diff mbox series

[v2,14/17] resource: Move devmem revoke code to resource framework

Message ID 20201009075934.3509076-15-daniel.vetter@ffwll.ch
State New, archived
Headers show
Series follow_pfn and other iomap races | expand

Commit Message

Daniel Vetter Oct. 9, 2020, 7:59 a.m. UTC
We want all iomem mmaps to consistently revoke ptes when the kernel
takes over and CONFIG_IO_STRICT_DEVMEM is enabled. This includes the
pci bar mmaps available through procfs and sysfs, which currently do
not revoke mappings.

To prepare for this, move the code from the /dev/kmem driver to
kernel/resource.c.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Kees Cook <keescook@chromium.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: linux-mm@kvack.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-samsung-soc@vger.kernel.org
Cc: linux-media@vger.kernel.org
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: David Hildenbrand <david@redhat.com>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
---
 drivers/char/mem.c     | 85 +------------------------------------
 include/linux/ioport.h |  6 +--
 kernel/resource.c      | 95 +++++++++++++++++++++++++++++++++++++++++-
 3 files changed, 96 insertions(+), 90 deletions(-)

Comments

Greg Kroah-Hartman Oct. 9, 2020, 10:59 a.m. UTC | #1
On Fri, Oct 09, 2020 at 09:59:31AM +0200, Daniel Vetter wrote:
> We want all iomem mmaps to consistently revoke ptes when the kernel
> takes over and CONFIG_IO_STRICT_DEVMEM is enabled. This includes the
> pci bar mmaps available through procfs and sysfs, which currently do
> not revoke mappings.
> 
> To prepare for this, move the code from the /dev/kmem driver to
> kernel/resource.c.
> 
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> Cc: Jason Gunthorpe <jgg@ziepe.ca>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: John Hubbard <jhubbard@nvidia.com>
> Cc: Jérôme Glisse <jglisse@redhat.com>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: linux-mm@kvack.org
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-samsung-soc@vger.kernel.org
> Cc: linux-media@vger.kernel.org
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Jason Gunthorpe Oct. 9, 2020, 12:31 p.m. UTC | #2
On Fri, Oct 09, 2020 at 09:59:31AM +0200, Daniel Vetter wrote:

> +struct address_space *iomem_get_mapping(void)
> +{
> +	return iomem_inode->i_mapping;

This should pair an acquire with the release below

> +	/*
> +	 * Publish /dev/mem initialized.
> +	 * Pairs with smp_load_acquire() in revoke_iomem().
> +	 */
> +	smp_store_release(&iomem_inode, inode);

However, this seems abnormal, initcalls rarely do this kind of stuff
with global data..

The kernel crashes if this fs_initcall is raced with
iomem_get_mapping() due to the unconditional dereference, so I think
it can be safely switched to a simple assignment.

Jason
Daniel Vetter Oct. 9, 2020, 2:24 p.m. UTC | #3
On Fri, Oct 9, 2020 at 2:31 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> On Fri, Oct 09, 2020 at 09:59:31AM +0200, Daniel Vetter wrote:
>
> > +struct address_space *iomem_get_mapping(void)
> > +{
> > +     return iomem_inode->i_mapping;
>
> This should pair an acquire with the release below
>
> > +     /*
> > +      * Publish /dev/mem initialized.
> > +      * Pairs with smp_load_acquire() in revoke_iomem().
> > +      */
> > +     smp_store_release(&iomem_inode, inode);
>
> However, this seems abnormal, initcalls rarely do this kind of stuff
> with global data..
>
> The kernel crashes if this fs_initcall is raced with
> iomem_get_mapping() due to the unconditional dereference, so I think
> it can be safely switched to a simple assignment.

Ah yes I checked this all, but forgot to correctly annotate the
iomem_get_mapping access. For reference, see b34e7e298d7a ("/dev/mem:
Add missing memory barriers for devmem_inode").

