637241a900
The dmesg_restrict sysctl currently covers the syslog method for access
dmesg, however /dev/kmsg isn't covered by the same protections. Most
people haven't noticed because util-linux dmesg(1) defaults to using the
syslog method for access in older versions. With util-linux dmesg(1)
defaults to reading directly from /dev/kmsg.
To fix /dev/kmsg, let's compare the existing interfaces and what they
allow:
- /proc/kmsg allows:
- open (SYSLOG_ACTION_OPEN) if CAP_SYSLOG since it uses a destructive
single-reader interface (SYSLOG_ACTION_READ).
- everything, after an open.
- syslog syscall allows:
- anything, if CAP_SYSLOG.
- SYSLOG_ACTION_READ_ALL and SYSLOG_ACTION_SIZE_BUFFER, if
dmesg_restrict==0.
- nothing else (EPERM).
The use-cases were:
- dmesg(1) needs to do non-destructive SYSLOG_ACTION_READ_ALLs.
- sysklog(1) needs to open /proc/kmsg, drop privs, and still issue the
destructive SYSLOG_ACTION_READs.
AIUI, dmesg(1) is moving to /dev/kmsg, and systemd-journald doesn't
clear the ring buffer.
Based on the comments in devkmsg_llseek, it sounds like actions besides
reading aren't going to be supported by /dev/kmsg (i.e.
SYSLOG_ACTION_CLEAR), so we have a strict subset of the non-destructive
syslog syscall actions.
To this end, move the check as Josh had done, but also rename the
constants to reflect their new uses (SYSLOG_FROM_CALL becomes
SYSLOG_FROM_READER, and SYSLOG_FROM_FILE becomes SYSLOG_FROM_PROC).
SYSLOG_FROM_READER allows non-destructive actions, and SYSLOG_FROM_PROC
allows destructive actions after a capabilities-constrained
SYSLOG_ACTION_OPEN check.
- /dev/kmsg allows:
- open if CAP_SYSLOG or dmesg_restrict==0
- reading/polling, after open
Addresses https://bugzilla.redhat.com/show_bug.cgi?id=903192
[akpm@linux-foundation.org: use pr_warn_once()]
Signed-off-by: Kees Cook <keescook@chromium.org>
Reported-by: Christian Kujau <lists@nerdbynature.de>
Tested-by: Josh Boyer <jwboyer@redhat.com>
Cc: Kay Sievers <kay@vrfy.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
65 lines
1.5 KiB
C
65 lines
1.5 KiB
C
/*
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* linux/fs/proc/kmsg.c
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*
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* Copyright (C) 1992 by Linus Torvalds
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*
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*/
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#include <linux/types.h>
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#include <linux/errno.h>
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#include <linux/time.h>
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#include <linux/kernel.h>
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#include <linux/poll.h>
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#include <linux/proc_fs.h>
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#include <linux/fs.h>
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#include <linux/syslog.h>
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#include <asm/uaccess.h>
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#include <asm/io.h>
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extern wait_queue_head_t log_wait;
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static int kmsg_open(struct inode * inode, struct file * file)
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{
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return do_syslog(SYSLOG_ACTION_OPEN, NULL, 0, SYSLOG_FROM_PROC);
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}
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static int kmsg_release(struct inode * inode, struct file * file)
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{
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(void) do_syslog(SYSLOG_ACTION_CLOSE, NULL, 0, SYSLOG_FROM_PROC);
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return 0;
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}
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static ssize_t kmsg_read(struct file *file, char __user *buf,
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size_t count, loff_t *ppos)
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{
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if ((file->f_flags & O_NONBLOCK) &&
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!do_syslog(SYSLOG_ACTION_SIZE_UNREAD, NULL, 0, SYSLOG_FROM_PROC))
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return -EAGAIN;
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return do_syslog(SYSLOG_ACTION_READ, buf, count, SYSLOG_FROM_PROC);
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}
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static unsigned int kmsg_poll(struct file *file, poll_table *wait)
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{
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poll_wait(file, &log_wait, wait);
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if (do_syslog(SYSLOG_ACTION_SIZE_UNREAD, NULL, 0, SYSLOG_FROM_PROC))
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return POLLIN | POLLRDNORM;
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return 0;
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}
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static const struct file_operations proc_kmsg_operations = {
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.read = kmsg_read,
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.poll = kmsg_poll,
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.open = kmsg_open,
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.release = kmsg_release,
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.llseek = generic_file_llseek,
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};
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static int __init proc_kmsg_init(void)
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{
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proc_create("kmsg", S_IRUSR, NULL, &proc_kmsg_operations);
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return 0;
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}
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module_init(proc_kmsg_init);
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