Re: [PATCH 1/1] mm: sysctl: add panic_on_inconsistent_mm sysctl

[Qian Cai]

> On Jan 29, 2020, at 1:08 PM, Grzegorz Halat <ghalat@xxxxxxxxxx> wrote:
> 
> Memory management subsystem performs various checks at runtime,
> if an inconsistency is detected then such event is being logged and kernel
> continues to run. While debugging such problems it is helpful to collect
> memory dump as early as possible. Currently, there is no easy way to panic
> kernel when such error is detected.
> 
> It was proposed[1] to panic the kernel if panic_on_oops is set but this
> approach was not accepted. One of alternative proposals was introduction of
> a new sysctl.
> 
> Add a new sysctl - panic_on_inconsistent_mm. If the sysctl is set then the
> kernel will be crashed when an inconsistency is detected by memory
> management. This currently means panic when bad page or bad PTE
> is detected(this may be extended to other places in MM).
> 
> Another use case of this sysctl may be in security-wise environments,
> it may be more desired to crash machine than continue to run with
> potentially damaged data structures.

It is annoying that I have to repeat my feedback, but I donât know why
admins want to enable this by allowing normal users to crash the systems
more easily through recoverable MM bugs where I am sure we have plenty.
How does that improve the security?

If this is mainly for debugging purpose, then debugfs would suite better?

> 
> Changes since v1 [2]:
> - rename the sysctl to panic_on_inconsistent_mm
> - move the sysctl from kernel to vm table
> - print modules in print_bad_pte() only before calling panic
> 
> [1] https://lore.kernel.org/linux-mm/1426495021-6408-1-git-send-email-borntraeger@xxxxxxxxxx/
> [2] https://lore.kernel.org/lkml/20200127101100.92588-1-ghalat@xxxxxxxxxx/
> 
> Signed-off-by: Grzegorz Halat <ghalat@xxxxxxxxxx>
> ---
> Documentation/admin-guide/sysctl/vm.rst | 14 ++++++++++++++
> include/linux/kernel.h                  |  1 +
> kernel/sysctl.c                         |  9 +++++++++
> mm/memory.c                             |  8 ++++++++
> mm/page_alloc.c                         |  4 +++-
> 5 files changed, 35 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
> index 64aeee1009ca..57f7926a64b8 100644
> --- a/Documentation/admin-guide/sysctl/vm.rst
> +++ b/Documentation/admin-guide/sysctl/vm.rst
> @@ -61,6 +61,7 @@ Currently, these files are in /proc/sys/vm:
> - overcommit_memory
> - overcommit_ratio
> - page-cluster
> +- panic_on_inconsistent_mm
> - panic_on_oom
> - percpu_pagelist_fraction
> - stat_interval
> @@ -741,6 +742,19 @@ extra faults and I/O delays for following faults if they would have been part of
> that consecutive pages readahead would have brought in.
> 
> 
> +panic_on_inconsistent_mm
> +========================
> +
> +Controls the kernel's behaviour when inconsistency is detected
> +by memory management code, for example bad page state or bad PTE.
> +
> +0: try to continue operation.
> +
> +1: panic immediately.
> +
> +The default value is 0.
> +
> +
> panic_on_oom
> ============
> 
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 0d9db2a14f44..b3bd94c558ab 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -518,6 +518,7 @@ extern int oops_in_progress;		/* If set, an oops, panic(), BUG() or die() is in
> extern int panic_timeout;
> extern unsigned long panic_print;
> extern int panic_on_oops;
> +extern int panic_on_inconsistent_mm;
> extern int panic_on_unrecovered_nmi;
> extern int panic_on_io_nmi;
> extern int panic_on_warn;
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 70665934d53e..a9733311e3a1 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1303,6 +1303,15 @@ static struct ctl_table vm_table[] = {
> 		.extra1		= SYSCTL_ZERO,
> 		.extra2		= &two,
> 	},
> +	{
> +		.procname	= "panic_on_inconsistent_mm",
> +		.data		= &panic_on_inconsistent_mm,
> +		.maxlen		= sizeof(int),
> +		.mode		= 0644,
> +		.proc_handler	= proc_dointvec_minmax,
> +		.extra1		= SYSCTL_ZERO,
> +		.extra2		= SYSCTL_ONE,
> +	},
> 	{
> 		.procname	= "panic_on_oom",
> 		.data		= &sysctl_panic_on_oom,
> diff --git a/mm/memory.c b/mm/memory.c
> index 45442d9a4f52..b29a18077a6a 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -71,6 +71,7 @@
> #include <linux/dax.h>
> #include <linux/oom.h>
> #include <linux/numa.h>
> +#include <linux/module.h>
> 
> #include <trace/events/kmem.h>
> 
> @@ -88,6 +89,8 @@
> #warning Unfortunate NUMA and NUMA Balancing config, growing page-frame for last_cpupid.
> #endif
> 
> +int panic_on_inconsistent_mm __read_mostly;
> +
> #ifndef CONFIG_NEED_MULTIPLE_NODES
> /* use the per-pgdat data instead for discontigmem - mbligh */
> unsigned long max_mapnr;
> @@ -543,6 +546,11 @@ static void print_bad_pte(struct vm_area_struct *vma, unsigned long addr,
> 		 vma->vm_ops ? vma->vm_ops->fault : NULL,
> 		 vma->vm_file ? vma->vm_file->f_op->mmap : NULL,
> 		 mapping ? mapping->a_ops->readpage : NULL);
> +
> +	if (panic_on_inconsistent_mm) {
> +		print_modules();
> +		panic("Bad page map detected");
> +	}
> 	dump_stack();
> 	add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
> }
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index d047bf7d8fd4..a20cd3ece5ba 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -643,9 +643,11 @@ static void bad_page(struct page *page, const char *reason,
> 	if (bad_flags)
> 		pr_alert("bad because of flags: %#lx(%pGp)\n",
> 						bad_flags, &bad_flags);
> -	dump_page_owner(page);
> 
> +	dump_page_owner(page);
> 	print_modules();
> +	if (panic_on_inconsistent_mm)
> +		panic("Bad page state detected");
> 	dump_stack();
> out:
> 	/* Leave bad fields for debug, except PageBuddy could make trouble */
> -- 
> 2.21.1
>