The reasons for the annotations is that iomem requests can happen
fairly early, way before fs_initcalls happen. That means revoke_iomem
needs to check for that and bail out if we race - nothing bad can
happen since userspace isn't running at this point anyway. And
apparently it needs to be a full acquire fence since we don't just
write a value, but need a barrier for the struct stuff.

Now iomem_get_mapping otoh can only be called after userspace is up &
running, so way after all the fs_initcalls are guaranteed to have
fininshed. Hence we don't really need anything there. But I expect the
kernel race checker thing to complain, plus that then gives me a good
spot to explain why we can't race and don't have to check for a NULL
iomem_inode.

I'll add that in v3.
-Daniel
Jason Gunthorpe Oct. 9, 2020, 2:32 p.m. UTC | #4
On Fri, Oct 09, 2020 at 04:24:45PM +0200, Daniel Vetter wrote:
> On Fri, Oct 9, 2020 at 2:31 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> >
> > On Fri, Oct 09, 2020 at 09:59:31AM +0200, Daniel Vetter wrote:
> >
> > > +struct address_space *iomem_get_mapping(void)
> > > +{
> > > +     return iomem_inode->i_mapping;
> >
> > This should pair an acquire with the release below
> >
> > > +     /*
> > > +      * Publish /dev/mem initialized.
> > > +      * Pairs with smp_load_acquire() in revoke_iomem().
> > > +      */
> > > +     smp_store_release(&iomem_inode, inode);
> >
> > However, this seems abnormal, initcalls rarely do this kind of stuff
> > with global data..
> >
> > The kernel crashes if this fs_initcall is raced with
> > iomem_get_mapping() due to the unconditional dereference, so I think
> > it can be safely switched to a simple assignment.
> 
> Ah yes I checked this all, but forgot to correctly annotate the
> iomem_get_mapping access. For reference, see b34e7e298d7a ("/dev/mem:
> Add missing memory barriers for devmem_inode").

Oh yikes, so revoke_iomem can run concurrently during early boot,
tricky.

> The reasons for the annotations is that iomem requests can happen
> fairly early, way before fs_initcalls happen. That means revoke_iomem
> needs to check for that and bail out if we race - nothing bad can
> happen since userspace isn't running at this point anyway. And
> apparently it needs to be a full acquire fence since we don't just
> write a value, but need a barrier for the struct stuff.

Yes, if that is what is happening it release/acquire is needed.

Jason
Dan Williams Oct. 9, 2020, 6:28 p.m. UTC | #5
On Fri, Oct 9, 2020 at 7:32 AM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> On Fri, Oct 09, 2020 at 04:24:45PM +0200, Daniel Vetter wrote:
> > On Fri, Oct 9, 2020 at 2:31 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> > >
> > > On Fri, Oct 09, 2020 at 09:59:31AM +0200, Daniel Vetter wrote:
> > >
> > > > +struct address_space *iomem_get_mapping(void)
> > > > +{
> > > > +     return iomem_inode->i_mapping;
> > >
> > > This should pair an acquire with the release below
> > >
> > > > +     /*
> > > > +      * Publish /dev/mem initialized.
> > > > +      * Pairs with smp_load_acquire() in revoke_iomem().
> > > > +      */
> > > > +     smp_store_release(&iomem_inode, inode);
> > >
> > > However, this seems abnormal, initcalls rarely do this kind of stuff
> > > with global data..
> > >
> > > The kernel crashes if this fs_initcall is raced with
> > > iomem_get_mapping() due to the unconditional dereference, so I think
> > > it can be safely switched to a simple assignment.
> >
> > Ah yes I checked this all, but forgot to correctly annotate the
> > iomem_get_mapping access. For reference, see b34e7e298d7a ("/dev/mem:
> > Add missing memory barriers for devmem_inode").
>
> Oh yikes, so revoke_iomem can run concurrently during early boot,
> tricky.

