[PATCH 4.9 095/167] um: Fix out-of-bounds read in LDT setup

Greg Kroah-Hartman
2022-06-13T10:25:31Z

From: Vincent Whitchurch <vincent.whitchurch@axis.com>

commit 2a4a62a14be1947fa945c5c11ebf67326381a568 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/um/ldt.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

--- a/arch/x86/um/ldt.c
+++ b/arch/x86/um/ldt.c
@@ -23,9 +23,11 @@ static long write_ldt_entry(struct mm_id
{
long res;
void *stub_addr;
+
+ BUILD_BUG_ON(sizeof(*desc) % sizeof(long));
+
res = syscall_stub_data(mm_idp, (unsigned long *)desc,
- (sizeof(*desc) + sizeof(long) - 1) &
- ~(sizeof(long) - 1),
+ sizeof(*desc) / sizeof(long),
addr, &stub_addr);
if (!res) {
unsigned long args[] = { func,