Re: tty, fbcon: use-after-free in fbcon_invert_region

On Tue, Oct 11, 2016 at 3:43 AM, Scot Doyle [off-list ref] wrote:

quoted

I wonder if the text selection is outside the newly resized vc?
Does this patch help?

--- vt.c        2016-10-11 00:32:43.079605599 -0000
+++ vt.c.new    2016-10-11 00:36:12.744650759 -0000

@@ -874,6 +874,9 @@

if (!newscreen)
return -ENOMEM;

+       if (vc = sel_cons)
+               clear_selection();
+
old_rows = vc->vc_rows;
old_row_size = vc->vc_size_row;

This helped with the use-after-frees and out-of-bounds.
Tested-by: Dmitry Vyukov <dvyukov@google.com>

However, now the test program hanged in D unkillable stack on some
kind of kernel deadlock. Don't know if it's induced by your patch, or
just another bug. At least there are no vc_do_resize in stacks.

# ps afxu | grep a.out
root      6163  6.5  0.0      0     0 pts/0    Zl   13:25   0:00  |
   \_ [a.out] <defunct>

# ls /proc/6163/task/
6163  6191  6193  6194 6201

# cat /proc/6163/task/*/stack
[<     inline     >] down_read_failed drivers/tty/tty_ldsem.c:241
[<ffffffff831b8da6>] __ldsem_down_read_nested+0x2a6/0x5b0 drivers/tty/tty_ldsem.c:332
[<ffffffff831b23f5>] tty_ldisc_ref_wait+0x35/0xb0 drivers/tty/tty_ldisc.c:274
[<ffffffff831962b7>] tty_write+0x177/0x840 drivers/tty/tty_io.c:1250
[<ffffffff8182c700>] __vfs_write+0x110/0x620 fs/read_write.c:510
[<ffffffff8182dc05>] vfs_write+0x175/0x4e0 fs/read_write.c:560
[<     inline     >] SYSC_write fs/read_write.c:607
[<ffffffff818314c9>] SyS_write+0xd9/0x1b0 fs/read_write.c:599
[<ffffffff86daf545>] entry_SYSCALL_64_fastpath+0x23/0xc6
arch/x86/entry/entry_64.S:208

The patch below removes the resize ioctl's from the first test program. 
Are there any use-after-free/out-of-bounds errors when running the patched 
test program on the unpatched kernel? If not, but there are still 
deadlocks, then perhaps they aren't caused by the proposed kernel patch?

--- test.c
+++ test.c.new

@@ -141,8 +141,6 @@
     NONFAILING(*(uint16_t*)0x20f77ff9 = (uint16_t)0x6);
     NONFAILING(*(uint16_t*)0x20f77ffb = (uint16_t)0x3f);
     NONFAILING(*(uint16_t*)0x20f77ffd = (uint16_t)0x0);
-    r[17] = execute_syscall(__NR_ioctl, r[8], 0x541cul, 0x20f77ff4ul, 0,
-                            0, 0, 0, 0, 0);
     break;
   case 8:
     NONFAILING(*(uint32_t*)0x20f6dffc = (uint32_t)0x5);

@@ -212,8 +210,6 @@
     NONFAILING(*(uint16_t*)0x20f78ffa = (uint16_t)0xeb8);
     NONFAILING(*(uint16_t*)0x20f78ffc = (uint16_t)0x9);
     NONFAILING(*(uint16_t*)0x20f78ffe = (uint16_t)0x7);
-    r2[17] = execute_syscall(__NR_ioctl, r2[5], 0x5609ul, 0x20f78ffaul, 0,
-                            0, 0, 0, 0, 0);
     break;
   case 8:
     r2[18] @@ -273,8 +269,6 @@
     NONFAILING(*(uint16_t*)0x20f70002 = (uint16_t)0x2);
     NONFAILING(*(uint16_t*)0x20f70004 = (uint16_t)0xd1e);
     NONFAILING(*(uint16_t*)0x20f70006 = (uint16_t)0x7);
-    r2[34] = execute_syscall(__NR_ioctl, r2[5], 0x5414ul, 0x20f70000ul, 0,
-                            0, 0, 0, 0, 0);
     break;
   }
   return 0;