It runs early because request_mem_region() can run before fs_initcall.
Rather than add an unnecessary lock just arrange for the revoke to be
skipped before the inode is initialized. The expectation is that any
early resource reservations will block future userspace mapping
attempts.
Jason Gunthorpe Oct. 15, 2020, 12:09 a.m. UTC | #6
On Fri, Oct 09, 2020 at 11:28:54AM -0700, Dan Williams wrote:
> On Fri, Oct 9, 2020 at 7:32 AM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> >
> > On Fri, Oct 09, 2020 at 04:24:45PM +0200, Daniel Vetter wrote:
> > > On Fri, Oct 9, 2020 at 2:31 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> > > >
> > > > On Fri, Oct 09, 2020 at 09:59:31AM +0200, Daniel Vetter wrote:
> > > >
> > > > > +struct address_space *iomem_get_mapping(void)
> > > > > +{
> > > > > +     return iomem_inode->i_mapping;
> > > >
> > > > This should pair an acquire with the release below
> > > >
> > > > > +     /*
> > > > > +      * Publish /dev/mem initialized.
> > > > > +      * Pairs with smp_load_acquire() in revoke_iomem().
> > > > > +      */
> > > > > +     smp_store_release(&iomem_inode, inode);
> > > >
> > > > However, this seems abnormal, initcalls rarely do this kind of stuff
> > > > with global data..
> > > >
> > > > The kernel crashes if this fs_initcall is raced with
> > > > iomem_get_mapping() due to the unconditional dereference, so I think
> > > > it can be safely switched to a simple assignment.
> > >
> > > Ah yes I checked this all, but forgot to correctly annotate the
> > > iomem_get_mapping access. For reference, see b34e7e298d7a ("/dev/mem:
> > > Add missing memory barriers for devmem_inode").
> >
> > Oh yikes, so revoke_iomem can run concurrently during early boot,
> > tricky.
> 
> It runs early because request_mem_region() can run before fs_initcall.
> Rather than add an unnecessary lock just arrange for the revoke to be
> skipped before the inode is initialized. The expectation is that any
> early resource reservations will block future userspace mapping
> attempts.

Actually, on this point a simple WRITE_ONCE/READ_ONCE pairing is OK,
Paul once explained that the pointer chase on the READ_ONCE side is
required to be like an acquire - this is why rcu_dereference is just
READ_ONCE

Jason
Daniel Vetter Oct. 15, 2020, 7:52 a.m. UTC | #7
On Thu, Oct 15, 2020 at 2:09 AM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> On Fri, Oct 09, 2020 at 11:28:54AM -0700, Dan Williams wrote:
> > On Fri, Oct 9, 2020 at 7:32 AM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> > >
> > > On Fri, Oct 09, 2020 at 04:24:45PM +0200, Daniel Vetter wrote:
> > > > On Fri, Oct 9, 2020 at 2:31 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> > > > >
> > > > > On Fri, Oct 09, 2020 at 09:59:31AM +0200, Daniel Vetter wrote:
> > > > >
> > > > > > +struct address_space *iomem_get_mapping(void)
> > > > > > +{
> > > > > > +     return iomem_inode->i_mapping;
> > > > >
> > > > > This should pair an acquire with the release below
> > > > >
> > > > > > +     /*
> > > > > > +      * Publish /dev/mem initialized.
> > > > > > +      * Pairs with smp_load_acquire() in revoke_iomem().
> > > > > > +      */
> > > > > > +     smp_store_release(&iomem_inode, inode);
> > > > >
> > > > > However, this seems abnormal, initcalls rarely do this kind of stuff
> > > > > with global data..
> > > > >
> > > > > The kernel crashes if this fs_initcall is raced with
> > > > > iomem_get_mapping() due to the unconditional dereference, so I think
> > > > > it can be safely switched to a simple assignment.
> > > >
> > > > Ah yes I checked this all, but forgot to correctly annotate the
> > > > iomem_get_mapping access. For reference, see b34e7e298d7a ("/dev/mem:
> > > > Add missing memory barriers for devmem_inode").
> > >
> > > Oh yikes, so revoke_iomem can run concurrently during early boot,
> > > tricky.
> >
> > It runs early because request_mem_region() can run before fs_initcall.
> > Rather than add an unnecessary lock just arrange for the revoke to be
> > skipped before the inode is initialized. The expectation is that any
> > early resource reservations will block future userspace mapping
> > attempts.
>
> Actually, on this point a simple WRITE_ONCE/READ_ONCE pairing is OK,
> Paul once explained that the pointer chase on the READ_ONCE side is
> required to be like an acquire - this is why rcu_dereference is just
> READ_ONCE

