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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
bfind.c 
// SPDX-License-Identifier: GPL-2.0
/*
 *  linux/fs/hfsplus/bfind.c
 *
 * Copyright (C) 2001
 * Brad Boyer (flar@allandria.com)
 * (C) 2003 Ardis Technologies <roman@ardistech.com>
 *
 * Search routines for btrees
 */

#include <linux/slab.h>
#include "hfsplus_fs.h"

int hfs_find_init(struct hfs_btree *tree, struct hfs_find_data *fd)
{
	void *ptr;

	fd->tree = tree;
	fd->bnode = NULL;
	ptr = kmalloc(tree->max_key_len * 2 + 4, GFP_KERNEL);
	if (!ptr)
		return -ENOMEM;
	fd->search_key = ptr;
	fd->key = ptr + tree->max_key_len + 2;
	hfs_dbg(BNODE_REFS, "find_init: %d (%p)\n",
		tree->cnid, __builtin_return_address(0));
	switch (tree->cnid) {
	case HFSPLUS_CAT_CNID:
		mutex_lock_nested(&tree->tree_lock, CATALOG_BTREE_MUTEX);
		break;
	case HFSPLUS_EXT_CNID:
		mutex_lock_nested(&tree->tree_lock, EXTENTS_BTREE_MUTEX);
		break;
	case HFSPLUS_ATTR_CNID:
		mutex_lock_nested(&tree->tree_lock, ATTR_BTREE_MUTEX);
		break;
	default:
		BUG();
	}
	return 0;
}

void hfs_find_exit(struct hfs_find_data *fd)
{
	hfs_bnode_put(fd->bnode);
	kfree(fd->search_key);
	hfs_dbg(BNODE_REFS, "find_exit: %d (%p)\n",
		fd->tree->cnid, __builtin_return_address(0));
	mutex_unlock(&fd->tree->tree_lock);
	fd->tree = NULL;
}

int hfs_find_1st_rec_by_cnid(struct hfs_bnode *bnode,
				struct hfs_find_data *fd,
				int *begin,
				int *end,
				int *cur_rec)
{
	__be32 cur_cnid;
	__be32 search_cnid;

	if (bnode->tree->cnid == HFSPLUS_EXT_CNID) {
		cur_cnid = fd->key->ext.cnid;
		search_cnid = fd->search_key->ext.cnid;
	} else if (bnode->tree->cnid == HFSPLUS_CAT_CNID) {
		cur_cnid = fd->key->cat.parent;
		search_cnid = fd->search_key->cat.parent;
	} else if (bnode->tree->cnid == HFSPLUS_ATTR_CNID) {
		cur_cnid = fd->key->attr.cnid;
		search_cnid = fd->search_key->attr.cnid;
	} else {
		cur_cnid = 0;	/* used-uninitialized warning */
		search_cnid = 0;
		BUG();
	}

	if (cur_cnid == search_cnid) {
		(*end) = (*cur_rec);
		if ((*begin) == (*end))
			return 1;
	} else {
		if (be32_to_cpu(cur_cnid) < be32_to_cpu(search_cnid))
			(*begin) = (*cur_rec) + 1;
		else
			(*end) = (*cur_rec) - 1;
	}

	return 0;
}

int hfs_find_rec_by_key(struct hfs_bnode *bnode,
				struct hfs_find_data *fd,
				int *begin,
				int *end,
				int *cur_rec)
{
	int cmpval;

	cmpval = bnode->tree->keycmp(fd->key, fd->search_key);
	if (!cmpval) {
		(*end) = (*cur_rec);
		return 1;
	}
	if (cmpval < 0)
		(*begin) = (*cur_rec) + 1;
	else
		*(end) = (*cur_rec) - 1;

	return 0;
}

/* Find the record in bnode that best matches key (not greater than...)*/
int __hfs_brec_find(struct hfs_bnode *bnode, struct hfs_find_data *fd,
					search_strategy_t rec_found)
{
	u16 off, len, keylen;
	int rec;
	int b, e;
	int res;

