Patch "MIPS: Prevent "restoration" of MSA context in non-MSA kernels" 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

    MIPS: Prevent "restoration" of MSA context in non-MSA kernels

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:
     mips-prevent-restoration-of-msa-context-in-non-msa-kernels.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 6533af4d4831c421cd9aa4dce7cfc19a3514cc09 Mon Sep 17 00:00:00 2001
From: Paul Burton <paul.burton@imgtec.com>
Date: Thu, 21 Apr 2016 18:04:53 +0100
Subject: MIPS: Prevent "restoration" of MSA context in non-MSA kernels

From: Paul Burton <paul.burton@imgtec.com>

commit 6533af4d4831c421cd9aa4dce7cfc19a3514cc09 upstream.

If a kernel doesn't support MSA context (ie. CONFIG_CPU_HAS_MSA=n) then
it will only keep 64 bits per FP register in thread context, and the
calls to set_fpr64 in restore_msa_extcontext will overrun the end of the
FP register context into the FCSR & MSACSR values. GCC 6.x has become
smart enough to detect this & complain like so:

    arch/mips/kernel/signal.c: In function 'protected_restore_fp_context':
    ./arch/mips/include/asm/processor.h:114:17: error: array subscript is above array bounds [-Werror=array-bounds]
      fpr->val##width[FPR_IDX(width, idx)] = val;   \
      ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
    ./arch/mips/include/asm/processor.h:118:1: note: in expansion of macro 'BUILD_FPR_ACCESS'
     BUILD_FPR_ACCESS(64)

The only way to trigger this code to run would be for a program to set
up an artificial extended MSA context structure following a sigframe &
execute sigreturn. Whilst this doesn't allow a program to write to any
state that it couldn't already, it makes little sense to allow this
"restoration" of MSA context in a system that doesn't support MSA.

Fix this by killing a program with SIGSYS if it tries something as crazy
as "restoring" fake MSA context in this way, also fixing the build error
& allowing for most of restore_msa_extcontext to be optimised out of
kernels without support for MSA.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Reported-by: Michal Toman <michal.toman@imgtec.com>
Fixes: bf82cb30c7e5 ("MIPS: Save MSA extended context around signals")
Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: Michal Toman <michal.toman@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/13164/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 arch/mips/kernel/signal.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

--- a/arch/mips/kernel/signal.c
+++ b/arch/mips/kernel/signal.c
@@ -195,6 +195,9 @@ static int restore_msa_extcontext(void _
 	unsigned int csr;
 	int i, err;
 
+	if (!config_enabled(CONFIG_CPU_HAS_MSA))
+		return SIGSYS;
+
 	if (size != sizeof(*msa))
 		return -EINVAL;
 
@@ -398,8 +401,8 @@ int protected_restore_fp_context(void __
 	}
 
 fp_done:
-	if (used & USED_EXTCONTEXT)
-		err |= restore_extcontext(sc_to_extcontext(sc));
+	if (!err && (used & USED_EXTCONTEXT))
+		err = restore_extcontext(sc_to_extcontext(sc));
 
 	return err ?: sig;
 }


Patches currently in stable-queue which might be from paul.burton@imgtec.com are

queue-4.4/mips-disable-preemption-during-prctl-pr_set_fp_mode.patch
queue-4.4/mips-fix-sigreturn-via-vdso-on-micromips-kernel.patch
queue-4.4/mips-handle-highmem-pages-in-__update_cache.patch
queue-4.4/mips-fix-watchpoint-restoration.patch
queue-4.4/mips-build-micromips-vdso-for-micromips-kernels.patch
queue-4.4/mips-math-emu-fix-jalr-emulation-when-rd-0.patch
queue-4.4/mips-sync-icache-dcache-in-set_pte_at.patch
queue-4.4/mips-fix-msa-ld_-st_-asm-macros-to-use-ptr_addu.patch
queue-4.4/mips-prevent-restoration-of-msa-context-in-non-msa-kernels.patch
queue-4.4/mips-use-copy_s.fmt-rather-than-copy_u.fmt.patch