Indeed this changed with the sm_read_barrier_depends() removal a year
ago. Before that READ_ONCE and rcu_dereference where not actually the
same. I guess I'll throw a patch on top to switch that over too.
-Daniel




--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
Daniel Vetter Oct. 15, 2020, 7:55 a.m. UTC | #8
On Thu, Oct 15, 2020 at 9:52 AM Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
>
> On Thu, Oct 15, 2020 at 2:09 AM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> >
> > On Fri, Oct 09, 2020 at 11:28:54AM -0700, Dan Williams wrote:
> > > On Fri, Oct 9, 2020 at 7:32 AM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> > > >
> > > > On Fri, Oct 09, 2020 at 04:24:45PM +0200, Daniel Vetter wrote:
> > > > > On Fri, Oct 9, 2020 at 2:31 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> > > > > >
> > > > > > On Fri, Oct 09, 2020 at 09:59:31AM +0200, Daniel Vetter wrote:
> > > > > >
> > > > > > > +struct address_space *iomem_get_mapping(void)
> > > > > > > +{
> > > > > > > +     return iomem_inode->i_mapping;
> > > > > >
> > > > > > This should pair an acquire with the release below
> > > > > >
> > > > > > > +     /*
> > > > > > > +      * Publish /dev/mem initialized.
> > > > > > > +      * Pairs with smp_load_acquire() in revoke_iomem().
> > > > > > > +      */
> > > > > > > +     smp_store_release(&iomem_inode, inode);
> > > > > >
> > > > > > However, this seems abnormal, initcalls rarely do this kind of stuff
> > > > > > with global data..
> > > > > >
> > > > > > The kernel crashes if this fs_initcall is raced with
> > > > > > iomem_get_mapping() due to the unconditional dereference, so I think
> > > > > > it can be safely switched to a simple assignment.
> > > > >
> > > > > Ah yes I checked this all, but forgot to correctly annotate the
> > > > > iomem_get_mapping access. For reference, see b34e7e298d7a ("/dev/mem:
> > > > > Add missing memory barriers for devmem_inode").
> > > >
> > > > Oh yikes, so revoke_iomem can run concurrently during early boot,
> > > > tricky.
> > >
> > > It runs early because request_mem_region() can run before fs_initcall.
> > > Rather than add an unnecessary lock just arrange for the revoke to be
> > > skipped before the inode is initialized. The expectation is that any
> > > early resource reservations will block future userspace mapping
> > > attempts.
> >
> > Actually, on this point a simple WRITE_ONCE/READ_ONCE pairing is OK,
> > Paul once explained that the pointer chase on the READ_ONCE side is
> > required to be like an acquire - this is why rcu_dereference is just
> > READ_ONCE
>
> Indeed this changed with the sm_read_barrier_depends() removal a year
> ago. Before that READ_ONCE and rcu_dereference where not actually the
> same. I guess I'll throw a patch on top to switch that over too.

Actually 2019 landed just the clean-up, the read change landed in 2017 already:

commit 76ebbe78f7390aee075a7f3768af197ded1bdfbb
Author: Will Deacon <will@kernel.org>
Date:   Tue Oct 24 11:22:47 2017 +0100

   locking/barriers: Add implicit smp_read_barrier_depends() to READ_ONCE()

