[Devel] [PATCH RHEL8 COMMIT] ms/vt: selection, close sel_buffer race #PSBM-120640
Konstantin Khorenko
khorenko at virtuozzo.com
Wed Oct 14 15:32:52 MSK 2020
The commit is pushed to "branch-rh8-4.18.0-193.6.3.vz8.4.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh8-4.18.0-193.6.3.vz8.4.12
------>
commit c69dd934d249b86e9b493bd89261b03f70f2357b
Author: Jiri Slaby <jslaby at suse.cz>
Date: Wed Oct 14 15:32:52 2020 +0300
ms/vt: selection, close sel_buffer race #PSBM-120640
syzkaller reported this UAF:
BUG: KASAN: use-after-free in n_tty_receive_buf_common+0x2481/0x2940 drivers/tty/n_tty.c:1741
Read of size 1 at addr ffff8880089e40e9 by task syz-executor.1/13184
CPU: 0 PID: 13184 Comm: syz-executor.1 Not tainted 5.4.7 #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
Call Trace:
...
kasan_report+0xe/0x20 mm/kasan/common.c:634
n_tty_receive_buf_common+0x2481/0x2940 drivers/tty/n_tty.c:1741
tty_ldisc_receive_buf+0xac/0x190 drivers/tty/tty_buffer.c:461
paste_selection+0x297/0x400 drivers/tty/vt/selection.c:372
tioclinux+0x20d/0x4e0 drivers/tty/vt/vt.c:3044
vt_ioctl+0x1bcf/0x28d0 drivers/tty/vt/vt_ioctl.c:364
tty_ioctl+0x525/0x15a0 drivers/tty/tty_io.c:2657
vfs_ioctl fs/ioctl.c:47 [inline]
It is due to a race between parallel paste_selection (TIOCL_PASTESEL)
and set_selection_user (TIOCL_SETSEL) invocations. One uses sel_buffer,
while the other frees it and reallocates a new one for another
selection. Add a mutex to close this race.
The mutex takes care properly of sel_buffer and sel_buffer_lth only. The
other selection global variables (like sel_start, sel_end, and sel_cons)
are protected only in set_selection_user. The other functions need quite
some more work to close the races of the variables there. This is going
to happen later.
This likely fixes (I am unsure as there is no reproducer provided) bug
206361 too. It was marked as CVE-2020-8648.
Signed-off-by: Jiri Slaby <jslaby at suse.cz>
Reported-by: syzbot+59997e8d5cbdc486e6f6 at syzkaller.appspotmail.com
References: https://bugzilla.kernel.org/show_bug.cgi?id=206361
Cc: stable <stable at vger.kernel.org>
Link: https://lore.kernel.org/r/20200210081131.23572-2-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
https://jira.sw.ru/browse/PSBM-120640
This is a backport of mainline commit 07e6124a1a46b4b5a9b3cacc0c306b50da87abf5:
the affected code is in set_selection() rather than set_selection_kernel().
Signed-off-by: Evgenii Shatokhin <eshatokhin at virtuozzo.com>
---
drivers/tty/vt/selection.c | 23 +++++++++++++++++------
1 file changed, 17 insertions(+), 6 deletions(-)
diff --git a/drivers/tty/vt/selection.c b/drivers/tty/vt/selection.c
index 90ea1cc52b7a..2a68d6fdb7b1 100644
--- a/drivers/tty/vt/selection.c
+++ b/drivers/tty/vt/selection.c
@@ -14,6 +14,7 @@
#include <linux/tty.h>
#include <linux/sched.h>
#include <linux/mm.h>
+#include <linux/mutex.h>
#include <linux/slab.h>
#include <linux/types.h>
@@ -41,6 +42,7 @@ static volatile int sel_start = -1; /* cleared by clear_selection */
static int sel_end;
static int sel_buffer_lth;
static char *sel_buffer;
+static DEFINE_MUTEX(sel_lock);
/* clear_selection, highlight and highlight_pointer can be called
from interrupt (via scrollback/front) */
@@ -161,7 +163,7 @@ int set_selection(const struct tiocl_selection __user *sel, struct tty_struct *t
char *bp, *obp;
int i, ps, pe, multiplier;
u16 c;
- int mode;
+ int mode, ret = 0;
poke_blanked_console();
if (copy_from_user(&v, sel, sizeof(*sel)))
@@ -188,6 +190,7 @@ int set_selection(const struct tiocl_selection __user *sel, struct tty_struct *t
if (ps > pe) /* make sel_start <= sel_end */
swap(ps, pe);
+ mutex_lock(&sel_lock);
if (sel_cons != vc_cons[fg_console].d) {
clear_selection();
sel_cons = vc_cons[fg_console].d;
@@ -233,9 +236,10 @@ int set_selection(const struct tiocl_selection __user *sel, struct tty_struct *t
break;
case TIOCL_SELPOINTER:
highlight_pointer(pe);
- return 0;
+ goto unlock;
default:
- return -EINVAL;
+ ret = -EINVAL;
+ goto unlock;
}
/* remove the pointer */
@@ -257,7 +261,7 @@ int set_selection(const struct tiocl_selection __user *sel, struct tty_struct *t
else if (new_sel_start == sel_start)
{
if (new_sel_end == sel_end) /* no action required */
- return 0;
+ goto unlock;
else if (new_sel_end > sel_end) /* extend to right */
highlight(sel_end + 2, new_sel_end);
else /* contract from right */
@@ -285,7 +289,8 @@ int set_selection(const struct tiocl_selection __user *sel, struct tty_struct *t
if (!bp) {
printk(KERN_WARNING "selection: kmalloc() failed\n");
clear_selection();
- return -ENOMEM;
+ ret = -ENOMEM;
+ goto unlock;
}
kfree(sel_buffer);
sel_buffer = bp;
@@ -310,7 +315,9 @@ int set_selection(const struct tiocl_selection __user *sel, struct tty_struct *t
}
}
sel_buffer_lth = bp - sel_buffer;
- return 0;
+unlock:
+ mutex_unlock(&sel_lock);
+ return ret;
}
/* Insert the contents of the selection buffer into the
@@ -338,10 +345,13 @@ int paste_selection(struct tty_struct *tty)
tty_buffer_lock_exclusive(&vc->port);
add_wait_queue(&vc->paste_wait, &wait);
+ mutex_lock(&sel_lock);
while (sel_buffer && sel_buffer_lth > pasted) {
set_current_state(TASK_INTERRUPTIBLE);
if (tty_throttled(tty)) {
+ mutex_unlock(&sel_lock);
schedule();
+ mutex_lock(&sel_lock);
continue;
}
__set_current_state(TASK_RUNNING);
@@ -350,6 +360,7 @@ int paste_selection(struct tty_struct *tty)
count);
pasted += count;
}
+ mutex_unlock(&sel_lock);
remove_wait_queue(&vc->paste_wait, &wait);
__set_current_state(TASK_RUNNING);
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