[PATCH v2] /dev/mem: Revoke mappings when a driver claims the region

[PATCH v2] /dev/mem: Revoke mappings when a driver claims the region

[Dan Williams]

Close the hole of holding a mapping over kernel driver takeover event of
a given address range.

Commit 90a545e98126 ("restrict /dev/mem to idle io memory ranges")
introduced CONFIG_IO_STRICT_DEVMEM with the goal of protecting the
kernel against scenarios where a /dev/mem user tramples memory that a
kernel driver owns. However, this protection only prevents *new* read(),
write() and mmap() requests. Established mappings prior to the driver
calling request_mem_region() are left alone.

Especially with persistent memory, and the core kernel metadata that is
stored there, there are plentiful scenarios for a /dev/mem user to
violate the expectations of the driver and cause amplified damage.

Teach request_mem_region() to find and shoot down active /dev/mem
mappings that it believes it has successfully claimed for the exclusive
use of the driver. Effectively a driver call to request_mem_region()
becomes a hole-punch on the /dev/mem device.

The typical usage of unmap_mapping_range() is part of
truncate_pagecache() to punch a hole in a file, but in this case the
implementation is only doing the "first half" of a hole punch. Namely it
is just evacuating current established mappings of the "hole", and it
relies on the fact that /dev/mem establishes mappings in terms of
absolute physical address offsets. Once existing mmap users are
invalidated they can attempt to re-establish the mapping, or attempt to
continue issuing read(2) / write(2) to the invalidated extent, but they
will then be subject to the CONFIG_IO_STRICT_DEVMEM checking that can
block those subsequent accesses.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Fixes: 90a545e98126 ("restrict /dev/mem to idle io memory ranges")
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
Changes since v1 [1]:

- updated the changelog to describe the usage of unmap_mapping_range().
  No other logic changes:

[1]: http://lore.kernel.org/r/158662721802.1893045.12301414116114602646.stgit@dwillia2-desk3.amr.corp.intel.com

Greg, Andrew,

I have a regression test for this case now. This was found by an
intermittent data corruption scenario on pmem from a test tool using
/dev/mem.


 drivers/char/mem.c         |  104 +++++++++++++++++++++++++++++++++++++++++++-
 include/linux/ioport.h     |    6 +++
 include/uapi/linux/magic.h |    1 
 kernel/resource.c          |    5 ++
 4 files changed, 114 insertions(+), 2 deletions(-)

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 43dd0891ca1e..3a3b7b37c5d2 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -31,11 +31,15 @@
 #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>
 #endif
 
+#define DEVMEM_MINOR	1
 #define DEVPORT_MINOR	4
 
 static inline unsigned long size_inside_page(unsigned long start,
@@ -805,12 +809,66 @@ 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)
+{
+	struct inode *inode = READ_ONCE(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.
+	 */
+	smp_rmb();
+	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;
+
 	if (!capable(CAP_SYS_RAWIO))
 		return -EPERM;
 
-	return security_locked_down(LOCKDOWN_DEV_MEM);
+	rc = security_locked_down(LOCKDOWN_DEV_MEM);
+	if (rc)
+		return rc;
+
+	if (iminor(inode) != DEVMEM_MINOR)
+		return 0;
+
+	/*
+	 * Use a unified address space to have a single point to manage
+	 * revocations when drivers want to take over a /dev/mem mapped
+	 * range.
+	 */
+	inode->i_mapping = devmem_inode->i_mapping;
+	inode->i_mapping->host = devmem_inode;
+	filp->f_mapping = inode->i_mapping;
+
+	return 0;
 }
 