Cheers, Daniel
Daniel Vetter Oct. 15, 2020, 3:29 p.m. UTC | #9
On Thu, Oct 15, 2020 at 2:09 AM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> On Fri, Oct 09, 2020 at 11:28:54AM -0700, Dan Williams wrote:
> > On Fri, Oct 9, 2020 at 7:32 AM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> > >
> > > On Fri, Oct 09, 2020 at 04:24:45PM +0200, Daniel Vetter wrote:
> > > > On Fri, Oct 9, 2020 at 2:31 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> > > > >
> > > > > On Fri, Oct 09, 2020 at 09:59:31AM +0200, Daniel Vetter wrote:
> > > > >
> > > > > > +struct address_space *iomem_get_mapping(void)
> > > > > > +{
> > > > > > +     return iomem_inode->i_mapping;
> > > > >
> > > > > This should pair an acquire with the release below
> > > > >
> > > > > > +     /*
> > > > > > +      * Publish /dev/mem initialized.
> > > > > > +      * Pairs with smp_load_acquire() in revoke_iomem().
> > > > > > +      */
> > > > > > +     smp_store_release(&iomem_inode, inode);
> > > > >
> > > > > However, this seems abnormal, initcalls rarely do this kind of stuff
> > > > > with global data..
> > > > >
> > > > > The kernel crashes if this fs_initcall is raced with
> > > > > iomem_get_mapping() due to the unconditional dereference, so I think
> > > > > it can be safely switched to a simple assignment.
> > > >
> > > > Ah yes I checked this all, but forgot to correctly annotate the
> > > > iomem_get_mapping access. For reference, see b34e7e298d7a ("/dev/mem:
> > > > Add missing memory barriers for devmem_inode").
> > >
> > > Oh yikes, so revoke_iomem can run concurrently during early boot,
> > > tricky.
> >
> > It runs early because request_mem_region() can run before fs_initcall.
> > Rather than add an unnecessary lock just arrange for the revoke to be
> > skipped before the inode is initialized. The expectation is that any
> > early resource reservations will block future userspace mapping
> > attempts.
>
> Actually, on this point a simple WRITE_ONCE/READ_ONCE pairing is OK,
> Paul once explained that the pointer chase on the READ_ONCE side is
> required to be like an acquire - this is why rcu_dereference is just
> READ_ONCE

Hm so WRITE_ONCE doesn't have any barriers, and we'd need that for
updating the pointer. That would leave things rather inconsistent, so
I think I'll just leave it as-is for symmetry reasons. None of this
code matters for performance anyway, so micro-optimizing barriers
seems a bit silly.
-Daniel
diff mbox series

Patch

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 5502f56f3655..53338aad8d28 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -31,9 +31,6 @@ 
 #include <linux/uio.h>
 #include <linux/uaccess.h>
 #include <linux/security.h>
-#include <linux/pseudo_fs.h>
-#include <uapi/linux/magic.h>
-#include <linux/mount.h>
 
 #ifdef CONFIG_IA64
 # include <linux/efi.h>
@@ -809,42 +806,6 @@  static loff_t memory_lseek(struct file *file, loff_t offset, int orig)
 	return ret;
 }
 
-static struct inode *devmem_inode;
-
-#ifdef CONFIG_IO_STRICT_DEVMEM
-void revoke_devmem(struct resource *res)
-{
-	/* pairs with smp_store_release() in devmem_init_inode() */
-	struct inode *inode = smp_load_acquire(&devmem_inode);
-
-	/*
-	 * Check that the initialization has completed. Losing the race
-	 * is ok because it means drivers are claiming resources before
-	 * the fs_initcall level of init and prevent /dev/mem from
-	 * establishing mappings.
-	 */
-	if (!inode)
-		return;
-
-	/*
-	 * The expectation is that the driver has successfully marked
-	 * the resource busy by this point, so devmem_is_allowed()
-	 * should start returning false, however for performance this
-	 * does not iterate the entire resource range.
-	 */
-	if (devmem_is_allowed(PHYS_PFN(res->start)) &&
-	    devmem_is_allowed(PHYS_PFN(res->end))) {
-		/*
-		 * *cringe* iomem=relaxed says "go ahead, what's the
-		 * worst that can happen?"
-		 */
-		return;
-	}
-
-	unmap_mapping_range(inode->i_mapping, res->start, resource_size(res), 1);
-}
-#endif
-
 static int open_port(struct inode *inode, struct file *filp)
 {
 	int rc;
@@ -864,7 +825,7 @@  static int open_port(struct inode *inode, struct file *filp)
 	 * revocations when drivers want to take over a /dev/mem mapped
 	 * range.
 	 */
-	filp->f_mapping = inode->i_mapping;
+	filp->f_mapping = iomem_get_mapping();
 
