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SubjectRe: [PATCH 5.5 027/150] vt: selection, close sel_buffer race
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On 27. 02. 20, 14:36, Greg Kroah-Hartman wrote:
> From: Jiri Slaby <jslaby@suse.cz>
>
> commit 07e6124a1a46b4b5a9b3cacc0c306b50da87abf5 upstream.
>
> syzkaller reported this UAF:

With this patch, syzkaller reports possible circular locking dependency:
https://lore.kernel.org/lkml/000000000000be57bf059f8aa7b9@google.com/

Could you drop the patch from stable until this is resolved?

> 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@suse.cz>
> Reported-by: syzbot+59997e8d5cbdc486e6f6@syzkaller.appspotmail.com
> Cc: stable <stable@vger.kernel.org>
> Link: https://lore.kernel.org/r/20200210081131.23572-2-jslaby@suse.cz
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>
> ---
> drivers/tty/vt/selection.c | 23 +++++++++++++++++------
> 1 file changed, 17 insertions(+), 6 deletions(-)
>
> --- a/drivers/tty/vt/selection.c
> +++ b/drivers/tty/vt/selection.c
> @@ -16,6 +16,7 @@
> #include <linux/tty.h>
> #include <linux/sched.h>
> #include <linux/mm.h>
> +#include <linux/mutex.h>
> #include <linux/slab.h>
> #include <linux/types.h>
>
> @@ -45,6 +46,7 @@ static volatile int sel_start = -1; /*
> 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) */
> @@ -186,7 +188,7 @@ int set_selection_kernel(struct tiocl_se
> char *bp, *obp;
> int i, ps, pe, multiplier;
> u32 c;
> - int mode;
> + int mode, ret = 0;
>
> poke_blanked_console();
>
> @@ -212,6 +214,7 @@ int set_selection_kernel(struct tiocl_se
> 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;
> @@ -257,9 +260,10 @@ int set_selection_kernel(struct tiocl_se
> break;
> case TIOCL_SELPOINTER:
> highlight_pointer(pe);
> - return 0;
> + goto unlock;
> default:
> - return -EINVAL;
> + ret = -EINVAL;
> + goto unlock;
> }
>
> /* remove the pointer */
> @@ -281,7 +285,7 @@ int set_selection_kernel(struct tiocl_se
> 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 */
> @@ -309,7 +313,8 @@ int set_selection_kernel(struct tiocl_se
> if (!bp) {
> printk(KERN_WARNING "selection: kmalloc() failed\n");
> clear_selection();
> - return -ENOMEM;
> + ret = -ENOMEM;
> + goto unlock;
> }
> kfree(sel_buffer);
> sel_buffer = bp;
> @@ -334,7 +339,9 @@ int set_selection_kernel(struct tiocl_se
> }
> }
> sel_buffer_lth = bp - sel_buffer;
> - return 0;
> +unlock:
> + mutex_unlock(&sel_lock);
> + return ret;
> }
> EXPORT_SYMBOL_GPL(set_selection_kernel);
>
> @@ -364,6 +371,7 @@ int paste_selection(struct tty_struct *t
> 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 (signal_pending(current)) {
> @@ -371,7 +379,9 @@ int paste_selection(struct tty_struct *t
> break;
> }
> if (tty_throttled(tty)) {
> + mutex_unlock(&sel_lock);
> schedule();
> + mutex_lock(&sel_lock);
> continue;
> }
> __set_current_state(TASK_RUNNING);
> @@ -380,6 +390,7 @@ int paste_selection(struct tty_struct *t
> count);
> pasted += count;
> }
> + mutex_unlock(&sel_lock);
> remove_wait_queue(&vc->paste_wait, &wait);
> __set_current_state(TASK_RUNNING);
>
>
>


--
js
suse labs

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