Re: [PATCH v1] mm: Fix access of uninitialized memmaps in fs/proc/page.c

[Naoya Horiguchi]

Hi David,

On Wed, Oct 09, 2019 at 11:12:04AM +0200, David Hildenbrand wrote:
> There are various places where we access uninitialized memmaps, namely:
> - /proc/kpagecount
> - /proc/kpageflags
> - /proc/kpagecgroup
> - memory_failure() - which reuses stable_page_flags() from fs/proc/page.c

Ah right, memory_failure is another victim of this bug.

> 
> We have initialized memmaps either when the section is online or when
> the page was initialized to the ZONE_DEVICE. Uninitialized memmaps contain
> garbage and in the worst case trigger kernel BUGs, especially with
> CONFIG_PAGE_POISONING.
> 
> For example, not onlining a DIMM during boot and calling /proc/kpagecount
> with CONFIG_PAGE_POISONING:
> :/# cat /proc/kpagecount > tmp.test
> [   95.600592] BUG: unable to handle page fault for address: fffffffffffffffe
> [   95.601238] #PF: supervisor read access in kernel mode
> [   95.601675] #PF: error_code(0x0000) - not-present page
> [   95.602116] PGD 114616067 P4D 114616067 PUD 114618067 PMD 0
> [   95.602596] Oops: 0000 [#1] SMP NOPTI
> [   95.602920] CPU: 0 PID: 469 Comm: cat Not tainted 5.4.0-rc1-next-20191004+ #11
> [   95.603547] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.4
> [   95.604521] RIP: 0010:kpagecount_read+0xce/0x1e0
> [   95.604917] Code: e8 09 83 e0 3f 48 0f a3 02 73 2d 4c 89 e7 48 c1 e7 06 48 03 3d ab 51 01 01 74 1d 48 8b 57 08 480
> [   95.606450] RSP: 0018:ffffa14e409b7e78 EFLAGS: 00010202
> [   95.606904] RAX: fffffffffffffffe RBX: 0000000000020000 RCX: 0000000000000000
> [   95.607519] RDX: 0000000000000001 RSI: 00007f76b5595000 RDI: fffff35645000000
> [   95.608128] RBP: 00007f76b5595000 R08: 0000000000000001 R09: 0000000000000000
> [   95.608731] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000140000
> [   95.609327] R13: 0000000000020000 R14: 00007f76b5595000 R15: ffffa14e409b7f08
> [   95.609924] FS:  00007f76b577d580(0000) GS:ffff8f41bd400000(0000) knlGS:0000000000000000
> [   95.610599] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [   95.611083] CR2: fffffffffffffffe CR3: 0000000078960000 CR4: 00000000000006f0
> [   95.611686] Call Trace:
> [   95.611906]  proc_reg_read+0x3c/0x60
> [   95.612228]  vfs_read+0xc5/0x180
> [   95.612505]  ksys_read+0x68/0xe0
> [   95.612785]  do_syscall_64+0x5c/0xa0
> [   95.613092]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
> Note that there are still two possible races as far as I can see:
> - pfn_to_online_page() succeeding but the memory getting offlined and
>   removed. get_online_mems() could help once we run into this.
> - pfn_zone_device() succeeding but the memmap not being fully
>   initialized yet. As the memmap is initialized outside of the memory
>   hoptlug lock, get_online_mems() can't help.
> 
> Let's keep the existing interfaces working with ZONE_DEVICE memory. We
> can later come back and fix these rare races and eventually speed-up the
> ZONE_DEVICE detection.

Actually, Toshiki is writing code to refactor and optimize the pfn walking
part, where we find the pfn ranges covered by zone devices by running over
xarray pgmap_array and use the range info to reduce pointer dereferences
to speed up pfn walk. I hope he will share it soon.