 	return 0;
 }
@@ -995,48 +956,6 @@  static char *mem_devnode(struct device *dev, umode_t *mode)
 
 static struct class *mem_class;
 
-static int devmem_fs_init_fs_context(struct fs_context *fc)
-{
-	return init_pseudo(fc, DEVMEM_MAGIC) ? 0 : -ENOMEM;
-}
-
-static struct file_system_type devmem_fs_type = {
-	.name		= "devmem",
-	.owner		= THIS_MODULE,
-	.init_fs_context = devmem_fs_init_fs_context,
-	.kill_sb	= kill_anon_super,
-};
-
-static int devmem_init_inode(void)
-{
-	static struct vfsmount *devmem_vfs_mount;
-	static int devmem_fs_cnt;
-	struct inode *inode;
-	int rc;
-
-	rc = simple_pin_fs(&devmem_fs_type, &devmem_vfs_mount, &devmem_fs_cnt);
-	if (rc < 0) {
-		pr_err("Cannot mount /dev/mem pseudo filesystem: %d\n", rc);
-		return rc;
-	}
-
-	inode = alloc_anon_inode(devmem_vfs_mount->mnt_sb);
-	if (IS_ERR(inode)) {
-		rc = PTR_ERR(inode);
-		pr_err("Cannot allocate inode for /dev/mem: %d\n", rc);
-		simple_release_fs(&devmem_vfs_mount, &devmem_fs_cnt);
-		return rc;
-	}
-
-	/*
-	 * Publish /dev/mem initialized.
-	 * Pairs with smp_load_acquire() in revoke_devmem().
-	 */
-	smp_store_release(&devmem_inode, inode);
-
-	return 0;
-}
-
 static int __init chr_dev_init(void)
 {
 	int minor;
@@ -1058,8 +977,6 @@  static int __init chr_dev_init(void)
 		 */
 		if ((minor == DEVPORT_MINOR) && !arch_has_dev_port())
 			continue;
-		if ((minor == DEVMEM_MINOR) && devmem_init_inode() != 0)
-			continue;
 
 		device_create(mem_class, NULL, MKDEV(MEM_MAJOR, minor),
 			      NULL, devlist[minor].name);
diff --git a/include/linux/ioport.h b/include/linux/ioport.h
index 6c2b06fe8beb..8ffb61b36606 100644
--- a/include/linux/ioport.h
+++ b/include/linux/ioport.h
@@ -302,11 +302,7 @@  struct resource *devm_request_free_mem_region(struct device *dev,
 struct resource *request_free_mem_region(struct resource *base,
 		unsigned long size, const char *name);
 
-#ifdef CONFIG_IO_STRICT_DEVMEM
-void revoke_devmem(struct resource *res);
-#else
-static inline void revoke_devmem(struct resource *res) { };
-#endif
+extern struct address_space *iomem_get_mapping(void);
 
 #endif /* __ASSEMBLY__ */
 #endif	/* _LINUX_IOPORT_H */
diff --git a/kernel/resource.c b/kernel/resource.c
index 841737bbda9e..22153fdec4f5 100644
--- a/kernel/resource.c
+++ b/kernel/resource.c
@@ -18,12 +18,15 @@ 
 #include <linux/spinlock.h>
 #include <linux/fs.h>
 #include <linux/proc_fs.h>
+#include <linux/pseudo_fs.h>
 #include <linux/sched.h>
 #include <linux/seq_file.h>
 #include <linux/device.h>
 #include <linux/pfn.h>
 #include <linux/mm.h>
+#include <linux/mount.h>
 #include <linux/resource_ext.h>
+#include <uapi/linux/magic.h>
 #include <asm/io.h>
 
