kernel: MIPS: math-emu Write-protect delay slot emulation pages

Backport https://git.kernel.org/pub/scm/linux/kernel/git/mips/linux.git/commit/?id=adcc81f148d733b7e8e641300c5590a2cdc13bf3

"Mapping the delay slot emulation page as both writeable & executable
presents a security risk, in that if an exploit can write to & jump into
the page then it can be used as an easy way to execute arbitrary code.

Prevent this by mapping the page read-only for userland, and using
access_process_vm() with the FOLL_FORCE flag to write to it from
mips_dsemul().

This will likely be less efficient due to copy_to_user_page() performing
cache maintenance on a whole page, rather than a single line as in the
previous use of flush_cache_sigtramp(). However this delay slot
emulation code ought not to be running in any performance critical paths
anyway so this isn't really a problem, and we can probably do better in
copy_to_user_page() anyway in future.

A major advantage of this approach is that the fix is small & simple to
backport to stable kernels.

Reported-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Fixes: 432c6bacbd0c ("MIPS: Use per-mm page to execute branch delay slot instructions")"

Without patch:

cat /proc/self/maps
00400000-0047a000 r-xp 00000000 1f:03 1823       /bin/busybox
00489000-0048a000 r-xp 00079000 1f:03 1823       /bin/busybox
0048a000-0048b000 rwxp 0007a000 1f:03 1823       /bin/busybox
77ec8000-77eed000 r-xp 00000000 1f:03 2296       /lib/libgcc_s.so.1
77eed000-77eee000 rwxp 00015000 1f:03 2296       /lib/libgcc_s.so.1
77eee000-77f81000 r-xp 00000000 1f:03 2470       /lib/libc.so
77f90000-77f92000 rwxp 00092000 1f:03 2470       /lib/libc.so
77f92000-77f94000 rwxp 00000000 00:00 0
7f946000-7f967000 rw-p 00000000 00:00 0          [stack]
7fefb000-7fefc000 rwxp 00000000 00:00 0
7ffac000-7ffad000 r--p 00000000 00:00 0          [vvar]
7ffad000-7ffae000 r-xp 00000000 00:00 0          [vdso]

Patch applied:

cat /proc/self/maps
00400000-0047a000 r-xp 00000000 1f:03 1825       /bin/busybox
00489000-0048a000 r-xp 00079000 1f:03 1825       /bin/busybox
0048a000-0048b000 rwxp 0007a000 1f:03 1825       /bin/busybox
77ed0000-77ef5000 r-xp 00000000 1f:03 2298       /lib/libgcc_s.so.1
77ef5000-77ef6000 rwxp 00015000 1f:03 2298       /lib/libgcc_s.so.1
77ef6000-77f89000 r-xp 00000000 1f:03 2474       /lib/libc.so
77f98000-77f9a000 rwxp 00092000 1f:03 2474       /lib/libc.so
77f9a000-77f9c000 rwxp 00000000 00:00 0
7fbed000-7fc0e000 rw-p 00000000 00:00 0          [stack]
7fefb000-7fefc000 r-xp 00000000 00:00 0
7fff6000-7fff7000 r--p 00000000 00:00 0          [vvar]
7fff7000-7fff8000 r-xp 00000000 00:00 0          [vdso]