 #define zero_lseek	null_lseek
@@ -885,7 +943,7 @@ static const struct memdev {
 	fmode_t fmode;
 } devlist[] = {
 #ifdef CONFIG_DEVMEM
-	 [1] = { "mem", 0, &mem_fops, FMODE_UNSIGNED_OFFSET },
+	 [DEVMEM_MINOR] = { "mem", 0, &mem_fops, FMODE_UNSIGNED_OFFSET },
 #endif
 #ifdef CONFIG_DEVKMEM
 	 [2] = { "kmem", 0, &kmem_fops, FMODE_UNSIGNED_OFFSET },
@@ -939,6 +997,46 @@ 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 */
+	WRITE_ONCE(devmem_inode, inode);
+	smp_wmb();
+
+	return 0;
+}
+
 static int __init chr_dev_init(void)
 {
 	int minor;
@@ -960,6 +1058,8 @@ 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 a9b9170b5dd2..6c3eca90cbc4 100644
--- a/include/linux/ioport.h
+++ b/include/linux/ioport.h
@@ -301,5 +301,11 @@ 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
+
 #endif /* __ASSEMBLY__ */
 #endif	/* _LINUX_IOPORT_H */
diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
index d78064007b17..f3956fc11de6 100644
--- a/include/uapi/linux/magic.h
+++ b/include/uapi/linux/magic.h
@@ -94,6 +94,7 @@
 #define BALLOON_KVM_MAGIC	0x13661366
 #define ZSMALLOC_MAGIC		0x58295829
 #define DMA_BUF_MAGIC		0x444d4142	/* "DMAB" */
+#define DEVMEM_MAGIC		0x454d444d	/* "DMEM" */
 #define Z3FOLD_MAGIC		0x33
 #define PPC_CMM_MAGIC		0xc7571590
 
diff --git a/kernel/resource.c b/kernel/resource.c
index 76036a41143b..841737bbda9e 100644
--- a/kernel/resource.c
+++ b/kernel/resource.c
@@ -1126,6 +1126,7 @@ struct resource * __request_region(struct resource *parent,
 {
 	DECLARE_WAITQUEUE(wait, current);
 	struct resource *res = alloc_resource(GFP_KERNEL);
+	struct resource *orig_parent = parent;
 
 	if (!res)
 		return NULL;
@@ -1176,6 +1177,10 @@ struct resource * __request_region(struct resource *parent,
 		break;
 	}
 	write_unlock(&resource_lock);
+
+	if (res && orig_parent == &iomem_resource)
+		revoke_devmem(res);
+
 	return res;
 }
 EXPORT_SYMBOL(__request_region);

Re: [PATCH v2] /dev/mem: Revoke mappings when a driver claims the region

[Greg KH]

On Tue, May 19, 2020 at 12:03:06AM -0700, Dan Williams wrote:
> Close the hole of holding a mapping over kernel driver takeover event of
> a given address range.
> 
> Commit 90a545e98126 ("restrict /dev/mem to idle io memory ranges")
> introduced CONFIG_IO_STRICT_DEVMEM with the goal of protecting the
> kernel against scenarios where a /dev/mem user tramples memory that a
> kernel driver owns. However, this protection only prevents *new* read(),
> write() and mmap() requests. Established mappings prior to the driver
> calling request_mem_region() are left alone.
> 
> Especially with persistent memory, and the core kernel metadata that is
> stored there, there are plentiful scenarios for a /dev/mem user to
> violate the expectations of the driver and cause amplified damage.
> 
> Teach request_mem_region() to find and shoot down active /dev/mem
> mappings that it believes it has successfully claimed for the exclusive
> use of the driver. Effectively a driver call to request_mem_region()
> becomes a hole-punch on the /dev/mem device.
> 
> The typical usage of unmap_mapping_range() is part of
> truncate_pagecache() to punch a hole in a file, but in this case the
> implementation is only doing the "first half" of a hole punch. Namely it
> is just evacuating current established mappings of the "hole", and it
> relies on the fact that /dev/mem establishes mappings in terms of
> absolute physical address offsets. Once existing mmap users are
> invalidated they can attempt to re-establish the mapping, or attempt to
> continue issuing read(2) / write(2) to the invalidated extent, but they
> will then be subject to the CONFIG_IO_STRICT_DEVMEM checking that can
> block those subsequent accesses.
> 
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Russell King <linux@arm.linux.org.uk>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Fixes: 90a545e98126 ("restrict /dev/mem to idle io memory ranges")
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
> Changes since v1 [1]:
> 
> - updated the changelog to describe the usage of unmap_mapping_range().
>   No other logic changes:
> 
> [1]: http://lore.kernel.org/r/158662721802.1893045.12301414116114602646.stgit@dwillia2-desk3.amr.corp.intel.com
> 
> Greg, Andrew,
> 
> I have a regression test for this case now. This was found by an
> intermittent data corruption scenario on pmem from a test tool using
> /dev/mem.