 
@@ -1112,6 +1115,52 @@  resource_size_t resource_alignment(struct resource *res)
 
 static DECLARE_WAIT_QUEUE_HEAD(muxed_resource_wait);
 
+static struct inode *iomem_inode;
+
+#ifdef CONFIG_IO_STRICT_DEVMEM
+static void revoke_iomem(struct resource *res)
+{
+	/* pairs with smp_store_release() in iomem_init_inode() */
+	struct inode *inode = smp_load_acquire(&iomem_inode);
+
+	/*
+	 * Check that the initialization has completed. Losing the race
+	 * is ok because it means drivers are claiming resources before
+	 * the fs_initcall level of init and prevent /dev/mem from
+	 * establishing mappings.
+	 */
+	if (!inode)
+		return;
+
+	/*
+	 * The expectation is that the driver has successfully marked
+	 * the resource busy by this point, so devmem_is_allowed()
+	 * should start returning false, however for performance this
+	 * does not iterate the entire resource range.
+	 */
+	if (devmem_is_allowed(PHYS_PFN(res->start)) &&
+	    devmem_is_allowed(PHYS_PFN(res->end))) {
+		/*
+		 * *cringe* iomem=relaxed says "go ahead, what's the
+		 * worst that can happen?"
+		 */
+		return;
+	}
+
+	unmap_mapping_range(inode->i_mapping, res->start, resource_size(res), 1);
+}
+struct address_space *iomem_get_mapping(void)
+{
+	return iomem_inode->i_mapping;
+}
+#else
+static void revoke_iomem(struct resource *res) {}
+struct address_space *iomem_get_mapping(void)
+{
+	return NULL;
+}
+#endif
+
 /**
  * __request_region - create a new busy resource region
  * @parent: parent resource descriptor
@@ -1179,7 +1228,7 @@  struct resource * __request_region(struct resource *parent,
 	write_unlock(&resource_lock);
 
 	if (res && orig_parent == &iomem_resource)
-		revoke_devmem(res);
+		revoke_iomem(res);
 
 	return res;
 }
@@ -1713,4 +1762,48 @@  static int __init strict_iomem(char *str)
 	return 1;
 }
 
+static int iomem_fs_init_fs_context(struct fs_context *fc)
+{
+	return init_pseudo(fc, DEVMEM_MAGIC) ? 0 : -ENOMEM;
+}
+
+static struct file_system_type iomem_fs_type = {
+	.name		= "iomem",
+	.owner		= THIS_MODULE,
+	.init_fs_context = iomem_fs_init_fs_context,
+	.kill_sb	= kill_anon_super,
+};
+
+static int __init iomem_init_inode(void)
+{
+	static struct vfsmount *iomem_vfs_mount;
+	static int iomem_fs_cnt;
+	struct inode *inode;
+	int rc;
+
+	rc = simple_pin_fs(&iomem_fs_type, &iomem_vfs_mount, &iomem_fs_cnt);
+	if (rc < 0) {
+		pr_err("Cannot mount iomem pseudo filesystem: %d\n", rc);
+		return rc;
+	}
+
+	inode = alloc_anon_inode(iomem_vfs_mount->mnt_sb);
+	if (IS_ERR(inode)) {
+		rc = PTR_ERR(inode);
+		pr_err("Cannot allocate inode for iomem: %d\n", rc);
+		simple_release_fs(&iomem_vfs_mount, &iomem_fs_cnt);
+		return rc;
+	}
+
+	/*
+	 * Publish /dev/mem initialized.
+	 * Pairs with smp_load_acquire() in revoke_iomem().
+	 */
+	smp_store_release(&iomem_inode, inode);
+
+	return 0;
+}
+
+fs_initcall(iomem_init_inode);
+
 __setup("iomem=", strict_iomem);