arm32 insecure W+X mapping

[Russell King (Oracle)]

On Tue, Sep 07, 2021 at 10:48:49AM -0700, Tim Harvey wrote:
> On Fri, Aug 20, 2021 at 11:41 AM Tim Harvey <tharvey at gateworks.com> wrote:
> > # uname -r
> > 5.13.12
> > # cat /proc/cmdline
> > console=ttymxc1,115200 no_hash_pointers
> > # echo 1 > /proc/sys/kernel/kptr_restrict
> > # dmesg | grep insecure
> > [   13.247957] arm/mm: Found insecure W+X mapping at address 0xf087d000
> > # cat /proc/vmallocinfo | grep 0xf087d000
> > 0xf0878000-0xf087d000   20480 of_iomap+0x44/0x68 phys=0x021b0000 ioremap
> > 0xf087d000-0xf087f000    8192 imx6_pm_common_init+0x118/0x36c
> > phys=0x00900000 ioremap
> >
> > Some debugging showed me that 0xf087d000 is 'suspend_ocram_base'
> > remapped from imx6q_suspend_init() (called form imx6_pm_common_init()
> > [1]
> > suspend_ocram_base = __arm_ioremap_exec(ocram_pbase,
> > MX6Q_SUSPEND_OCRAM_SIZE, false);
> >
> > This should be throwing 'Checked W+X mappings: FAILED, 1 W+X pages
> > found' messages for all IMX6 users that have CONFIG_SUSPEND and
> > CONFIG_DEBUG_WX enabled so I'm adding the IMX6 players to the thread
> > to see if they know why this happens.
> >
> 
> Shawn, Fabio and Pengutronix Kernel team,
> 
> Do you know why we get 'Checked W+X mappings: FAILED, 1 W+X pages
> found' messages for IMX6 with CONFIG_SUSPEND and CONFIG_DEBUG_WX
> enabled due to to __arm_ioremap_exec call remapping ocram? [1]

The current situation looks like the OCRAM is used to store some
suspend/resume code (see arch/arm/mach-imx/suspend-imx6.S), along
with some data.

It looks like once the code has been copied and the data has been
written, the mapping is left as-is - it isn't changed to be
read-only-execute. However, I don't think we have any APIs to do
that on iomem.

set_memory_ro() could be leveraged to do it _if_ we are certain
the memory is not mapped using a section mapping, but that would
depend on the size and alignment of the mapping.

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