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SubjectRe: [PATCH 1/1] mm: sysctl: add panic_on_inconsistent_mm sysctl
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> On Jan 29, 2020, at 1:08 PM, Grzegorz Halat <ghalat@redhat.com> 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 don’t 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@de.ibm.com/
> [2] https://lore.kernel.org/lkml/20200127101100.92588-1-ghalat@redhat.com/
>
> Signed-off-by: Grzegorz Halat <ghalat@redhat.com>
> ---
> 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
>

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