Ick, why are test tools messing around in /dev/mem :)

Anyway, this seems sane to me, want me to take it through my tree?

thanks,

greg k-h

Re: [PATCH v2] /dev/mem: Revoke mappings when a driver claims the region

[Dan Williams]

On Tue, May 19, 2020 at 5:11 AM Greg KH <gregkh@linuxfoundation.org> wrote:
>
> On Tue, May 19, 2020 at 12:03:06AM -0700, Dan Williams wrote:
> > Close the hole of holding a mapping over kernel driver takeover event of
> > a given address range.
> >
> > Commit 90a545e98126 ("restrict /dev/mem to idle io memory ranges")
> > introduced CONFIG_IO_STRICT_DEVMEM with the goal of protecting the
> > kernel against scenarios where a /dev/mem user tramples memory that a
> > kernel driver owns. However, this protection only prevents *new* read(),
> > write() and mmap() requests. Established mappings prior to the driver
> > calling request_mem_region() are left alone.
> >
> > Especially with persistent memory, and the core kernel metadata that is
> > stored there, there are plentiful scenarios for a /dev/mem user to
> > violate the expectations of the driver and cause amplified damage.
> >
> > Teach request_mem_region() to find and shoot down active /dev/mem
> > mappings that it believes it has successfully claimed for the exclusive
> > use of the driver. Effectively a driver call to request_mem_region()
> > becomes a hole-punch on the /dev/mem device.
> >
> > The typical usage of unmap_mapping_range() is part of
> > truncate_pagecache() to punch a hole in a file, but in this case the
> > implementation is only doing the "first half" of a hole punch. Namely it
> > is just evacuating current established mappings of the "hole", and it
> > relies on the fact that /dev/mem establishes mappings in terms of
> > absolute physical address offsets. Once existing mmap users are
> > invalidated they can attempt to re-establish the mapping, or attempt to
> > continue issuing read(2) / write(2) to the invalidated extent, but they
> > will then be subject to the CONFIG_IO_STRICT_DEVMEM checking that can
> > block those subsequent accesses.
> >
> > Cc: Arnd Bergmann <arnd@arndb.de>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: Kees Cook <keescook@chromium.org>
> > Cc: Russell King <linux@arm.linux.org.uk>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Fixes: 90a545e98126 ("restrict /dev/mem to idle io memory ranges")
> > Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> > ---
> > Changes since v1 [1]:
> >
> > - updated the changelog to describe the usage of unmap_mapping_range().
> >   No other logic changes:
> >
> > [1]: http://lore.kernel.org/r/158662721802.1893045.12301414116114602646.stgit@dwillia2-desk3.amr.corp.intel.com
> >
> > Greg, Andrew,
> >
> > I have a regression test for this case now. This was found by an
> > intermittent data corruption scenario on pmem from a test tool using
> > /dev/mem.
>
> Ick, why are test tools messing around in /dev/mem :)

Yeah, I'm all for useful tools, just not at the expense of kernel integrity.

> Anyway, this seems sane to me, want me to take it through my tree?

Yes please, seems to belong with the driver core.

Thanks!

Re: [PATCH v2] /dev/mem: Revoke mappings when a driver claims the region

[Matthew Wilcox]

On Tue, May 19, 2020 at 12:03:06AM -0700, Dan Williams wrote:
> +void revoke_devmem(struct resource *res)
> +{
> +	struct inode *inode = READ_ONCE(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.
> +	 */
> +	smp_rmb();
> +	if (!inode)
> +		return;

Which wmb() is this pairing with?

> +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 */
> +	WRITE_ONCE(devmem_inode, inode);
> +	smp_wmb();
> +
> +	return 0;

... is that this one?  I don't see what it's guarding against.  Surely if
it's needed to ensure that the writes to 'inode' have happened before
the write of the inode pointer, the smp_wmb() needs to be before the
WRITE_ONCE, not after it?

