tty/vt: fix write/write race in ioctl(KDSKBSENT) handler

The bug manifests as an attempt to access deallocated memory:

BUG: unable to handle kernel paging request at ffff9c8735448000
#PF error: [PROT] [WRITE]
PGD 288a05067 P4D 288a05067 PUD 288a07067 PMD 7f60c2063 PTE 80000007f5448161
Oops: 0003 [#1] PREEMPT SMP
CPU: 6 PID: 388 Comm: loadkeys Tainted: G C 5.0.0-rc6-00153-g5ded5871030e #91
Hardware name: Gigabyte Technology Co., Ltd. To be filled by O.E.M./H77M-D3H, BIOS F12 11/14/2013
RIP: 0010:__memmove+0x81/0x1a0
Code: 4c 89 4f 10 4c 89 47 18 48 8d 7f 20 73 d4 48 83 c2 20 e9 a2 00 00 00 66 90 48 89 d1 4c 8b 5c 16 f8 4c 8d 54 17 f8 48 c1 e9 03 <f3> 48 a5 4d 89 1a e9 0c 01 00 00 0f 1f 40 00 48 89 d1 4c 8b 1e 49
RSP: 0018:ffffa1b9002d7d08 EFLAGS: 00010203
RAX: ffff9c873541af43 RBX: ffff9c873541af43 RCX: 00000c6f105cd6bf
RDX: 0000637882e986b6 RSI: ffff9c8735447ffb RDI: ffff9c8735447ffb
RBP: ffff9c8739cd3800 R08: ffff9c873b802f00 R09: 00000000fffff73b
R10: ffffffffb82b35f1 R11: 00505b1b004d5b1b R12: 0000000000000000
R13: ffff9c873541af3d R14: 000000000000000b R15: 000000000000000c
FS: 00007f450c390580(0000) GS:ffff9c873f180000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffff9c8735448000 CR3: 00000007e213c002 CR4: 00000000000606e0
Call Trace:
vt_do_kdgkb_ioctl+0x34d/0x440
vt_ioctl+0xba3/0x1190
? __bpf_prog_run32+0x39/0x60
? mem_cgroup_commit_charge+0x7b/0x4e0
tty_ioctl+0x23f/0x920
? preempt_count_sub+0x98/0xe0
? __seccomp_filter+0x67/0x600
do_vfs_ioctl+0xa2/0x6a0
? syscall_trace_enter+0x192/0x2d0
ksys_ioctl+0x3a/0x70
__x64_sys_ioctl+0x16/0x20
do_syscall_64+0x54/0xe0
entry_SYSCALL_64_after_hwframe+0x49/0xbe

The bug manifests on systemd systems with multiple vtcon devices:
# cat /sys/devices/virtual/vtconsole/vtcon0/name
(S) dummy device
# cat /sys/devices/virtual/vtconsole/vtcon1/name
(M) frame buffer device

There systemd runs 'loadkeys' tool in tapallel for each vtcon
instance. This causes two parallel ioctl(KDSKBSENT) calls to
race into adding the same entry into 'func_table' array at:

drivers/tty/vt/keyboard.c:vt_do_kdgkb_ioctl()

The function has no locking around writes to 'func_table'.

The simplest reproducer is to have initrams with the following
init on a 8-CPU machine x86_64:

#!/bin/sh

loadkeys -q windowkeys ru4 &
loadkeys -q windowkeys ru4 &
loadkeys -q windowkeys ru4 &
loadkeys -q windowkeys ru4 &

loadkeys -q windowkeys ru4 &
loadkeys -q windowkeys ru4 &
loadkeys -q windowkeys ru4 &
loadkeys -q windowkeys ru4 &
wait

The change adds lock on write path only. Reads are still racy.

CC: Greg Kroah-Hartman <[e-mail]>
CC: Jiri Slaby <[e-mail]>
Link: https://lkml.org/lkml/2019/2/17/256
Signed-off-by: Sergei Trofimovich <[e-mail]>
Cc: stable <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/vt/keyboard.c

···
 static struct input_handler kbd_handler;
 static DEFINE_SPINLOCK(kbd_event_lock);
 static DEFINE_SPINLOCK(led_lock);
+static DEFINE_SPINLOCK(func_buf_lock); /* guard 'func_buf'  and friends */
 static unsigned long key_down[BITS_TO_LONGS(KEY_CNT)];	/* keyboard key bitmap */
 static unsigned char shift_down[NR_SHIFT];		/* shift state counters.. */
 static bool dead_key_next;
···
 	char *p;
 	u_char *q;
 	u_char __user *up;
-	int sz;
+	int sz, fnw_sz;
 	int delta;
 	char *first_free, *fj, *fnw;
 	int i, j, k;
 	int ret;
+	unsigned long flags;
 
 	if (!capable(CAP_SYS_TTY_CONFIG))
 		perm = 0;
···
 			goto reterr;
 		}
 
+		fnw = NULL;
+		fnw_sz = 0;
+		/* race aginst other writers */
+		again:
+		spin_lock_irqsave(&func_buf_lock, flags);
 		q = func_table[i];
+
+		/* fj pointer to next entry after 'q' */
 		first_free = funcbufptr + (funcbufsize - funcbufleft);
 		for (j = i+1; j < MAX_NR_FUNC && !func_table[j]; j++)
 			;
···
 			fj = func_table[j];
 		else
 			fj = first_free;
-
+		/* buffer usage increase by new entry */
 		delta = (q ? -strlen(q) : 1) + strlen(kbs->kb_string);
+
 		if (delta <= funcbufleft) { 	/* it fits in current buf */
 		    if (j < MAX_NR_FUNC) {
+			/* make enough space for new entry at 'fj' */
 			memmove(fj + delta, fj, first_free - fj);
 			for (k = j; k < MAX_NR_FUNC; k++)
 			    if (func_table[k])
···
 		    sz = 256;
 		    while (sz < funcbufsize - funcbufleft + delta)
 		      sz <<= 1;
-		    fnw = kmalloc(sz, GFP_KERNEL);
-		    if(!fnw) {
-		      ret = -ENOMEM;
-		      goto reterr;
+		    if (fnw_sz != sz) {
+		      spin_unlock_irqrestore(&func_buf_lock, flags);
+		      kfree(fnw);
+		      fnw = kmalloc(sz, GFP_KERNEL);
+		      fnw_sz = sz;
+		      if (!fnw) {
+			ret = -ENOMEM;
+			goto reterr;
+		      }
+		      goto again;
 		    }
 
 		    if (!q)
 		      func_table[i] = fj;
+		    /* copy data before insertion point to new location */
 		    if (fj > funcbufptr)
 			memmove(fnw, funcbufptr, fj - funcbufptr);
 		    for (k = 0; k < j; k++)
 		      if (func_table[k])
 			func_table[k] = fnw + (func_table[k] - funcbufptr);
 
+		    /* copy data after insertion point to new location */
 		    if (first_free > fj) {
 			memmove(fnw + (fj - funcbufptr) + delta, fj, first_free - fj);
 			for (k = j; k < MAX_NR_FUNC; k++)
···
 		    funcbufleft = funcbufleft - delta + sz - funcbufsize;
 		    funcbufsize = sz;
 		}
+		/* finally insert item itself */
 		strcpy(func_table[i], kbs->kb_string);
+		spin_unlock_irqrestore(&func_buf_lock, flags);
 		break;
 	}
 	ret = 0;