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Revision bbcd53c960713507ae764bf81970651b5577b95a authored by David Hildenbrand on 07 May 2021, 01:05:55 UTC, committed by Linus Torvalds on 07 May 2021, 07:26:34 UTC 
drivers/char: remove /dev/kmem for good
 
Patch series "drivers/char: remove /dev/kmem for good".

Exploring /dev/kmem and /dev/mem in the context of memory hot(un)plug and
memory ballooning, I started questioning the existence of /dev/kmem.

Comparing it with the /proc/kcore implementation, it does not seem to be
able to deal with things like

a) Pages unmapped from the direct mapping (e.g., to be used by secretmem)
  -> kern_addr_valid(). virt_addr_valid() is not sufficient.

b) Special cases like gart aperture memory that is not to be touched
  -> mem_pfn_is_ram()

Unless I am missing something, it's at least broken in some cases and might
fault/crash the machine.

Looks like its existence has been questioned before in 2005 and 2010 [1],
after ~11 additional years, it might make sense to revive the discussion.

CONFIG_DEVKMEM is only enabled in a single defconfig (on purpose or by
mistake?).  All distributions disable it: in Ubuntu it has been disabled
for more than 10 years, in Debian since 2.6.31, in Fedora at least
starting with FC3, in RHEL starting with RHEL4, in SUSE starting from
15sp2, and OpenSUSE has it disabled as well.

1) /dev/kmem was popular for rootkits [2] before it got disabled
   basically everywhere. Ubuntu documents [3] "There is no modern user of
   /dev/kmem any more beyond attackers using it to load kernel rootkits.".
   RHEL documents in a BZ [5] "it served no practical purpose other than to
   serve as a potential security problem or to enable binary module drivers
   to access structures/functions they shouldn't be touching"

2) /proc/kcore is a decent interface to have a controlled way to read
   kernel memory for debugging puposes. (will need some extensions to
   deal with memory offlining/unplug, memory ballooning, and poisoned
   pages, though)

3) It might be useful for corner case debugging [1]. KDB/KGDB might be a
   better fit, especially, to write random memory; harder to shoot
   yourself into the foot.

4) "Kernel Memory Editor" [4] hasn't seen any updates since 2000 and seems
   to be incompatible with 64bit [1]. For educational purposes,
   /proc/kcore might be used to monitor value updates -- or older
   kernels can be used.

5) It's broken on arm64, and therefore, completely disabled there.

Looks like it's essentially unused and has been replaced by better
suited interfaces for individual tasks (/proc/kcore, KDB/KGDB). Let's
just remove it.

[1] https://lwn.net/Articles/147901/
[2] https://www.linuxjournal.com/article/10505
[3] https://wiki.ubuntu.com/Security/Features#A.2Fdev.2Fkmem_disabled
[4] https://sourceforge.net/projects/kme/
[5] https://bugzilla.redhat.com/show_bug.cgi?id=154796

Link: https://lkml.kernel.org/r/20210324102351.6932-1-david@redhat.com
Link: https://lkml.kernel.org/r/20210324102351.6932-2-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Alexander A. Klimov" <grandmaster@al2klimov.de>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Andrey Zhizhikin <andrey.zhizhikin@leica-geosystems.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Brian Cain <bcain@codeaurora.org>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Chris Zankel <chris@zankel.net>
Cc: Corentin Labbe <clabbe@baylibre.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Cc: Greentime Hu <green.hu@gmail.com>
Cc: Gregory Clement <gregory.clement@bootlin.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Helge Deller <deller@gmx.de>
Cc: Hillf Danton <hdanton@sina.com>
Cc: huang ying <huang.ying.caritas@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>
Cc: James Troup <james.troup@canonical.com>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: Jonas Bonn <jonas@southpole.se>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kairui Song <kasong@redhat.com>
Cc: Krzysztof Kozlowski <krzk@kernel.org>
Cc: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Niklas Schnelle <schnelle@linux.ibm.com>
Cc: Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Cc: openrisc@lists.librecores.org
Cc: Palmer Dabbelt <palmerdabbelt@google.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: "Pavel Machek (CIP)" <pavel@denx.de>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Cc: Pierre Morel <pmorel@linux.ibm.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Rich Felker <dalias@libc.org>
Cc: Robert Richter <rric@kernel.org>
Cc: Rob Herring <robh@kernel.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Cc: sparclinux@vger.kernel.org
Cc: Stafford Horne <shorne@gmail.com>
Cc: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Cc: Theodore Dubois <tblodt@icloud.com>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: William Cohen <wcohen@redhat.com>
Cc: Xiaoming Ni <nixiaoming@huawei.com>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1 parent cb152a1
Raw File
xattr.c 
/*
 * Copyright IBM Corporation, 2010
 * Author Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
 *
 * This program is free software; you can redistribute it and/or modify it
 * under the terms of version 2.1 of the GNU Lesser General Public License
 * as published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it would be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 *
 */

#include <linux/module.h>
#include <linux/fs.h>
#include <linux/sched.h>
#include <linux/uio.h>
#include <net/9p/9p.h>
#include <net/9p/client.h>

