Re: [PATCH v2 1/2] mm: Don't access uninitialized memmaps in fs/proc/page.c

[Michal Hocko <mhocko@kernel.org>]

On Wed 09-10-19 16:24:34, David Hildenbrand wrote:
> There are three places where we access uninitialized memmaps, namely:
> - /proc/kpagecount
> - /proc/kpageflags
> - /proc/kpagecgroup
> 
> 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
> 
> For now, let's drop support for ZONE_DEVICE from the three pseudo files
> in order to fix this. To distinguish offline memory (with garbage memmap)
> from ZONE_DEVICE memory with properly initialized memmaps, we would have to
> check get_dev_pagemap() and pfn_zone_device_reserved() right now. The usage
> of both (especially, special casing devmem) is frowned upon and needs to
> be reworked. The fundamental issue we have is:
> 
> 	if (pfn_to_online_page(pfn)) {
> 		/* memmap initialized */
> 	} else if (pfn_valid(pfn)) {
> 		/*
> 		 * ???
> 		 * a) offline memory. memmap garbage.
> 		 * b) devmem: memmap initialized to ZONE_DEVICE.
> 		 * c) devmem: reserved for driver. memmap garbage.
> 		 * (d) devmem: memmap currently initializing - garbage)
> 		 */
> 	}
> 
> We'll leave the pfn_zone_device_reserved() check in stable_page_flags()
> in place as that function is also used from memory failure. We now
> no longer dump information about pages that are not in use anymore -
> offline.
> 
> Reported-by: Qian Cai <cai@lca.pw>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Alexey Dobriyan <adobriyan@gmail.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Stephen Rothwell <sfr@canb.auug.org.au>
> Cc: Toshiki Fukasawa <t-fukasawa@vx.jp.nec.com>
> Cc: Pankaj gupta <pagupta@redhat.com>
> Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
> Cc: Anthony Yznaga <anthony.yznaga@oracle.com>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
> Cc: linux-fsdevel@vger.kernel.org
> Signed-off-by: David Hildenbrand <david@redhat.com>
Fixes: f1dd2cd13c4b ("mm, memory_hotplug: do not associate hotadded memory to zones until online") # visible after d0dc12e86b319

usage of pfn_to_online_page is the right way to dereference the pfn
here. My understanding of the zone device internals is limited to see
whether this fixes it as well.

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  fs/proc/page.c | 28 ++++++++++++++++------------
>  1 file changed, 16 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/proc/page.c b/fs/proc/page.c
> index decd3fe39674..e40dbfe1168e 100644
> --- a/fs/proc/page.c
> +++ b/fs/proc/page.c
> @@ -42,10 +42,12 @@ static ssize_t kpagecount_read(struct file *file, char __user *buf,
>  		return -EINVAL;
>  
>  	while (count > 0) {
> -		if (pfn_valid(pfn))
> -			ppage = pfn_to_page(pfn);
> -		else
> -			ppage = NULL;
> +		/*
> +		 * TODO: ZONE_DEVICE support requires to identify
> +		 * memmaps that were actually initialized.
> +		 */
> +		ppage = pfn_to_online_page(pfn);
> +
>  		if (!ppage || PageSlab(ppage) || page_has_type(ppage))
>  			pcount = 0;
>  		else
> @@ -218,10 +220,11 @@ static ssize_t kpageflags_read(struct file *file, char __user *buf,
>  		return -EINVAL;
>  
>  	while (count > 0) {
> -		if (pfn_valid(pfn))
> -			ppage = pfn_to_page(pfn);
> -		else
> -			ppage = NULL;
> +		/*
> +		 * TODO: ZONE_DEVICE support requires to identify
> +		 * memmaps that were actually initialized.
> +		 */
> +		ppage = pfn_to_online_page(pfn);
>  
>  		if (put_user(stable_page_flags(ppage), out)) {
>  			ret = -EFAULT;
> @@ -263,10 +266,11 @@ static ssize_t kpagecgroup_read(struct file *file, char __user *buf,
>  		return -EINVAL;
>  
>  	while (count > 0) {
> -		if (pfn_valid(pfn))
> -			ppage = pfn_to_page(pfn);
> -		else
> -			ppage = NULL;
> +		/*
> +		 * TODO: ZONE_DEVICE support requires to identify
> +		 * memmaps that were actually initialized.
> +		 */
> +		ppage = pfn_to_online_page(pfn);
>  
>  		if (ppage)
>  			ino = page_cgroup_ino(ppage);
> -- 
> 2.21.0

-- 
Michal Hocko
SUSE Labs