Thanks,
Naoya Horiguchi

> This patch now also makes sure we don't dump data
> about memory blocks that are already offline again.
> 
> Reported-by: Qian Cai <cai@xxxxxx>
> Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
> Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Toshiki Fukasawa <t-fukasawa@xxxxxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Konstantin Khlebnikov <koct9i@xxxxxxxxx>
> Cc: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx>
> Cc: Anthony Yznaga <anthony.yznaga@xxxxxxxxxx>
> Cc: Jason Gunthorpe <jgg@xxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Logan Gunthorpe <logang@xxxxxxxxxxxx>
> Cc: Ira Weiny <ira.weiny@xxxxxxxxx>
> Cc: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxx>
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>  fs/proc/page.c           | 12 ++++++------
>  include/linux/memremap.h | 11 +++++++++--
>  mm/memory-failure.c      | 22 ++++++++++++++++------
>  mm/memremap.c            | 19 +++++++++++--------
>  4 files changed, 42 insertions(+), 22 deletions(-)
> 
> diff --git a/fs/proc/page.c b/fs/proc/page.c
> index decd3fe39674..76502af461e2 100644
> --- a/fs/proc/page.c
> +++ b/fs/proc/page.c
> @@ -42,7 +42,8 @@ static ssize_t kpagecount_read(struct file *file, char __user *buf,
>  		return -EINVAL;
>  
>  	while (count > 0) {
> -		if (pfn_valid(pfn))
> +		if (pfn_valid(pfn) &&
> +		    (pfn_to_online_page(pfn) || pfn_zone_device(pfn)))
>  			ppage = pfn_to_page(pfn);
>  		else
>  			ppage = NULL;
> @@ -97,9 +98,6 @@ u64 stable_page_flags(struct page *page)
>  	if (!page)
>  		return BIT_ULL(KPF_NOPAGE);
>  
> -	if (pfn_zone_device_reserved(page_to_pfn(page)))
> -		return BIT_ULL(KPF_RESERVED);
> -
>  	k = page->flags;
>  	u = 0;
>  
> @@ -218,7 +216,8 @@ static ssize_t kpageflags_read(struct file *file, char __user *buf,
>  		return -EINVAL;
>  
>  	while (count > 0) {
> -		if (pfn_valid(pfn))
> +		if (pfn_valid(pfn) &&
> +		    (pfn_to_online_page(pfn) || pfn_zone_device(pfn)))
>  			ppage = pfn_to_page(pfn);
>  		else
>  			ppage = NULL;
> @@ -263,7 +262,8 @@ static ssize_t kpagecgroup_read(struct file *file, char __user *buf,
>  		return -EINVAL;
>  
>  	while (count > 0) {
> -		if (pfn_valid(pfn))
> +		if (pfn_valid(pfn) &&
> +		    (pfn_to_online_page(pfn) || pfn_zone_device(pfn)))
>  			ppage = pfn_to_page(pfn);
>  		else
>  			ppage = NULL;
> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> index c676e33205d3..c076bb163c2f 100644
> --- a/include/linux/memremap.h
> +++ b/include/linux/memremap.h
> @@ -123,7 +123,8 @@ static inline struct vmem_altmap *pgmap_altmap(struct dev_pagemap *pgmap)
>  }
>  
>  #ifdef CONFIG_ZONE_DEVICE
> -bool pfn_zone_device_reserved(unsigned long pfn);
> +bool pfn_zone_device(unsigned long pfn);
> +bool __pfn_zone_device(unsigned long pfn, struct dev_pagemap *pgmap);
>  void *memremap_pages(struct dev_pagemap *pgmap, int nid);
>  void memunmap_pages(struct dev_pagemap *pgmap);
>  void *devm_memremap_pages(struct device *dev, struct dev_pagemap *pgmap);
> @@ -134,7 +135,13 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
>  unsigned long vmem_altmap_offset(struct vmem_altmap *altmap);
>  void vmem_altmap_free(struct vmem_altmap *altmap, unsigned long nr_pfns);
>  #else
> -static inline bool pfn_zone_device_reserved(unsigned long pfn)
> +static inline bool pfn_zone_device(unsigned long pfn)
> +{
> +	return false;
> +}
> +
> +static inline bool __pfn_zone_device(unsigned long pfn,
> +				     struct dev_pagemap *pgmap)
>  {
>  	return false;
>  }
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 7ef849da8278..2b4cc6b67720 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1161,6 +1161,14 @@ static int memory_failure_dev_pagemap(unsigned long pfn, int flags,
>  	loff_t start;
>  	dax_entry_t cookie;
>  
> +	/* memmaps of driver reserved memory is not initialized */
> +	if (!__pfn_zone_device(pfn, pgmap)) {
> +		pr_err("Memory failure: %#lx: driver reserved memory\n",
> +			pfn);
> +		rc = -ENXIO;
> +		goto out;
> +	}
> +
>  	/*
>  	 * Prevent the inode from being freed while we are interrogating
>  	 * the address_space, typically this would be handled by
> @@ -1253,17 +1261,19 @@ int memory_failure(unsigned long pfn, int flags)
>  	if (!sysctl_memory_failure_recovery)
>  		panic("Memory failure on page %lx", pfn);
>  
> -	if (!pfn_valid(pfn)) {
> +	p = pfn_to_online_page(pfn);
> +	if (!p) {
> +		if (pfn_valid(pfn)) {
> +			pgmap = get_dev_pagemap(pfn, NULL);
> +			if (pgmap)
> +				return memory_failure_dev_pagemap(pfn, flags,
> +								  pgmap);
> +		}
>  		pr_err("Memory failure: %#lx: memory outside kernel control\n",
>  			pfn);
>  		return -ENXIO;
>  	}
>  
> -	pgmap = get_dev_pagemap(pfn, NULL);
> -	if (pgmap)
> -		return memory_failure_dev_pagemap(pfn, flags, pgmap);
> -
> -	p = pfn_to_page(pfn);
>  	if (PageHuge(p))
>  		return memory_failure_hugetlb(pfn, flags);
>  	if (TestSetPageHWPoison(p)) {
> diff --git a/mm/memremap.c b/mm/memremap.c
> index 7fed8bd32a18..9f3bb223aec7 100644
> --- a/mm/memremap.c
> +++ b/mm/memremap.c
> @@ -73,21 +73,24 @@ static unsigned long pfn_next(unsigned long pfn)
>  	return pfn + 1;
>  }
>  
> +bool __pfn_zone_device(unsigned long pfn, struct dev_pagemap *pgmap)
> +{
> +	return pfn >= pfn_first(pgmap) && pfn <= pfn_end(pgmap);
> +}
> +
>  /*
> - * This returns true if the page is reserved by ZONE_DEVICE driver.
> + * Returns true if the page was initialized to the ZONE_DEVICE (especially,
> + * is not reserved for driver usage).
>   */
> -bool pfn_zone_device_reserved(unsigned long pfn)
> +bool pfn_zone_device(unsigned long pfn)
>  {
>  	struct dev_pagemap *pgmap;
> -	struct vmem_altmap *altmap;
> -	bool ret = false;
> +	bool ret;
>  
>  	pgmap = get_dev_pagemap(pfn, NULL);
>  	if (!pgmap)
> -		return ret;
> -	altmap = pgmap_altmap(pgmap);
> -	if (altmap && pfn < (altmap->base_pfn + altmap->reserve))
> -		ret = true;
> +		return false;
> +	ret = __pfn_zone_device(pfn, pgmap);
>  	put_dev_pagemap(pgmap);
>  
>  	return ret;
> -- 
> 2.21.0
> 
>