linux-kernel - Re: [PATCH] x86: mm: Check if PUD is large when validating a kernel address

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Date:	Tue, 12 Feb 2013 18:43:14 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] x86: mm: Check if PUD is large when validating a kernel
 address

On Mon 11-02-13 14:52:36, Mel Gorman wrote:
> A user reported the following oops when a backup process read
> /proc/kcore.
> 
>  BUG: unable to handle kernel paging request at ffffbb00ff33b000
>  IP: [<ffffffff8103157e>] kern_addr_valid+0xbe/0x110
>  PGD 0
>  Oops: 0000 [#1] SMP
>  CPU 6
>  Modules linked in: af_packet nfs lockd fscache auth_rpcgss nfs_acl sunrpc 8021q garp stp llc cpufreq_conservative cpufreq_userspace cpufreq_powersave acpi_cpufreq mperf microcode fuse nls_iso8859_1 nls_cp437 vfat fat loop dm_mod ioatdma ipv6 ipv6_lib igb dca i7core_edac edac_core i2c_i801 i2c_core cdc_ether usbnet bnx2 mii iTCO_wdt iTCO_vendor_support shpchp rtc_cmos pci_hotplug tpm_tis sg tpm pcspkr tpm_bios serio_raw button ext3 jbd mbcache uhci_hcd ehci_hcd usbcore sd_mod crc_t10dif usb_common processor thermal_sys hwmon scsi_dh_emc scsi_dh_rdac scsi_dh_alua scsi_dh_hp_sw scsi_dh ata_generic ata_piix libata megaraid_sas scsi_mod
> 
>  Pid: 16196, comm: Hibackp Not tainted 3.0.13-0.27-default #1 IBM System x3550 M3 -[7944 K3G]-/94Y7614
>  RIP: 0010:[<ffffffff8103157e>]  [<ffffffff8103157e>] kern_addr_valid+0xbe/0x110
>  RSP: 0018:ffff88094165fe80  EFLAGS: 00010246
>  RAX: 00003300ff33b000 RBX: ffff880100000000 RCX: 0000000000000000
>  RDX: 0000000100000000 RSI: ffff880000000000 RDI: ff32b300ff33b400
>  RBP: 0000000000001000 R08: 00003ffffffff000 R09: 0000000000000000
>  R10: 22302e31223d6e6f R11: 0000000000000246 R12: 0000000000001000
>  R13: 0000000000003000 R14: 0000000000571be0 R15: ffff88094165ff50
>  FS:  00007ff152d33700(0000) GS:ffff88097f2c0000(0000) knlGS:0000000000000000
>  CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
>  CR2: ffffbb00ff33b000 CR3: 00000009405a3000 CR4: 00000000000006e0
>  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>  DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
>  Process Hibackp (pid: 16196, threadinfo ffff88094165e000, task ffff8808eb9ba600)
>  Stack:
>   ffffffff811b8aaa 0000000000004000 ffff880943fea480 ffff8808ef2bae50
>   ffff880943d32980 fffffffffffffffb ffff8808ef2bae40 ffff88094165ff50
>   0000000000004000 000000000056ebe0 ffffffff811ad847 000000000056ebe0
>  Call Trace:
>   [<ffffffff811b8aaa>] read_kcore+0x17a/0x370
>   [<ffffffff811ad847>] proc_reg_read+0x77/0xc0
>   [<ffffffff81151687>] vfs_read+0xc7/0x130
>   [<ffffffff811517f3>] sys_read+0x53/0xa0
>   [<ffffffff81449692>] system_call_fastpath+0x16/0x1b
> 
> Investigation determined that the bug triggered when reading system RAM
> at the 4G mark. On this system, that was the first address using 1G pages
> for the virt->phys direct mapping so the PUD is pointing to a physical
> address, not a PMD page.  The problem is that the page table walker in
> kern_addr_valid() is not checking pud_large() and treats the physical
> address as if it was a PMD.  If it happens to look like pmd_none then it'll
> silently fail, probably returning zeros instead of real data. If the data
> happens to look like a present PMD though, it will be walked resulting in
> the oops above. This patch adds the necessary pud_large() check.
> 
> Unfortunately the problem was not readily reproducible and now they are
> running the backup program without accessing /proc/kcore so the patch has
> not been validated but I think it makes sense. If reviewers agree then it
> should also be included in -stable back as far as 3.0-stable.
> 
> Cc: stable@...r.kernel.org
> Signed-off-by: Mel Gorman <mgorman@...e.de>

Reviewed-by: Michal Hocko <mhocko@...e.cz>

> ---
>  arch/x86/include/asm/pgtable.h |    5 +++++
>  arch/x86/mm/init_64.c          |    3 +++
>  2 files changed, 8 insertions(+)
> 
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index 5199db2..1c1a955 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -142,6 +142,11 @@ static inline unsigned long pmd_pfn(pmd_t pmd)
>  	return (pmd_val(pmd) & PTE_PFN_MASK) >> PAGE_SHIFT;
>  }
>  
> +static inline unsigned long pud_pfn(pud_t pud)
> +{
> +	return (pud_val(pud) & PTE_PFN_MASK) >> PAGE_SHIFT;
> +}
> +
>  #define pte_page(pte)	pfn_to_page(pte_pfn(pte))
>  
>  static inline int pmd_large(pmd_t pte)
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index 2ead3c8..75c9a6a 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -831,6 +831,9 @@ int kern_addr_valid(unsigned long addr)
>  	if (pud_none(*pud))
>  		return 0;
>  
> +	if (pud_large(*pud))
> +		return pfn_valid(pud_pfn(*pud));
> +
>  	pmd = pmd_offset(pud, addr);
>  	if (pmd_none(*pmd))
>  		return 0;
> 
> --
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-- 
Michal Hocko
SUSE Labs
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