Date:   Mon, 29 Jul 2019 08:29:50 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Steven Price <steven.price@....com>, linux-mm@...ck.org
Cc:     Andy Lutomirski <luto@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        James Morse <james.morse@....com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Mark Rutland <Mark.Rutland@....com>,
        "Liang, Kan" <kan.liang@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v9 19/21] mm: Add generic ptdump


On 07/22/2019 09:12 PM, Steven Price wrote:
> Add a generic version of page table dumping that architectures can
> opt-in to
> 
> Signed-off-by: Steven Price <steven.price@....com>
> ---
>  include/linux/ptdump.h |  19 +++++
>  mm/Kconfig.debug       |  21 ++++++
>  mm/Makefile            |   1 +
>  mm/ptdump.c            | 161 +++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 202 insertions(+)
>  create mode 100644 include/linux/ptdump.h
>  create mode 100644 mm/ptdump.c
> 
> diff --git a/include/linux/ptdump.h b/include/linux/ptdump.h
> new file mode 100644
> index 000000000000..eb8e78154be3
> --- /dev/null
> +++ b/include/linux/ptdump.h
> @@ -0,0 +1,19 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef _LINUX_PTDUMP_H
> +#define _LINUX_PTDUMP_H
> +
> +struct ptdump_range {
> +	unsigned long start;
> +	unsigned long end;
> +};
> +
> +struct ptdump_state {
> +	void (*note_page)(struct ptdump_state *st, unsigned long addr,
> +			  int level, unsigned long val);
> +	const struct ptdump_range *range;
> +};
> +
> +void ptdump_walk_pgd(struct ptdump_state *st, struct mm_struct *mm);
> +
> +#endif /* _LINUX_PTDUMP_H */
> diff --git a/mm/Kconfig.debug b/mm/Kconfig.debug
> index 82b6a20898bd..7ad939b7140f 100644
> --- a/mm/Kconfig.debug
> +++ b/mm/Kconfig.debug
> @@ -115,3 +115,24 @@ config DEBUG_RODATA_TEST
>      depends on STRICT_KERNEL_RWX
>      ---help---
>        This option enables a testcase for the setting rodata read-only.
> +
> +config GENERIC_PTDUMP
> +	bool
> +
> +config PTDUMP_CORE
> +	bool
> +
> +config PTDUMP_DEBUGFS
> +	bool "Export kernel pagetable layout to userspace via debugfs"
> +	depends on DEBUG_KERNEL
> +	depends on DEBUG_FS
> +	depends on GENERIC_PTDUMP
> +	select PTDUMP_CORE

So PTDUMP_DEBUGFS depends on GENERIC_PTDUMP but selects PTDUMP_CORE. So any arch
subscribing this new generic PTDUMP by selecting GENERIC_PTDUMP needs to provide
some functions for PTDUMP_DEBUGFS which does not really have any code in generic
MM. Also ptdump_walk_pgd() is wrapped in PTDUMP_CORE not GENERIC_PTDUMP. Then what
does PTDUMP_GENERIC really indicate ? Bit confusing here.

The new ptdump_walk_pgd() symbol needs to be wrapped in a config symbol for sure
which should be selected in all platforms wishing to use it. GENERIC_PTDUMP can
be that config.

PTDUMP_DEBUGFS will require a full implementation (i.e PTDUMP_CORE) irrespective
of whether the platform subscribes GENERIC_PTDUMP or not. It should be something
like this.

config PTDUMP_DEBUGFS
	bool "Export kernel pagetable layout to userspace via debugfs"
	depends on DEBUG_KERNEL
	depends on DEBUG_FS
	select PTDUMP_CORE

PTDUMP_DEBUGFS need not depend on GENERIC_PTDUMP. All it requires is a PTDUMP_CORE
implementation which can optionally use ptdump_walk_pgd() through GENERIC_PTDUMP.
s/GENERIC_PTDUMP/PTDUMP_GENERIC to match and group with other configs.

DEBUG_WX can also be moved to generic MM like PTDUMP_DEBUGFS ?

> +	help
> +	  Say Y here if you want to show the kernel pagetable layout in a
> +	  debugfs file. This information is only useful for kernel developers
> +	  who are working in architecture specific areas of the kernel.
> +	  It is probably not a good idea to enable this feature in a production
> +	  kernel.
> +
> +	  If in doubt, say N.
> diff --git a/mm/Makefile b/mm/Makefile
> index 338e528ad436..750a4c12d5da 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -104,3 +104,4 @@ obj-$(CONFIG_HARDENED_USERCOPY) += usercopy.o
>  obj-$(CONFIG_PERCPU_STATS) += percpu-stats.o
>  obj-$(CONFIG_HMM_MIRROR) += hmm.o
>  obj-$(CONFIG_MEMFD_CREATE) += memfd.o
> +obj-$(CONFIG_PTDUMP_CORE) += ptdump.o

Should be GENERIC_PTDUMP instead ?

