[PATCH] x86: mm: Check if PUD is large when validating a kerneladdress

From: Mel Gorman
Date: Mon Feb 11 2013 - 09:52:48 EST

A user reported the following oops when a backup process read

BUG: unable to handle kernel paging request at ffffbb00ff33b000
IP: [<ffffffff8103157e>] kern_addr_valid+0xbe/0x110
Oops: 0000 [#1] SMP
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)
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@xxxxxxxxxxxxxxx
Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
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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