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>

diff --git a/arch/x86/um/ldt.c b/arch/x86/um/ldt.c
index 3ee234b..255a44d 100644 (file)
--- a/arch/x86/um/ldt.c
+++ b/arch/x86/um/ldt.c
@@ -23,9 +23,11 @@ static long write_ldt_entry(struct mm_id *mm_idp, int func,
 {
        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,