[PATCH 3.16.y-ckt 208/254] ARM: 8198/1: make kuser helpers depend on MMU

[Luis Henriques]

3.16.7-ckt2 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Nathan Lynch <nathan_lynch@xxxxxxxxxx>

commit 08b964ff3c51b10aaf2e6ba639f40054c09f0f7a upstream.

The kuser helpers page is not set up on non-MMU systems, so it does
not make sense to allow CONFIG_KUSER_HELPERS to be enabled when
CONFIG_MMU=n.  Allowing it to be set on !MMU results in an oops in
set_tls (used in execve and the arm_syscall trap handler):

Unhandled exception: IPSR = 00000005 LR = fffffff1
CPU: 0 PID: 1 Comm: swapper Not tainted 3.18.0-rc1-00041-ga30465a #216
task: 8b838000 ti: 8b82a000 task.ti: 8b82a000
PC is at flush_thread+0x32/0x40
LR is at flush_thread+0x21/0x40
pc : [<8f00157a>]    lr : [<8f001569>]    psr: 4100000b
sp : 8b82be20  ip : 00000000  fp : 8b83c000
r10: 00000001  r9 : 88018c84  r8 : 8bb85000
r7 : 8b838000  r6 : 00000000  r5 : 8bb77400  r4 : 8b82a000
r3 : ffff0ff0  r2 : 8b82a000  r1 : 00000000  r0 : 88020354
xPSR: 4100000b
CPU: 0 PID: 1 Comm: swapper Not tainted 3.18.0-rc1-00041-ga30465a #216
[<8f002bc1>] (unwind_backtrace) from [<8f002033>] (show_stack+0xb/0xc)
[<8f002033>] (show_stack) from [<8f00265b>] (__invalid_entry+0x4b/0x4c)

As best I can tell this issue existed for the set_tls ARM syscall
before commit fbfb872f5f41 "ARM: 8148/1: flush TLS and thumbee
register state during exec" consolidated the TLS manipulation code
into the set_tls helper function, but now that we're using it to flush
register state during execve, !MMU users encounter the oops at the
first exec.

Prevent CONFIG_MMU=n configurations from enabling
CONFIG_KUSER_HELPERS.

Fixes: fbfb872f5f41 (ARM: 8148/1: flush TLS and thumbee register state during exec)

Signed-off-by: Nathan Lynch <nathan_lynch@xxxxxxxxxx>
Reported-by: Stefan Agner <stefan@xxxxxxxx>
Acked-by: Uwe Kleine-KÃnig <u.kleine-koenig@xxxxxxxxxxxxxx>
Signed-off-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxxx>
Signed-off-by: Luis Henriques <luis.henriques@xxxxxxxxxxxxx>
---
 arch/arm/mm/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm/mm/Kconfig b/arch/arm/mm/Kconfig
index c348eaee7ee2..b781cc7c7b6a 100644
--- a/arch/arm/mm/Kconfig
+++ b/arch/arm/mm/Kconfig
@@ -798,6 +798,7 @@ config NEED_KUSER_HELPERS
 
 config KUSER_HELPERS
 	bool "Enable kuser helpers in vector page" if !NEED_KUSER_HELPERS
+	depends on MMU
 	default y
 	help
 	  Warning: disabling this option may break user programs.
-- 
2.1.0

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