#include "fid.h"
#include "xattr.h"

ssize_t v9fs_fid_xattr_get(struct p9_fid *fid, const char *name,
			   void *buffer, size_t buffer_size)
{
	ssize_t retval;
	u64 attr_size;
	struct p9_fid *attr_fid;
	struct kvec kvec = {.iov_base = buffer, .iov_len = buffer_size};
	struct iov_iter to;
	int err;

	iov_iter_kvec(&to, READ, &kvec, 1, buffer_size);

	attr_fid = p9_client_xattrwalk(fid, name, &attr_size);
	if (IS_ERR(attr_fid)) {
		retval = PTR_ERR(attr_fid);
		p9_debug(P9_DEBUG_VFS, "p9_client_attrwalk failed %zd\n",
			 retval);
		return retval;
	}
	if (attr_size > buffer_size) {
		if (!buffer_size) /* request to get the attr_size */
			retval = attr_size;
		else
			retval = -ERANGE;
	} else {
		iov_iter_truncate(&to, attr_size);
		retval = p9_client_read(attr_fid, 0, &to, &err);
		if (err)
			retval = err;
	}
	p9_client_clunk(attr_fid);
	return retval;
}


/*
 * v9fs_xattr_get()
 *
 * Copy an extended attribute into the buffer
 * provided, or compute the buffer size required.
 * Buffer is NULL to compute the size of the buffer required.
 *
 * Returns a negative error number on failure, or the number of bytes
 * used / required on success.
 */
ssize_t v9fs_xattr_get(struct dentry *dentry, const char *name,
		       void *buffer, size_t buffer_size)
{
	struct p9_fid *fid;
	int ret;

	p9_debug(P9_DEBUG_VFS, "name = %s value_len = %zu\n",
		 name, buffer_size);
	fid = v9fs_fid_lookup(dentry);
	if (IS_ERR(fid))
		return PTR_ERR(fid);
	ret = v9fs_fid_xattr_get(fid, name, buffer, buffer_size);
	p9_client_clunk(fid);

	return ret;
}

/*
 * v9fs_xattr_set()
 *
 * Create, replace or remove an extended attribute for this inode. Buffer
 * is NULL to remove an existing extended attribute, and non-NULL to
 * either replace an existing extended attribute, or create a new extended
 * attribute. The flags XATTR_REPLACE and XATTR_CREATE
 * specify that an extended attribute must exist and must not exist
 * previous to the call, respectively.
 *
 * Returns 0, or a negative error number on failure.
 */
int v9fs_xattr_set(struct dentry *dentry, const char *name,
		   const void *value, size_t value_len, int flags)
{
	int ret;
	struct p9_fid *fid;

	fid  = v9fs_fid_lookup(dentry);
	if (IS_ERR(fid))
		return PTR_ERR(fid);
	ret = v9fs_fid_xattr_set(fid, name, value, value_len, flags);
	p9_client_clunk(fid);
	return ret;
}

int v9fs_fid_xattr_set(struct p9_fid *fid, const char *name,
		   const void *value, size_t value_len, int flags)
{
	struct kvec kvec = {.iov_base = (void *)value, .iov_len = value_len};
	struct iov_iter from;
	int retval, err;

	iov_iter_kvec(&from, WRITE, &kvec, 1, value_len);

	p9_debug(P9_DEBUG_VFS, "name = %s value_len = %zu flags = %d\n",
		 name, value_len, flags);

	/* Clone it */
	fid = clone_fid(fid);
	if (IS_ERR(fid))
		return PTR_ERR(fid);

	/*
	 * On success fid points to xattr
	 */
	retval = p9_client_xattrcreate(fid, name, value_len, flags);
	if (retval < 0)
		p9_debug(P9_DEBUG_VFS, "p9_client_xattrcreate failed %d\n",
			 retval);
	else
		p9_client_write(fid, 0, &from, &retval);
	err = p9_client_clunk(fid);
	if (!retval && err)
		retval = err;
	return retval;
}

ssize_t v9fs_listxattr(struct dentry *dentry, char *buffer, size_t buffer_size)
{
	return v9fs_xattr_get(dentry, NULL, buffer, buffer_size);
}

static int v9fs_xattr_handler_get(const struct xattr_handler *handler,
				  struct dentry *dentry, struct inode *inode,
				  const char *name, void *buffer, size_t size)
{
	const char *full_name = xattr_full_name(handler, name);

	return v9fs_xattr_get(dentry, full_name, buffer, size);
}

static int v9fs_xattr_handler_set(const struct xattr_handler *handler,
				  struct user_namespace *mnt_userns,
				  struct dentry *dentry, struct inode *inode,
				  const char *name, const void *value,
				  size_t size, int flags)
{
	const char *full_name = xattr_full_name(handler, name);

	return v9fs_xattr_set(dentry, full_name, value, size, flags);
}

static struct xattr_handler v9fs_xattr_user_handler = {
	.prefix	= XATTR_USER_PREFIX,
	.get	= v9fs_xattr_handler_get,
	.set	= v9fs_xattr_handler_set,
};

static struct xattr_handler v9fs_xattr_trusted_handler = {
	.prefix	= XATTR_TRUSTED_PREFIX,
	.get	= v9fs_xattr_handler_get,
	.set	= v9fs_xattr_handler_set,
};

#ifdef CONFIG_9P_FS_SECURITY
static struct xattr_handler v9fs_xattr_security_handler = {
	.prefix	= XATTR_SECURITY_PREFIX,
	.get	= v9fs_xattr_handler_get,
	.set	= v9fs_xattr_handler_set,
};
#endif

const struct xattr_handler *v9fs_xattr_handlers[] = {
	&v9fs_xattr_user_handler,
	&v9fs_xattr_trusted_handler,
#ifdef CONFIG_9P_FS_POSIX_ACL
	&v9fs_xattr_acl_access_handler,
	&v9fs_xattr_acl_default_handler,
#endif
#ifdef CONFIG_9P_FS_SECURITY
	&v9fs_xattr_security_handler,
#endif
	NULL
};
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