Note lack of write permission to 7fefb000-7fefc000

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>

1 files changed, 119 insertions, 0 deletions

diff --git a/target/linux/generic/backport-4.9/096-mips-math-emu-Write-protect-delay-slot-emulation-pages.patch b/target/linux/generic/backport-4.9/096-mips-math-emu-Write-protect-delay-slot-emulation-pages.patch
new file mode 100644
index 0000000000..69cc493bba
--- /dev/null
+++ b/target/linux/generic/backport-4.9/096-mips-math-emu-Write-protect-delay-slot-emulation-pages.patch
@@ -0,0 +1,119 @@
+From adcc81f148d733b7e8e641300c5590a2cdc13bf3 Mon Sep 17 00:00:00 2001
+From: Paul Burton <paul.burton@mips.com>
+Date: Thu, 20 Dec 2018 17:45:43 +0000
+Subject: MIPS: math-emu: Write-protect delay slot emulation pages
+
+Mapping the delay slot emulation page as both writeable & executable
+presents a security risk, in that if an exploit can write to & jump into
+the page then it can be used as an easy way to execute arbitrary code.
+
+Prevent this by mapping the page read-only for userland, and using
+access_process_vm() with the FOLL_FORCE flag to write to it from
+mips_dsemul().
+
+This will likely be less efficient due to copy_to_user_page() performing
+cache maintenance on a whole page, rather than a single line as in the
+previous use of flush_cache_sigtramp(). However this delay slot
+emulation code ought not to be running in any performance critical paths
+anyway so this isn't really a problem, and we can probably do better in
+copy_to_user_page() anyway in future.
+
+A major advantage of this approach is that the fix is small & simple to
+backport to stable kernels.
+
+Reported-by: Andy Lutomirski <luto@kernel.org>
+Signed-off-by: Paul Burton <paul.burton@mips.com>
+Fixes: 432c6bacbd0c ("MIPS: Use per-mm page to execute branch delay slot instructions")
+Cc: stable@vger.kernel.org # v4.8+
+Cc: linux-mips@vger.kernel.org
+Cc: linux-kernel@vger.kernel.org
+Cc: Rich Felker <dalias@libc.org>
+Cc: David Daney <david.daney@cavium.com>
+---
+ arch/mips/kernel/vdso.c     |  4 ++--
+ arch/mips/math-emu/dsemul.c | 38 ++++++++++++++++++++------------------
+ 2 files changed, 22 insertions(+), 20 deletions(-)
+
+--- a/arch/mips/kernel/vdso.c
++++ b/arch/mips/kernel/vdso.c
+@@ -111,8 +111,8 @@ int arch_setup_additional_pages(struct l
+ 
+ 	/* Map delay slot emulation page */
+ 	base = mmap_region(NULL, STACK_TOP, PAGE_SIZE,
+-			   VM_READ|VM_WRITE|VM_EXEC|
+-			   VM_MAYREAD|VM_MAYWRITE|VM_MAYEXEC,
++			   VM_READ | VM_EXEC |
++			   VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC,
+ 			   0);
+ 	if (IS_ERR_VALUE(base)) {
+ 		ret = base;
+--- a/arch/mips/math-emu/dsemul.c
++++ b/arch/mips/math-emu/dsemul.c
+@@ -211,8 +211,9 @@ int mips_dsemul(struct pt_regs *regs, mi
+ {
+ 	int isa16 = get_isa16_mode(regs->cp0_epc);
+ 	mips_instruction break_math;
+-	struct emuframe __user *fr;
+-	int err, fr_idx;
++	unsigned long fr_uaddr;
++	struct emuframe fr;
++	int fr_idx, ret;
+ 
+ 	/* NOP is easy */
+ 	if (ir == 0)
+@@ -247,27 +248,31 @@ int mips_dsemul(struct pt_regs *regs, mi
+ 		fr_idx = alloc_emuframe();
+ 	if (fr_idx == BD_EMUFRAME_NONE)
+ 		return SIGBUS;
+-	fr = &dsemul_page()[fr_idx];
+ 
+ 	/* Retrieve the appropriately encoded break instruction */
+ 	break_math = BREAK_MATH(isa16);
+ 
+ 	/* Write the instructions to the frame */
+ 	if (isa16) {
+-		err = __put_user(ir >> 16,
+-				 (u16 __user *)(&fr->emul));
+-		err |= __put_user(ir & 0xffff,
+-				  (u16 __user *)((long)(&fr->emul) + 2));
+-		err |= __put_user(break_math >> 16,
+-				  (u16 __user *)(&fr->badinst));
+-		err |= __put_user(break_math & 0xffff,
+-				  (u16 __user *)((long)(&fr->badinst) + 2));
++		union mips_instruction _emul = {
++			.halfword = { ir >> 16, ir }
++		};
++		union mips_instruction _badinst = {
++			.halfword = { break_math >> 16, break_math }
++		};
++
++		fr.emul = _emul.word;
++		fr.badinst = _badinst.word;
+ 	} else {
+-		err = __put_user(ir, &fr->emul);
+-		err |= __put_user(break_math, &fr->badinst);
++		fr.emul = ir;
++		fr.badinst = break_math;
+ 	}
+ 
+-	if (unlikely(err)) {
++	/* Write the frame to user memory */
++	fr_uaddr = (unsigned long)&dsemul_page()[fr_idx];
++	ret = access_process_vm(current, fr_uaddr, &fr, sizeof(fr),
++				FOLL_FORCE | FOLL_WRITE);
++	if (unlikely(ret != sizeof(fr))) {
+ 		MIPS_FPU_EMU_INC_STATS(errors);
+ 		free_emuframe(fr_idx, current->mm);
+ 		return SIGBUS;
+@@ -279,10 +284,7 @@ int mips_dsemul(struct pt_regs *regs, mi
+ 	atomic_set(&current->thread.bd_emu_frame, fr_idx);
+ 
+ 	/* Change user register context to execute the frame */
+-	regs->cp0_epc = (unsigned long)&fr->emul | isa16;
+-
+-	/* Ensure the icache observes our newly written frame */
+-	flush_cache_sigtramp((unsigned long)&fr->emul);
++	regs->cp0_epc = fr_uaddr | isa16;
+ 
+ 	return 0;
+ }