Re: [Crash-utility] [PATCH] Fix live debugging with lockdown=integrity

Hi Lianbo, On Wed, 10 Nov 2021 14:07:50 +0800 lijiang <lijiang(a)redhat.com&gt; wrote: > Hi, Philipp > Thank you for the fix. > > Date: Tue, 9 Nov 2021 14:52:22 +0100 > > From: Philipp Rudo <prudo(a)redhat.com&gt; > > To: crash-utility(a)redhat.com > > Subject: [Crash-utility] [PATCH] Fix live debugging with > > lockdown=integrity > > Message-ID: <20211109135222.51636-1-prudo(a)redhat.com&gt; > > > > With kernel lockdown the access to kernel interfaces that allow to > > extract confidential information (lockdown=confidentiality) or modify a > > running kernel (lockdown=integrity) can be restricted. Two of the > > interfaces that can be restricted are /dev/mem (integrity & > > confidentiality) and /proc/kcore (confidentiality). With > > lockdown=integrity this leads to a situation where /dev/mem exists but > > is not readable while /proc/kcore exists and is readable. This breaks > > crash's live debugging when it is invoked without argument, i.e. > > > > $ crash > > [...] > > crash: /dev/mem: Operation not permitted > > > > while passing /proc/kcore as image succeeds. The reason for this is that > > crash always picks /dev/mem as source when it exits but doesn't check if > > it is readable. Fix this by only selecting /dev/mem when it is readable. > > > > Signed-off-by: Philipp Rudo <prudo(a)redhat.com&gt; > > --- > > filesys.c | 2 +- > > main.c | 2 +- > > 2 files changed, 2 insertions(+), 2 deletions(-) > > > > diff --git a/filesys.c b/filesys.c > > index 3361b6c..43cbe82 100644 > > --- a/filesys.c > > +++ b/filesys.c > > @@ -3666,7 +3666,7 @@ get_live_memory_source(void) > > if (pc->live_memsrc) > > goto live_report; > > > > - if (file_exists("/dev/mem", NULL)) > > + if (file_readable("/dev/mem")) > > pc->live_memsrc = "/dev/mem"; > > else if (file_exists("/proc/kcore", NULL)) { > > pc->flags &= ~DEVMEM; > > diff --git a/main.c b/main.c > > index 71c59d2..b278c22 100644 > > --- a/main.c > > +++ b/main.c > > @@ -1119,7 +1119,7 @@ setup_environment(int argc, char **argv) > > pc->flags2 |= REDZONE; > > pc->confd = -2; > > pc->machine_type = MACHINE_TYPE; > > - if (file_exists("/dev/mem", NULL)) { /* defaults until argv[] > > is parsed */ > > + if (file_readable("/dev/mem")) { /* defaults until argv[] is > > parsed */ > > pc->readmem = read_dev_mem; > > pc->writemem = write_dev_mem; > > } else if (file_exists("/proc/kcore", NULL)) { > > -- > > 2.31.1 > > > > After applying this patch, it works, but redundant information is > displayed in the crash prompt as below. I marked it twice, is that expected? > > [root@testvm crash]# ./crash > [69580.039885] Lockdown: crash: /dev/mem,kmem,port is restricted; see man > kernel_lockdown.7 > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > > crash 7.3.0++ > Copyright (C) 2002-2021 Red Hat, Inc. > Copyright (C) 2004, 2005, 2006, 2010 IBM Corporation > Copyright (C) 1999-2006 Hewlett-Packard Co > Copyright (C) 2005, 2006, 2011, 2012 Fujitsu Limited > Copyright (C) 2006, 2007 VA Linux Systems Japan K.K. > Copyright (C) 2005, 2011, 2020-2021 NEC Corporation > Copyright (C) 1999, 2002, 2007 Silicon Graphics, Inc. > Copyright (C) 1999, 2000, 2001, 2002 Mission Critical Linux, Inc. > Copyright (C) 2015, 2021 VMware, Inc. > This program is free software, covered by the GNU General Public License, > and you are welcome to change it and/or distribute copies of it under > certain conditions. Enter "help copying" to see the conditions. > This program has absolutely no warranty. Enter "help warranty" for details. > > [69580.662388] Lockdown: crash: /dev/mem,kmem,port is restricted; see man > kernel_lockdown.7 > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > GNU gdb (GDB) 10.2 > ... > crash> I assume you are using a serial console as I cannot see the messages when connecting via ssh. They do appear in dmesg though.

The message is emitted by the kernel every time someone opens /dev/mem (or one of the other two files). This is done in file_readable as it checks if a file can be read from by actually opening and reading from it. Unfortunately I don't see a way around it. At least stat still shows /dev/mem as readable # stat /dev/mem [...] Access: (0640/crw-r-----) Uid: ( 0/ root) Gid: ( 9/ kmem) [...] So yes, seeing those messages is expected.