CVE-2015-4170 (published 2016-05-02T10:59Z)

Race condition in the ldsem_cmpxchg function in drivers/tty/tty_ldsem.c in the Linux kernel before 3.13-rc4-next-20131218 allows local users to cause a denial of service (ldsem_down_read and ldsem_down_write deadlock) by establishing a new tty thread during shutdown of a previous tty thread.

Severity: MEDIUM
CVSS v2 base score: 4.7 (AV:L/AC:M/Au:N/C:N/I:N/A:C)
CVSS v3 base score: 4.7, MEDIUM (CVSS:3.0/AV:L/AC:H/PR:N/UI:R/S:U/C:N/I:N/A:H)

Weakness information:
cwe_id: CWE-362
cwe_name: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
description: The product contains a code sequence that can run concurrently with other code, and the code sequence requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence that is operating concurrently.
extended_description: 
            This can have security implications when the expected synchronization is in security-critical code, such as recording whether a user is authenticated or modifying important state information that should not be influenced by an outsider.
            A race condition occurs within concurrent environments, and is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc.
            A race condition violates these properties, which are closely related:
               
                  Exclusivity - the code sequence is given exclusive access to the shared resource, i.e., no other code sequence can modify properties of the shared resource before the original sequence has completed execution.
                  Atomicity - the code sequence is behaviorally atomic, i.e., no other thread or process can concurrently execute the same sequence of instructions (or a subset) against the same resource.
               
            A race condition exists when an "interfering code sequence" can still access the shared resource, violating exclusivity. Programmers may assume that certain code sequences execute too quickly to be affected by an interfering code sequence; when they are not, this violates atomicity. For example, the single "x++" statement may appear atomic at the code layer, but it is actually non-atomic at the instruction layer, since it involves a read (the original value of x), followed by a computation (x+1), followed by a write (save the result to x).
            The interfering code sequence could be "trusted" or "untrusted." A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.
         
         

Fix information:
repo_url: https://github.com/torvalds/linux
commit_details: repo_url: https://github.com/torvalds/linux
author: Peter Hurley
author_date: 2013-12-11 21:11:58-05:00
msg: tty: Fix hang at ldsem_down_read()

When a controlling tty is being hung up and the hang up is
waiting for a just-signalled tty reader or writer to exit, and a new tty
reader/writer tries to acquire an ldisc reference concurrently with the
ldisc reference release from the signalled reader/writer, the hangup
can hang. The new reader/writer is sleeping in ldsem_down_read() and the
hangup is sleeping in ldsem_down_write() [1].

The new reader/writer fails to wakeup the waiting hangup because the
wrong lock count value is checked (the old lock count rather than the new
lock count) to see if the lock is unowned.

Change helper function to return the new lock count if the cmpxchg was
successful; document this behavior.

