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Subject[PATCH] vgacon: Fix an out-of-bounds in vgacon_scrollback_update()
Date
I got a slab-out-of-bounds report when I doing fuzz test.

[ 334.989515] ==================================================================
[ 334.989577] BUG: KASAN: slab-out-of-bounds in vgacon_scroll+0x57a/0x8ed
[ 334.989588] Write of size 1766 at addr ffff8883de69ff3e by task test/2658
[ 334.989593]
[ 334.989608] CPU: 3 PID: 2658 Comm: test Not tainted 5.7.0-rc5-00005-g152036d1379f #789
[ 334.989617] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.org 04/01/2014
[ 334.989624] Call Trace:
[ 334.989646] dump_stack+0xe4/0x14e
[ 334.989676] print_address_description.constprop.5+0x3f/0x60
[ 334.989699] ? vgacon_scroll+0x57a/0x8ed
[ 334.989710] __kasan_report.cold.8+0x92/0xaf
[ 334.989735] ? vgacon_scroll+0x57a/0x8ed
[ 334.989761] kasan_report+0x37/0x50
[ 334.989789] check_memory_region+0x1c1/0x1e0
[ 334.989806] memcpy+0x38/0x60
[ 334.989824] vgacon_scroll+0x57a/0x8ed
[ 334.989876] con_scroll+0x4ef/0x5e0
[ 334.989904] ? lockdep_hardirqs_on+0x5e0/0x5e0
[ 334.989934] lf+0x24f/0x2a0
[ 334.989951] ? con_scroll+0x5e0/0x5e0
[ 334.989975] ? find_held_lock+0x33/0x1c0
[ 334.990005] do_con_trol+0x313/0x5ff0
[ 334.990027] ? lock_downgrade+0x730/0x730
[ 334.990045] ? reset_palette+0x440/0x440
[ 334.990070] ? _raw_spin_unlock_irqrestore+0x4b/0x60
[ 334.990095] ? notifier_call_chain+0x120/0x170
[ 334.990132] ? __atomic_notifier_call_chain+0xf0/0x180
[ 334.990160] do_con_write.part.16+0xb2b/0x1b20
[ 334.990238] ? do_con_trol+0x5ff0/0x5ff0
[ 334.990258] ? mutex_lock_io_nested+0x1280/0x1280
[ 334.990269] ? rcu_read_unlock+0x50/0x50
[ 334.990315] ? __mutex_unlock_slowpath+0xd9/0x670
[ 334.990340] ? lockdep_hardirqs_on+0x3a2/0x5e0
[ 334.990368] con_write+0x36/0xc0
[ 334.990389] do_output_char+0x561/0x780
[ 334.990414] n_tty_write+0x58e/0xd30
[ 334.990478] ? n_tty_read+0x1800/0x1800
[ 334.990500] ? prepare_to_wait_exclusive+0x300/0x300
[ 334.990525] ? __might_fault+0x17a/0x1c0
[ 334.990557] tty_write+0x430/0x960
[ 334.990568] ? n_tty_read+0x1800/0x1800
[ 334.990600] ? tty_release+0x1280/0x1280
[ 334.990622] __vfs_write+0x81/0x100
[ 334.990648] vfs_write+0x1ce/0x510
[ 334.990676] ksys_write+0x104/0x200
[ 334.990691] ? __ia32_sys_read+0xb0/0xb0
[ 334.990708] ? trace_hardirqs_on_thunk+0x1a/0x1c
[ 334.990725] ? trace_hardirqs_off_caller+0x40/0x1a0
[ 334.990744] ? do_syscall_64+0x3b/0x5e0
[ 334.990775] do_syscall_64+0xc8/0x5e0
[ 334.990798] entry_SYSCALL_64_after_hwframe+0x49/0xb3
[ 334.990811] RIP: 0033:0x44f369
[ 334.990827] Code: 00 00 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 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 73 01 c3 48 c7 c1 c0 ff ff ff f7 d8 64 89 01 48
[ 334.990834] RSP: 002b:00007ffe9ace0968 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
[ 334.990848] RAX: ffffffffffffffda RBX: 0000000000400418 RCX: 000000000044f369
[ 334.990856] RDX: 0000000000000381 RSI: 0000000020003500 RDI: 0000000000000003
[ 334.990865] RBP: 00007ffe9ace0980 R08: 0000000020003530 R09: 00007ffe9ace0980
[ 334.990873] R10: 0000000000000001 R11: 0000000000000246 R12: 0000000000402110
[ 334.990881] R13: 0000000000000000 R14: 00000000006bf018 R15: 0000000000000000
[ 334.990937]
[ 334.990943] The buggy address belongs to the page:
[ 334.990962] page:ffffea000f79a400 refcount:1 mapcount:0 mapping:000000002bff47b3 index:0x0 head:ffffea000f79a400 order:4 compound_mapcount:0 compound_pincount:0
[ 334.990973] flags: 0x2fffff80010000(head)
[ 334.990992] raw: 002fffff80010000 dead000000000100 dead000000000122 0000000000000000
[ 334.991006] raw: 0000000000000000 0000000000000000 00000001ffffffff 0000000000000000
[ 334.991013] page dumped because: kasan: bad access detected
[ 334.991017]
[ 334.991023] Memory state around the buggy address:
[ 334.991034] ffff8883de6a0000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 334.991044] ffff8883de6a0080: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 334.991054] >ffff8883de6a0100: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 fc fc
[ 334.991061] ^
[ 334.991071] ffff8883de6a0180: fc fc fc fc fc fc 00 00 00 00 00 00 00 00 00 00
[ 334.991082] ffff8883de6a0200: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 334.991088] ==================================================================

Because vgacon_scrollback_cur->tail plus memcpy size is greater than
vgacon_scrollback_cur->size. Fix this by checking the memcpy size.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
---
drivers/video/console/vgacon.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/video/console/vgacon.c b/drivers/video/console/vgacon.c
index 998b0de1812f..b51ffb9a208d 100644
--- a/drivers/video/console/vgacon.c
+++ b/drivers/video/console/vgacon.c
@@ -243,6 +243,7 @@ static void vgacon_scrollback_startup(void)
static void vgacon_scrollback_update(struct vc_data *c, int t, int count)
{
void *p;
+ int size;

if (!vgacon_scrollback_cur->data || !vgacon_scrollback_cur->size ||
c->vc_num != fg_console)
@@ -251,13 +252,17 @@ static void vgacon_scrollback_update(struct vc_data *c, int t, int count)
p = (void *) (c->vc_origin + t * c->vc_size_row);

while (count--) {
+ size = vgacon_scrollback_cur->size - vgacon_scrollback_cur->tail;
+ if (size > c->vc_size_row)
+ size = c->vc_size_row;
+
scr_memcpyw(vgacon_scrollback_cur->data +
vgacon_scrollback_cur->tail,
- p, c->vc_size_row);
+ p, size);

vgacon_scrollback_cur->cnt++;
- p += c->vc_size_row;
- vgacon_scrollback_cur->tail += c->vc_size_row;
+ p += size;
+ vgacon_scrollback_cur->tail += size;

if (vgacon_scrollback_cur->tail >= vgacon_scrollback_cur->size)
vgacon_scrollback_cur->tail = 0;
--
2.25.1
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