f0
kasan_check_range+0x3a3/0x440
memcpy+0x52/0x140
syscall_stub_data+0x70/0xe0
write_ldt_entry+0xac/0x190
init_new_ldt+0x515/0x960
init_new_context+0x2c4/0x4d0
mm_init.constprop.0+0x5ed/0x760
mm_alloc+0x118/0x170
0x60033f48
do_one_initcall+0x1d7/0x860
0x60003e7b
kernel_init+0x6e/0x3d4
new_thread_handler+0x1e7/0x2c0

The buggy address belongs to stack of task swapper/1
and is located at offset 64 in frame:
init_new_ldt+0x0/0x960

This frame has 2 objects:
[32, 40) 'addr'
[64, 80) 'desc'
==================================================================

Fixes: 858259cf7d1c443c83 ("uml: maintain own LDT entries")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Cc: stable@vger.kernel.org
Signed-off-by: Richard Weinberger <richard@nod.at>

x86/entry: Use SYSCALL_DEFINE() macros for sys_modify_ldt()

We do not have tracepoints for sys_modify_ldt() because we define
it directly instead of using the normal SYSCALL_DEFINEx() macros.

However, there is a reason sys_modify_ldt() does not use the macros:
it has an 'int' return type instead of 'unsigned long'. This is
a bug, but it's a bug cemented in the ABI.

What does this mean? If we return -EINVAL from a function that
returns 'int', we have 0x00000000ffffffea in %rax. But, if we
return -EINVAL from a function returning 'unsigned long', we end
up with 0xffffffffffffffea in %rax, which is wrong.

To work around this and maintain the 'int' behavior while using
the SYSCALL_DEFINEx() macros, so we add a cast to 'unsigned int'
in both implementations of sys_modify_ldt().

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Andy Lutomirski <luto@kernel.org>
Reviewed-by: Brian Gerst <brgerst@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20171018172107.1A79C532@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

um: Do not rely on libc to provide modify_ldt()

modify_ldt() was declared as an external symbol. Despite the man
page for this syscall telling that there is no wrapper in glibc,
since version 2.1 there actually is, so linking to the glibc
works.

Since modify_ldt() is not a POSIX interface, other libc
implementations do not always provide a wrapper function.
Even glibc headers do not provide a corresponding declaration.

So go the recommended way to call this using syscall().

Signed-off-by: Hans-Werner Hilse <hwhilse@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

um: Add uaccess.h to ldt.c

...for userspace memory access.

Signed-off-by: Richard Weinberger <richard@nod.at>

um: Remove SKAS3/4 support

Before we had SKAS0 UML had two modes of operation
TT (tracing thread) and SKAS3/4 (separated kernel address space).
TT was known to be insecure and got removed a long time ago.
SKAS3/4 required a few (3 or 4) patches on the host side which never went
mainline. The last host patch is 10 years old.

With SKAS0 mode (separated kernel address space using 0 host patches),
default since 2005, SKAS3/4 is obsolete and can be removed.

Signed-off-by: Richard Weinberger <richard@nod.at>

um: get rid of pointless include "..." where include <...> will do

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>

um: take ldt.h to arch/x86/um/asm/mm_context.h

it's x86-only and we have no business playing with it in asm/mmu.h; make
the latter have
struct uml_arch_mm_context arch;
instead of
struct uml_ldt ldt;
and let arch/<subarch>/um/asm/mm_context.h decide what'll be in there.
While we are at it, kill host_ldt.h - it's not needed in part of places
that include it (we want asm/ldt.h in those) and it can be trivially
expanded into the single remaining one.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>

um: take arch/um/sys-x86 to arch/x86/um

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>