Re: [PATCH v2] /dev/mem: Revoke mappings when a driver claims the region

[Dan Williams]

On Tue, May 19, 2020 at 11:46 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Tue, May 19, 2020 at 12:03:06AM -0700, Dan Williams wrote:
> > +void revoke_devmem(struct resource *res)
> > +{
> > +     struct inode *inode = READ_ONCE(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.
> > +      */
> > +     smp_rmb();
> > +     if (!inode)
> > +             return;
>
> Which wmb() is this pairing with?
>
> > +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 */
> > +     WRITE_ONCE(devmem_inode, inode);
> > +     smp_wmb();
> > +
> > +     return 0;
>
> ... is that this one?  I don't see what it's guarding against.  Surely if
> it's needed to ensure that the writes to 'inode' have happened before
> the write of the inode pointer, the smp_wmb() needs to be before the
> WRITE_ONCE, not after it?

Whoops, yes. Thanks for the catch.

Re: [PATCH v2] /dev/mem: Revoke mappings when a driver claims the region

[Greg KH]

On Tue, May 19, 2020 at 11:27:02AM -0700, Dan Williams wrote:
> On Tue, May 19, 2020 at 5:11 AM Greg KH <gregkh@linuxfoundation.org> wrote:
> >
> > On Tue, May 19, 2020 at 12:03:06AM -0700, Dan Williams wrote:
> > > Close the hole of holding a mapping over kernel driver takeover event of
> > > a given address range.
> > >
> > > Commit 90a545e98126 ("restrict /dev/mem to idle io memory ranges")
> > > introduced CONFIG_IO_STRICT_DEVMEM with the goal of protecting the
> > > kernel against scenarios where a /dev/mem user tramples memory that a
> > > kernel driver owns. However, this protection only prevents *new* read(),
> > > write() and mmap() requests. Established mappings prior to the driver
> > > calling request_mem_region() are left alone.
> > >
> > > Especially with persistent memory, and the core kernel metadata that is
> > > stored there, there are plentiful scenarios for a /dev/mem user to
> > > violate the expectations of the driver and cause amplified damage.
> > >
> > > Teach request_mem_region() to find and shoot down active /dev/mem
> > > mappings that it believes it has successfully claimed for the exclusive
> > > use of the driver. Effectively a driver call to request_mem_region()
> > > becomes a hole-punch on the /dev/mem device.
> > >
> > > The typical usage of unmap_mapping_range() is part of
> > > truncate_pagecache() to punch a hole in a file, but in this case the
> > > implementation is only doing the "first half" of a hole punch. Namely it
> > > is just evacuating current established mappings of the "hole", and it
> > > relies on the fact that /dev/mem establishes mappings in terms of
> > > absolute physical address offsets. Once existing mmap users are
> > > invalidated they can attempt to re-establish the mapping, or attempt to
> > > continue issuing read(2) / write(2) to the invalidated extent, but they
> > > will then be subject to the CONFIG_IO_STRICT_DEVMEM checking that can
> > > block those subsequent accesses.
> > >
> > > Cc: Arnd Bergmann <arnd@arndb.de>
> > > Cc: Ingo Molnar <mingo@redhat.com>
> > > Cc: Kees Cook <keescook@chromium.org>
> > > Cc: Russell King <linux@arm.linux.org.uk>
> > > Cc: Andrew Morton <akpm@linux-foundation.org>
> > > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > > Fixes: 90a545e98126 ("restrict /dev/mem to idle io memory ranges")
> > > Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> > > ---
> > > Changes since v1 [1]:
> > >
> > > - updated the changelog to describe the usage of unmap_mapping_range().
> > >   No other logic changes:
> > >
> > > [1]: http://lore.kernel.org/r/158662721802.1893045.12301414116114602646.stgit@dwillia2-desk3.amr.corp.intel.com
> > >
> > > Greg, Andrew,
> > >
> > > I have a regression test for this case now. This was found by an
> > > intermittent data corruption scenario on pmem from a test tool using
> > > /dev/mem.
> >
> > Ick, why are test tools messing around in /dev/mem :)
> 
> Yeah, I'm all for useful tools, just not at the expense of kernel integrity.
> 
> > Anyway, this seems sane to me, want me to take it through my tree?
> 
> Yes please, seems to belong with the driver core.

Ok, will wait for a v3 to handle the issue that was just found in
review.

thanks,

greg k-h