KASAN: use-after-free Read in vt_do_kdgkb_ioctl

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 54dedb5b Merge tag 'for-linus-5.6-rc3-tag' of git://git.ke..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=100403d9e00000
kernel config: https://syzkaller.appspot.com/x/.config?x=3b8906eb6a7d6028
dashboard link: https://syzkaller.appspot.com/bug?extid=a58bfd517766aef83043
compiler: gcc (GCC) 9.0.0 20181231 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+a58bfd...@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: use-after-free in vt_do_kdgkb_ioctl+0xb24/0xb50 drivers/tty/vt/keyboard.c:2025
Read of size 1 at addr ffff8880a4a8c801 by task syz-executor.4/27890

CPU: 1 PID: 27890 Comm: syz-executor.4 Not tainted 5.6.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x197/0x210 lib/dump_stack.c:118
print_address_description.constprop.0.cold+0xd4/0x30b mm/kasan/report.c:374
__kasan_report.cold+0x1b/0x32 mm/kasan/report.c:506
kasan_report+0x12/0x20 mm/kasan/common.c:641
__asan_report_load1_noabort+0x14/0x20 mm/kasan/generic_report.c:132
vt_do_kdgkb_ioctl+0xb24/0xb50 drivers/tty/vt/keyboard.c:2025
vt_ioctl+0x47e/0x26c0 drivers/tty/vt/vt_ioctl.c:549
tty_ioctl+0xa37/0x14f0 drivers/tty/tty_io.c:2660
vfs_ioctl fs/ioctl.c:47 [inline]
ksys_ioctl+0x123/0x180 fs/ioctl.c:763
__do_sys_ioctl fs/ioctl.c:772 [inline]
__se_sys_ioctl fs/ioctl.c:770 [inline]
__x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:770
do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x45c429
Code: ad b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 7b b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f0f2868cc78 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f0f2868d6d4 RCX: 000000000045c429
RDX: 0000000020000200 RSI: 0000000000004b48 RDI: 0000000000000003
RBP: 000000000076bf20 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
R13: 000000000000041a R14: 00000000004c676d R15: 000000000076bf2c

Allocated by task 1548:
save_stack+0x23/0x90 mm/kasan/common.c:72
set_track mm/kasan/common.c:80 [inline]
__kasan_kmalloc mm/kasan/common.c:515 [inline]
__kasan_kmalloc.constprop.0+0xcf/0xe0 mm/kasan/common.c:488
kasan_kmalloc+0x9/0x10 mm/kasan/common.c:529
__do_kmalloc mm/slab.c:3656 [inline]
__kmalloc+0x163/0x770 mm/slab.c:3665
kmalloc include/linux/slab.h:560 [inline]
vt_do_kdgkb_ioctl+0x3d1/0xb50 drivers/tty/vt/keyboard.c:2078
vt_ioctl+0x47e/0x26c0 drivers/tty/vt/vt_ioctl.c:549
tty_ioctl+0xa37/0x14f0 drivers/tty/tty_io.c:2660
vfs_ioctl fs/ioctl.c:47 [inline]
ksys_ioctl+0x123/0x180 fs/ioctl.c:763
__do_sys_ioctl fs/ioctl.c:772 [inline]
__se_sys_ioctl fs/ioctl.c:770 [inline]
__x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:770
do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x49/0xbe

Freed by task 27892:
save_stack+0x23/0x90 mm/kasan/common.c:72
set_track mm/kasan/common.c:80 [inline]
kasan_set_free_info mm/kasan/common.c:337 [inline]
__kasan_slab_free+0x102/0x150 mm/kasan/common.c:476
kasan_slab_free+0xe/0x10 mm/kasan/common.c:485
__cache_free mm/slab.c:3426 [inline]
kfree+0x10a/0x2c0 mm/slab.c:3757
vt_do_kdgkb_ioctl+0x5b4/0xb50 drivers/tty/vt/keyboard.c:2104
vt_ioctl+0x47e/0x26c0 drivers/tty/vt/vt_ioctl.c:549
tty_ioctl+0xa37/0x14f0 drivers/tty/tty_io.c:2660
vfs_ioctl fs/ioctl.c:47 [inline]
ksys_ioctl+0x123/0x180 fs/ioctl.c:763
__do_sys_ioctl fs/ioctl.c:772 [inline]
__se_sys_ioctl fs/ioctl.c:770 [inline]
__x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:770
do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x49/0xbe

