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

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Feb 11, 2013 at 02:52:36PM +0000, 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@vger.kernel.org
> Signed-off-by: Mel Gorman <mgorman@suse.de>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Agreed also on the backporting to -stable as far as possible.

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