linux-kernel - Re: [PATCH] x86: do not overrun page table ranges in gup

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Date:	Tue, 29 Jul 2008 02:18:07 +0200
From:	Johannes Weiner <hannes@...urebad.de>
To:	Alexey Dobriyan <adobriyan@...il.com>
Cc:	akpm@...l.org, torvalds@...l.org, npiggin@...e.de,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: do not overrun page table ranges in gup

Johannes Weiner <hannes@...urebad.de> writes:

> Hi,
>
> Alexey Dobriyan <adobriyan@...il.com> writes:
>
>> On Mon, Jul 28, 2008 at 10:49:47PM +0400, Alexey Dobriyan wrote:
>>> Version: 2.6.26-837b41b5de356aa67abb2cadb5eef3efc7776f91
>>> Core2 Duo, x86_64, 4 GB of RAM.
>>> 
>>> Kernel is "tainted" with ZFS driver, but it can so little, and
>>> probability of screwup is very little too. :-)
>>> 
>>> 
>>> Long LTP session finally ended with
>>> 
>>> BUG: unable to handle kernel paging request at ffff88012b60c000
>>> IP: [<ffffffff80223ff4>] gup_pte_range+0x54/0x120
>>> PGD 202063 PUD a067 PMD 17cedc163 PTE 800000012b60c160
>>> Oops: 0000 [1] PREEMPT SMP DEBUG_PAGEALLOC
>>> CPU 0 
>>> Modules linked in: zfs iptable_raw xt_state iptable_filter ipt_MASQUERADE iptable_nat nf_nat nf_conntrack_ipv4 ip_tables x_tables nf_conntrack_irc nf_conntrack fuse usblp uhci_hcd ehci_hcd usbcore sr_mod cdrom [last unloaded: zfs]
>>> Pid: 16863, comm: vmsplice01 Tainted: G        W 2.6.26-zfs #2
>>> RIP: 0010:[<ffffffff80223ff4>]  [<ffffffff80223ff4>] gup_pte_range+0x54/0x120
>>> RSP: 0018:ffff88012ff57c68  EFLAGS: 00010096
>>> RAX: 0000000000000008 RBX: 00007fff4a800000 RCX: 0000000000000001
>>> RDX: ffffe200040b5f00 RSI: 00007fff4a800310 RDI: ffff88012b60c000
>>> RBP: ffff88012ff57c78 R08: 0000000000000005 R09: ffff88012ff57cec
>>> R10: 0000000000000024 R11: 0000000000000205 R12: ffff88012ff57e58
>>> R13: 00007fff4a807310 R14: 00007fff4a80730f R15: ffff88012ff57e58
>>> FS:  00007fbb4280b6f0(0000) GS:ffffffff805dec40(0000) knlGS:0000000000000000
>>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>> CR2: ffff88012b60c000 CR3: 000000017e294000 CR4: 00000000000006e0
>>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>>> DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
>>> Process vmsplice01 (pid: 16863, threadinfo ffff88012ff56000, task ffff88015f9db360)
>>> Stack:  00007fff4a800000 ffff88010e6cf298 ffff88012ff57d18 ffffffff802243cb
>>>  0000000000000002 ffff88015f9db360 0000000004f23a08 00007fff4a7f7310
>>>  ffff88017d582880 00007fff4a807310 00007fff4a807310 ffff88017e2947f8
>>> Call Trace:
>>>  [<ffffffff802243cb>] get_user_pages_fast+0x1db/0x300
>>>  [<ffffffff802b1bfd>] sys_vmsplice+0x32d/0x420
>>>  [<ffffffff80262acd>] ? unlock_page+0x2d/0x40
>>>  [<ffffffff80275d78>] ? __do_fault+0x1c8/0x450
>>>  [<ffffffff8030e20c>] ? __up_read+0x4c/0xb0
>>>  [<ffffffff802495c6>] ? up_read+0x26/0x30
>>>  [<ffffffff802b0780>] ? spd_release_page+0x0/0x20
>>>  [<ffffffff80463f0d>] ? lockdep_sys_exit_thunk+0x35/0x67
>>>  [<ffffffff8020b65b>] system_call_fastpath+0x16/0x1b
>>
>> Very reproducible and ZFS driver doesn't matter:
>>
>> 	# vmsplice01 from LTP 20080630
>> 	# while true; do ./vmsplice01; done
>
> I think this is the right fix, but my thinking might be buggy, please
> verify.
>
> ---
> From: Johannes Weiner <hannes@...urebad.de>
> Subject: x86: do not overrun page table ranges in gup
>
> Walking the addresses in PAGE_SIZE steps and checking for addr != end
> assumes that the distance between addr and end is a multiple of
> PAGE_SIZE.
>
> This is not garuanteed, though, if the end of this level walk is not the
> total end (for which we know that the distance to the starting address
> is a multiple of PAGE_SIZE) but that of the range the upper level entry
> maps.

Actually, I think the prettier fix would be to just establish that
garuantee:

--- a/arch/x86/mm/gup.c
+++ b/arch/x86/mm/gup.c
@@ -223,7 +223,7 @@ int get_user_pages_fast(unsigned long start, int nr_pages, int write,
 			struct page **pages)
 {
 	struct mm_struct *mm = current->mm;
-	unsigned long end = start + (nr_pages << PAGE_SHIFT);
+	unsigned long end = PAGE_ALIGN(start + (nr_pages << PAGE_SHIFT));
 	unsigned long addr = start;
 	unsigned long next;
 	pgd_t *pgdp;

What do you think, Nick?

	Hannes
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