CVE-2020-28941 (published 2020-11-19T19:15Z)

An issue was discovered in drivers/accessibility/speakup/spk_ttyio.c in the Linux kernel through 5.9.9. Local attackers on systems with the speakup driver could cause a local denial of service attack, aka CID-d41227544427. This occurs because of an invalid free when the line discipline is used more than once.

Severity: MEDIUM
CVSS v2 base score: 4.9 (AV:L/AC:L/Au:N/C:N/I:N/A:C)
CVSS v3 base score: 5.5, MEDIUM (CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H)

Weakness information:
cwe_id: CWE-763
cwe_name: Release of Invalid Pointer or Reference
description: The product attempts to return a memory resource to the system, but it calls the wrong release function or calls the appropriate release function incorrectly.
extended_description: 
            This weakness can take several forms, such as:
               
                  The memory was allocated, explicitly or implicitly, via one memory management method and deallocated using a different, non-compatible function (CWE-762).
                  The function calls or memory management routines chosen are appropriate, however they are used incorrectly, such as in CWE-761.
               
         
         

Fix information:
repo_url: https://github.com/torvalds/linux
commit_details: repo_url: https://github.com/torvalds/linux
author: Samuel Thibault
author_date: 2020-11-10 19:35:41+01:00
msg: speakup: Do not let the line discipline be used several times

Speakup has only one speakup_tty variable to store the tty it is managing. This
makes sense since its codebase currently assumes that there is only one user who
controls the screen reading.

That however means that we have to forbid using the line discipline several
times, otherwise the second closure would try to free a NULL ldisc_data, leading to

general protection fault: 0000 [#1] SMP KASAN PTI
RIP: 0010:spk_ttyio_ldisc_close+0x2c/0x60
Call Trace:
 tty_ldisc_release+0xa2/0x340
 tty_release_struct+0x17/0xd0
 tty_release+0x9d9/0xcc0
 __fput+0x231/0x740
 task_work_run+0x12c/0x1a0
 do_exit+0x9b5/0x2230
 ? release_task+0x1240/0x1240
 ? __do_page_fault+0x562/0xa30
 do_group_exit+0xd5/0x2a0
 __x64_sys_exit_group+0x35/0x40
 do_syscall_64+0x89/0x2b0
 ? page_fault+0x8/0x30
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Cc: stable@vger.kernel.org
Reported-by: 秦世松 <qinshisong1205@gmail.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Tested-by: Shisong Qin <qinshisong1205@gmail.com>
Link: https://lore.kernel.org/r/20201110183541.fzgnlwhjpgqzjeth@function
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
repository: repo_url: https://github.com/torvalds/linux
repo_name: torvalds/linux
description: Linux kernel source tree
repo_language: C
owner: torvalds
file_changes: filename: spk_ttyio.c
change_type: ModificationType.MODIFY
diff: @@ -49,15 +49,25 @@ static int spk_ttyio_ldisc_open(struct tty_struct *tty)
 
 	if (!tty->ops->write)
 		return -EOPNOTSUPP;
+
+	mutex_lock(&speakup_tty_mutex);
+	if (speakup_tty) {
+		mutex_unlock(&speakup_tty_mutex);
+		return -EBUSY;
+	}
 	speakup_tty = tty;
 
 	ldisc_data = kmalloc(sizeof(*ldisc_data), GFP_KERNEL);
-	if (!ldisc_data)
+	if (!ldisc_data) {
+		speakup_tty = NULL;
+		mutex_unlock(&speakup_tty_mutex);
 		return -ENOMEM;
+	}
 
 	init_completion(&ldisc_data->completion);
 	ldisc_data->buf_free = true;
 	speakup_tty->disc_data = ldisc_data;
+	mutex_unlock(&speakup_tty_mutex);
 
 	return 0;
 }

programming_language: C