The buggy address belongs to the object at ffff8880a4a8c800
which belongs to the cache kmalloc-1k of size 1024
The buggy address is located 1 bytes inside of
1024-byte region [ffff8880a4a8c800, ffff8880a4a8cc00)
The buggy address belongs to the page:
page:ffffea000292a300 refcount:1 mapcount:0 mapping:ffff8880aa400c40 index:0x0
flags: 0xfffe0000000200(slab)
raw: 00fffe0000000200 ffffea00026aa6c8 ffffea000210b1c8 ffff8880aa400c40
raw: 0000000000000000 ffff8880a4a8c000 0000000100000002 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff8880a4a8c700: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff8880a4a8c780: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff8880a4a8c800: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
^
ffff8880a4a8c880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8880a4a8c900: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================


---
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[Hillf Danton]

On Mon, 24 Feb 2020 01:18:14 -0800

Allocate a tmp buf for duplicating the string under lock and use it
outside lock.

--- a/drivers/tty/vt/keyboard.c
+++ b/drivers/tty/vt/keyboard.c
@@ -1995,7 +1995,7 @@ int vt_do_kdgkb_ioctl(int cmd, struct kb
int delta;
char *first_free, *fj, *fnw;
int i, j, k;
- int ret;
+ int ret = 0;
unsigned long flags;

if (!capable(CAP_SYS_TTY_CONFIG))
@@ -2021,18 +2021,45 @@ int vt_do_kdgkb_ioctl(int cmd, struct kb
a struct member */
up = user_kdgkb->kb_string;
p = func_table[i];
- if(p)
+ if (p) {
+ char *tmp_buf = kzalloc(sz + 1, GFP_KERNEL);
+
+ if (!tmp_buf) {
+ ret = -ENOMEM;
+ goto reterr;
+ }
+
+ spin_lock_irqsave(&func_buf_lock, flags);
+ p = func_table[i];
+ if (p) {
+ __kernel_size_t len = strlen(p);
+ if (sz < len) {
+ ret = -EOVERFLOW;
+ p = NULL;
+ } else {
+ memcpy(tmp_buf, p, len);
+ p = tmp_buf;
+ }
+ }
+ spin_unlock_irqrestore(&func_buf_lock, flags);
+ if (!p)
+ goto free_buf;
+
for ( ; *p && sz; p++, sz--)
if (put_user(*p, up++)) {
ret = -EFAULT;
- goto reterr;
+ goto free_buf;
}
- if (put_user('\0', up)) {
- ret = -EFAULT;
- goto reterr;
+
+free_buf:
+ kfree(tmp_buf);
+ if (ret)
+ goto reterr;
}
- kfree(kbs);
- return ((p && *p) ? -EOVERFLOW : 0);
+
+ if (put_user('\0', up))
+ ret = -EFAULT;
+ goto reterr;
case KDSKBSENT:
if (!perm) {
ret = -EPERM;

[Greg KH]

What ever happened to this? Did it fix the issue? If so, can you
resend this as a "real" patch I can apply?

thanks,

greg k-h

[Hillf Danton]

On Wed, 18 Mar 2020 11:06:50 +0100 Greg KH wrote:
> On Mon, Feb 24, 2020 at 09:00:28PM +0800, Hillf Danton wrote:
> >

> What ever happened to this?

This is the first echo I see so far.

> Did it fix the issue?

No, Sir. It was posted by and large for helping to reproduce the uaf.

Hillf

[syzbot]

Auto-closing this bug as obsolete.
Crashes did not happen for a while, no reproducer and no activity.