	BUG_ON(!rec_found);
	b = 0;
	e = bnode->num_recs - 1;
	res = -ENOENT;
	do {
		rec = (e + b) / 2;
		len = hfs_brec_lenoff(bnode, rec, &off);
		keylen = hfs_brec_keylen(bnode, rec);
		if (keylen == 0) {
			res = -EINVAL;
			goto fail;
		}
		hfs_bnode_read(bnode, fd->key, off, keylen);
		if (rec_found(bnode, fd, &b, &e, &rec)) {
			res = 0;
			goto done;
		}
	} while (b <= e);

	if (rec != e && e >= 0) {
		len = hfs_brec_lenoff(bnode, e, &off);
		keylen = hfs_brec_keylen(bnode, e);
		if (keylen == 0) {
			res = -EINVAL;
			goto fail;
		}
		hfs_bnode_read(bnode, fd->key, off, keylen);
	}

done:
	fd->record = e;
	fd->keyoffset = off;
	fd->keylength = keylen;
	fd->entryoffset = off + keylen;
	fd->entrylength = len - keylen;

fail:
	return res;
}

/* Traverse a B*Tree from the root to a leaf finding best fit to key */
/* Return allocated copy of node found, set recnum to best record */
int hfs_brec_find(struct hfs_find_data *fd, search_strategy_t do_key_compare)
{
	struct hfs_btree *tree;
	struct hfs_bnode *bnode;
	u32 nidx, parent;
	__be32 data;
	int height, res;

	tree = fd->tree;
	if (fd->bnode)
		hfs_bnode_put(fd->bnode);
	fd->bnode = NULL;
	nidx = tree->root;
	if (!nidx)
		return -ENOENT;
	height = tree->depth;
	res = 0;
	parent = 0;
	for (;;) {
		bnode = hfs_bnode_find(tree, nidx);
		if (IS_ERR(bnode)) {
			res = PTR_ERR(bnode);
			bnode = NULL;
			break;
		}
		if (bnode->height != height)
			goto invalid;
		if (bnode->type != (--height ? HFS_NODE_INDEX : HFS_NODE_LEAF))
			goto invalid;
		bnode->parent = parent;

		res = __hfs_brec_find(bnode, fd, do_key_compare);
		if (!height)
			break;
		if (fd->record < 0)
			goto release;

		parent = nidx;
		hfs_bnode_read(bnode, &data, fd->entryoffset, 4);
		nidx = be32_to_cpu(data);
		hfs_bnode_put(bnode);
	}
	fd->bnode = bnode;
	return res;

invalid:
	pr_err("inconsistency in B*Tree (%d,%d,%d,%u,%u)\n",
		height, bnode->height, bnode->type, nidx, parent);
	res = -EIO;
release:
	hfs_bnode_put(bnode);
	return res;
}

int hfs_brec_read(struct hfs_find_data *fd, void *rec, int rec_len)
{
	int res;

	res = hfs_brec_find(fd, hfs_find_rec_by_key);
	if (res)
		return res;
	if (fd->entrylength > rec_len)
		return -EINVAL;
	hfs_bnode_read(fd->bnode, rec, fd->entryoffset, fd->entrylength);
	return 0;
}

int hfs_brec_goto(struct hfs_find_data *fd, int cnt)
{
	struct hfs_btree *tree;
	struct hfs_bnode *bnode;
	int idx, res = 0;
	u16 off, len, keylen;

	bnode = fd->bnode;
	tree = bnode->tree;

	if (cnt < 0) {
		cnt = -cnt;
		while (cnt > fd->record) {
			cnt -= fd->record + 1;
			fd->record = bnode->num_recs - 1;
			idx = bnode->prev;
			if (!idx) {
				res = -ENOENT;
				goto out;
			}
			hfs_bnode_put(bnode);
			bnode = hfs_bnode_find(tree, idx);
			if (IS_ERR(bnode)) {
				res = PTR_ERR(bnode);
				bnode = NULL;
				goto out;
			}
		}
		fd->record -= cnt;
	} else {
		while (cnt >= bnode->num_recs - fd->record) {
			cnt -= bnode->num_recs - fd->record;
			fd->record = 0;
			idx = bnode->next;
			if (!idx) {
				res = -ENOENT;
				goto out;
			}
			hfs_bnode_put(bnode);
			bnode = hfs_bnode_find(tree, idx);
			if (IS_ERR(bnode)) {
				res = PTR_ERR(bnode);
				bnode = NULL;
				goto out;
			}
		}
		fd->record += cnt;
	}

	len = hfs_brec_lenoff(bnode, fd->record, &off);
	keylen = hfs_brec_keylen(bnode, fd->record);
	if (keylen == 0) {
		res = -EINVAL;
		goto out;
	}
	fd->keyoffset = off;
	fd->keylength = keylen;
	fd->entryoffset = off + keylen;
	fd->entrylength = len - keylen;
	hfs_bnode_read(bnode, fd->key, off, keylen);
out:
	fd->bnode = bnode;
	return res;
}
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