[3.19.y-ckt stable] Patch "arm64: entry: fix context tracking for el0_sp_pc" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    arm64: entry: fix context tracking for el0_sp_pc

to the linux-3.19.y-queue branch of the 3.19.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.19.y-queue

This patch is scheduled to be released in version 3.19.y-ckt4.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.19.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 36b22aed822b170eda0f50a2ae79c238e0c38162 Mon Sep 17 00:00:00 2001
From: Mark Rutland <mark.rutland at arm.com>
Date: Mon, 15 Jun 2015 16:40:27 +0100
Subject: arm64: entry: fix context tracking for el0_sp_pc

commit 46b0567c851cf85d6ba6f23eef385ec9111d09bc upstream.

Commit 6c81fe7925cc4c42 ("arm64: enable context tracking") did not
update el0_sp_pc to use ct_user_exit, but this appears to have been
unintentional. In commit 6ab6463aeb5fbc75 ("arm64: adjust el0_sync so
that a function can be called") we made x0 available, and in the return
to userspace we call ct_user_enter in the kernel_exit macro.

Due to this, we currently don't correctly inform RCU of the user->kernel
transition, and may erroneously account for time spent in the kernel as
if we were in an extended quiescent state when CONFIG_CONTEXT_TRACKING
is enabled.

As we do record the kernel->user transition, a userspace application
making accesses from an unaligned stack pointer can demonstrate the
imbalance, provoking the following warning:

------------[ cut here ]------------
WARNING: CPU: 2 PID: 3660 at kernel/context_tracking.c:75 context_tracking_enter+0xd8/0xe4()
Modules linked in:
CPU: 2 PID: 3660 Comm: a.out Not tainted 4.1.0-rc7+ #8
Hardware name: ARM Juno development board (r0) (DT)
Call trace:
[<ffffffc000089914>] dump_backtrace+0x0/0x124
[<ffffffc000089a48>] show_stack+0x10/0x1c
[<ffffffc0005b3cbc>] dump_stack+0x84/0xc8
[<ffffffc0000b3214>] warn_slowpath_common+0x98/0xd0
[<ffffffc0000b330c>] warn_slowpath_null+0x14/0x20
[<ffffffc00013ada4>] context_tracking_enter+0xd4/0xe4
[<ffffffc0005b534c>] preempt_schedule_irq+0xd4/0x114
[<ffffffc00008561c>] el1_preempt+0x4/0x28
[<ffffffc0001b8040>] exit_files+0x38/0x4c
[<ffffffc0000b5b94>] do_exit+0x430/0x978
[<ffffffc0000b614c>] do_group_exit+0x40/0xd4
[<ffffffc0000c0208>] get_signal+0x23c/0x4f4
[<ffffffc0000890b4>] do_signal+0x1ac/0x518
[<ffffffc000089650>] do_notify_resume+0x5c/0x68
---[ end trace 963c192600337066 ]---

This patch adds the missing ct_user_exit to the el0_sp_pc entry path,
correcting the context tracking for this case.

Signed-off-by: Mark Rutland <mark.rutland at arm.com>
Acked-by: Will Deacon <will.deacon at arm.com>
Fixes: 6c81fe7925cc ("arm64: enable context tracking")
Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 arch/arm64/kernel/entry.S | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 9b870a2..158b076 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -517,6 +517,7 @@ el0_sp_pc:
 	mrs	x26, far_el1
 	// enable interrupts before calling the main handler
 	enable_dbg_and_irq
+	ct_user_exit
 	mov	x0, x26
 	mov	x1, x25
 	mov	x2, sp
--
1.9.1