Patch "ARM: BUG if jumping to usermode address in kernel mode" has been added to the 4.4-stable tree

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

    ARM: BUG if jumping to usermode address in kernel mode

to the 4.4-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     arm-bug-if-jumping-to-usermode-address-in-kernel-mode.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 8bafae202c82dc257f649ea3c275a0f35ee15113 Mon Sep 17 00:00:00 2001
From: Russell King <rmk+kernel@armlinux.org.uk>
Date: Fri, 24 Nov 2017 23:49:34 +0000
Subject: ARM: BUG if jumping to usermode address in kernel mode

From: Russell King <rmk+kernel@armlinux.org.uk>

commit 8bafae202c82dc257f649ea3c275a0f35ee15113 upstream.

Detect if we are returning to usermode via the normal kernel exit paths
but the saved PSR value indicates that we are in kernel mode.  This
could occur due to corrupted stack state, which has been observed with
"ftracetest".

This ensures that we catch the problem case before we get to user code.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Cc: Alex Shi <alex.shi@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 arch/arm/include/asm/assembler.h |   18 ++++++++++++++++++
 arch/arm/kernel/entry-header.S   |    6 ++++++
 2 files changed, 24 insertions(+)

--- a/arch/arm/include/asm/assembler.h
+++ b/arch/arm/include/asm/assembler.h
@@ -512,4 +512,22 @@ THUMB(	orr	\reg , \reg , #PSR_T_BIT	)
 #endif
 	.endm
 
+	.macro	bug, msg, line
+#ifdef CONFIG_THUMB2_KERNEL
+1:	.inst	0xde02
+#else
+1:	.inst	0xe7f001f2
+#endif
+#ifdef CONFIG_DEBUG_BUGVERBOSE
+	.pushsection .rodata.str, "aMS", %progbits, 1
+2:	.asciz	"\msg"
+	.popsection
+	.pushsection __bug_table, "aw"
+	.align	2
+	.word	1b, 2b
+	.hword	\line
+	.popsection
+#endif
+	.endm
+
 #endif /* __ASM_ASSEMBLER_H__ */
--- a/arch/arm/kernel/entry-header.S
+++ b/arch/arm/kernel/entry-header.S
@@ -295,6 +295,8 @@
 	mov	r2, sp
 	ldr	r1, [r2, #\offset + S_PSR]	@ get calling cpsr
 	ldr	lr, [r2, #\offset + S_PC]!	@ get pc
+	tst	r1, #0xcf
+	bne	1f
 	msr	spsr_cxsf, r1			@ save in spsr_svc
 #if defined(CONFIG_CPU_V6) || defined(CONFIG_CPU_32v6K)
 	@ We must avoid clrex due to Cortex-A15 erratum #830321
@@ -309,6 +311,7 @@
 						@ after ldm {}^
 	add	sp, sp, #\offset + S_FRAME_SIZE
 	movs	pc, lr				@ return & move spsr_svc into cpsr
+1:	bug	"Returning to usermode but unexpected PSR bits set?", \@
 #elif defined(CONFIG_CPU_V7M)
 	@ V7M restore.
 	@ Note that we don't need to do clrex here as clearing the local
@@ -324,6 +327,8 @@
 	ldr	r1, [sp, #\offset + S_PSR]	@ get calling cpsr
 	ldr	lr, [sp, #\offset + S_PC]	@ get pc
 	add	sp, sp, #\offset + S_SP
+	tst	r1, #0xcf
+	bne	1f
 	msr	spsr_cxsf, r1			@ save in spsr_svc
 
 	@ We must avoid clrex due to Cortex-A15 erratum #830321
@@ -336,6 +341,7 @@
 	.endif
 	add	sp, sp, #S_FRAME_SIZE - S_SP
 	movs	pc, lr				@ return & move spsr_svc into cpsr
+1:	bug	"Returning to usermode but unexpected PSR bits set?", \@
 #endif	/* !CONFIG_THUMB2_KERNEL */
 	.endm
 


Patches currently in stable-queue which might be from rmk+kernel@armlinux.org.uk are

queue-4.4/arm-avoid-faulting-on-qemu.patch
queue-4.4/arm-bug-if-jumping-to-usermode-address-in-kernel-mode.patch