> diff --git a/mm/ptdump.c b/mm/ptdump.c
> new file mode 100644
> index 000000000000..39befc9088b8
> --- /dev/null
> +++ b/mm/ptdump.c
> @@ -0,0 +1,161 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/mm.h>
> +#include <linux/ptdump.h>
> +#include <linux/kasan.h>
> +
> +static int ptdump_pgd_entry(pgd_t *pgd, unsigned long addr,
> +			    unsigned long next, struct mm_walk *walk)
> +{
> +	struct ptdump_state *st = walk->private;
> +	pgd_t val = READ_ONCE(*pgd);
> +
> +	if (pgd_leaf(val))
> +		st->note_page(st, addr, 1, pgd_val(val));
> +
> +	return 0;
> +}
> +
> +static int ptdump_p4d_entry(p4d_t *p4d, unsigned long addr,
> +			    unsigned long next, struct mm_walk *walk)
> +{
> +	struct ptdump_state *st = walk->private;
> +	p4d_t val = READ_ONCE(*p4d);
> +
> +	if (p4d_leaf(val))
> +		st->note_page(st, addr, 2, p4d_val(val));
> +
> +	return 0;
> +}
> +
> +static int ptdump_pud_entry(pud_t *pud, unsigned long addr,
> +			    unsigned long next, struct mm_walk *walk)
> +{
> +	struct ptdump_state *st = walk->private;
> +	pud_t val = READ_ONCE(*pud);
> +
> +	if (pud_leaf(val))
> +		st->note_page(st, addr, 3, pud_val(val));
> +
> +	return 0;
> +}
> +
> +static int ptdump_pmd_entry(pmd_t *pmd, unsigned long addr,
> +			    unsigned long next, struct mm_walk *walk)
> +{
> +	struct ptdump_state *st = walk->private;
> +	pmd_t val = READ_ONCE(*pmd);
> +
> +	if (pmd_leaf(val))
> +		st->note_page(st, addr, 4, pmd_val(val));
> +
> +	return 0;
> +}
> +
> +static int ptdump_pte_entry(pte_t *pte, unsigned long addr,
> +			    unsigned long next, struct mm_walk *walk)
> +{
> +	struct ptdump_state *st = walk->private;
> +
> +	st->note_page(st, addr, 5, pte_val(READ_ONCE(*pte)));
> +
> +	return 0;
> +}
> +
> +#ifdef CONFIG_KASAN
> +/*
> + * This is an optimization for KASAN=y case. Since all kasan page tables
> + * eventually point to the kasan_early_shadow_page we could call note_page()
> + * right away without walking through lower level page tables. This saves
> + * us dozens of seconds (minutes for 5-level config) while checking for
> + * W+X mapping or reading kernel_page_tables debugfs file.
> + */
> +static inline bool kasan_page_table(struct ptdump_state *st, void *pt,
> +				    unsigned long addr)
> +{
> +	if (__pa(pt) == __pa(kasan_early_shadow_pmd) ||
> +#ifdef CONFIG_X86
> +	    (pgtable_l5_enabled() &&
> +			__pa(pt) == __pa(kasan_early_shadow_p4d)) ||
> +#endif
> +	    __pa(pt) == __pa(kasan_early_shadow_pud)) {
> +		st->note_page(st, addr, 5, pte_val(kasan_early_shadow_pte[0]));
> +		return true;
> +	}
> +	return false;
> +}
> +#else
> +static inline bool kasan_page_table(struct ptdump_state *st, void *pt,
> +				    unsigned long addr)
> +{
> +	return false;
> +}
> +#endif
> +
> +static int ptdump_test_p4d(unsigned long addr, unsigned long next,
> +			   p4d_t *p4d, struct mm_walk *walk)
> +{
> +	struct ptdump_state *st = walk->private;
> +
> +	if (kasan_page_table(st, p4d, addr))
> +		return 1;
> +	return 0;
> +}
> +
> +static int ptdump_test_pud(unsigned long addr, unsigned long next,
> +			   pud_t *pud, struct mm_walk *walk)
> +{
> +	struct ptdump_state *st = walk->private;
> +
> +	if (kasan_page_table(st, pud, addr))
> +		return 1;
> +	return 0;
> +}
> +
> +static int ptdump_test_pmd(unsigned long addr, unsigned long next,
> +			   pmd_t *pmd, struct mm_walk *walk)
> +{
> +	struct ptdump_state *st = walk->private;
> +
> +	if (kasan_page_table(st, pmd, addr))
> +		return 1;
> +	return 0;
> +}
> +
> +static int ptdump_hole(unsigned long addr, unsigned long next,
> +		       struct mm_walk *walk)
> +{
> +	struct ptdump_state *st = walk->private;
> +
> +	st->note_page(st, addr, -1, 0);
> +
> +	return 0;
> +}
> +
> +void ptdump_walk_pgd(struct ptdump_state *st, struct mm_struct *mm)
> +{
> +	struct mm_walk walk = {
> +		.mm		= mm,
> +		.pgd_entry	= ptdump_pgd_entry,
> +		.p4d_entry	= ptdump_p4d_entry,
> +		.pud_entry	= ptdump_pud_entry,
> +		.pmd_entry	= ptdump_pmd_entry,
> +		.pte_entry	= ptdump_pte_entry,
> +		.test_p4d	= ptdump_test_p4d,
> +		.test_pud	= ptdump_test_pud,
> +		.test_pmd	= ptdump_test_pmd,
> +		.pte_hole	= ptdump_hole,
> +		.private	= st
> +	};
> +	const struct ptdump_range *range = st->range;
> +
> +	down_read(&mm->mmap_sem);
> +	while (range->start != range->end) {
> +		walk_page_range(range->start, range->end, &walk);
> +		range++;
> +	}
> +	up_read(&mm->mmap_sem);

Does walk_page_range() really needed here when it is definitely walking a
kernel page table. Why not directly use walk_pgd_range() instead which can
save some cycles avoiding going over VMAs, checking for HugeTLB, taking the
mmap_sem lock etc. AFAICS only thing it will miss is the opportunity to call
walk->test_walk() via walk_page_test(). IIUC test_walk() callback is primarily
for testing a VMA for it's eligibility and for kernel page table now there are
test callbacks like p?d_test() for individual levels anyway.

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