[1] edited dmesg log from reporter

SysRq : Show Blocked State
  task                        PC stack   pid father
systemd         D ffff88040c4f0000     0     1      0 0x00000000
 ffff88040c49fbe0 0000000000000046 ffff88040c4a0000 ffff88040c49ffd8
 00000000001d3980 00000000001d3980 ffff88040c4a0000 ffff88040593d840
 ffff88040c49fb40 ffffffff810a4cc0 0000000000000006 0000000000000023
Call Trace:
 [<ffffffff810a4cc0>] ? sched_clock_cpu+0x9f/0xe4
 [<ffffffff810a4cc0>] ? sched_clock_cpu+0x9f/0xe4
 [<ffffffff810a4cc0>] ? sched_clock_cpu+0x9f/0xe4
 [<ffffffff810a4cc0>] ? sched_clock_cpu+0x9f/0xe4
 [<ffffffff817a6649>] schedule+0x24/0x5e
 [<ffffffff817a588b>] schedule_timeout+0x15b/0x1ec
 [<ffffffff810a4cc0>] ? sched_clock_cpu+0x9f/0xe4
 [<ffffffff817aa691>] ? _raw_spin_unlock_irq+0x24/0x26
 [<ffffffff817aa10c>] down_read_failed+0xe3/0x1b9
 [<ffffffff817aa26d>] ldsem_down_read+0x8b/0xa5
 [<ffffffff8142b5ca>] ? tty_ldisc_ref_wait+0x1b/0x44
 [<ffffffff8142b5ca>] tty_ldisc_ref_wait+0x1b/0x44
 [<ffffffff81423f5b>] tty_write+0x7d/0x28a
 [<ffffffff814241f5>] redirected_tty_write+0x8d/0x98
 [<ffffffff81424168>] ? tty_write+0x28a/0x28a
 [<ffffffff8115d03f>] do_loop_readv_writev+0x56/0x79
 [<ffffffff8115e604>] do_readv_writev+0x1b0/0x1ff
 [<ffffffff8116ea0b>] ? do_vfs_ioctl+0x32a/0x489
 [<ffffffff81167d9d>] ? final_putname+0x1d/0x3a
 [<ffffffff8115e6c7>] vfs_writev+0x2e/0x49
 [<ffffffff8115e7d3>] SyS_writev+0x47/0xaa
 [<ffffffff817ab822>] system_call_fastpath+0x16/0x1b
bash            D ffffffff81c104c0     0  5469   5302 0x00000082
 ffff8800cf817ac0 0000000000000046 ffff8804086b22a0 ffff8800cf817fd8
 00000000001d3980 00000000001d3980 ffff8804086b22a0 ffff8800cf817a48
 000000000000b9a0 ffff8800cf817a78 ffffffff81004675 ffff8800cf817a44
Call Trace:
 [<ffffffff81004675>] ? dump_trace+0x165/0x29c
 [<ffffffff810a4cc0>] ? sched_clock_cpu+0x9f/0xe4
 [<ffffffff8100edda>] ? save_stack_trace+0x26/0x41
 [<ffffffff817a6649>] schedule+0x24/0x5e
 [<ffffffff817a588b>] schedule_timeout+0x15b/0x1ec
 [<ffffffff810a4cc0>] ? sched_clock_cpu+0x9f/0xe4
 [<ffffffff817a9f03>] ? down_write_failed+0xa3/0x1c9
 [<ffffffff817aa691>] ? _raw_spin_unlock_irq+0x24/0x26
 [<ffffffff817a9f0b>] down_write_failed+0xab/0x1c9
 [<ffffffff817aa300>] ldsem_down_write+0x79/0xb1
 [<ffffffff817aada3>] ? tty_ldisc_lock_pair_timeout+0xa5/0xd9
 [<ffffffff817aada3>] tty_ldisc_lock_pair_timeout+0xa5/0xd9
 [<ffffffff8142bf33>] tty_ldisc_hangup+0xc4/0x218
 [<ffffffff81423ab3>] __tty_hangup+0x2e2/0x3ed
 [<ffffffff81424a76>] disassociate_ctty+0x63/0x226
 [<ffffffff81078aa7>] do_exit+0x79f/0xa11
 [<ffffffff81086bdb>] ? get_signal_to_deliver+0x206/0x62f
 [<ffffffff810b4bfb>] ? lock_release_holdtime.part.8+0xf/0x16e
 [<ffffffff81079b05>] do_group_exit+0x47/0xb5
 [<ffffffff81086c16>] get_signal_to_deliver+0x241/0x62f
 [<ffffffff810020a7>] do_signal+0x43/0x59d
 [<ffffffff810f2af7>] ? __audit_syscall_exit+0x21a/0x2a8
 [<ffffffff810b4bfb>] ? lock_release_holdtime.part.8+0xf/0x16e
 [<ffffffff81002655>] do_notify_resume+0x54/0x6c
 [<ffffffff817abaf8>] int_signal+0x12/0x17

Reported-by: Sami Farin <sami.farin@gmail.com>
Cc: <stable@vger.kernel.org> # 3.12.x
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
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: tty_ldsem.c
change_type: ModificationType.MODIFY
diff: @@ -86,11 +86,21 @@ static inline long ldsem_atomic_update(long delta, struct ld_semaphore *sem)
 	return atomic_long_add_return(delta, (atomic_long_t *)&sem->count);
 }
 
+/*
+ * ldsem_cmpxchg() updates @*old with the last-known sem->count value.
+ * Returns 1 if count was successfully changed; @*old will have @new value.
+ * Returns 0 if count was not changed; @*old will have most recent sem->count
+ */
 static inline int ldsem_cmpxchg(long *old, long new, struct ld_semaphore *sem)
 {
-	long tmp = *old;
-	*old = atomic_long_cmpxchg(&sem->count, *old, new);
-	return *old == tmp;
+	long tmp = atomic_long_cmpxchg(&sem->count, *old, new);
+	if (tmp == *old) {
+		*old = new;
+		return 1;
+	} else {
+		*old = tmp;
+		return 0;
+	}
 }
 
 /*

programming_language: C