Re: [PATCH v2] fs/proc/page: Skip uninitialized page when iterating page structures

[Waiman Long <longman@redhat.com>]

On 8/26/19 9:25 AM, Matthew Wilcox wrote:
> On Mon, Aug 26, 2019 at 08:43:36AM -0400, Waiman Long wrote:
>> It was found that on a dual-socket x86-64 system with nvdimm, reading
>> /proc/kpagecount may cause the system to panic:
>>
>> ===================
>> [   79.917682] BUG: unable to handle page fault for address: fffffffffffffffe
>> [   79.924558] #PF: supervisor read access in kernel mode
>> [   79.929696] #PF: error_code(0x0000) - not-present page
>> [   79.934834] PGD 87b60d067 P4D 87b60d067 PUD 87b60f067 PMD 0
>> [   79.940494] Oops: 0000 [#1] SMP NOPTI
>> [   79.944157] CPU: 89 PID: 3455 Comm: cp Not tainted 5.3.0-rc5-test+ #14
>> [   79.950682] Hardware name: Dell Inc. PowerEdge R740/07X9K0, BIOS 2.2.11 06/13/2019
>> [   79.958246] RIP: 0010:kpagecount_read+0xdb/0x1a0
>> [   79.962859] Code: e8 09 83 e0 3f 48 0f a3 02 73 2d 4c 89 f7 48 c1 e7 06 48 03 3d fe da de 00 74 1d 48 8b 57 08 48 8d 42 ff 83 e2 01 48 0f 44 c7 <48> 8b 00 f6 c4 02 75 06 83 7f 30 80 7d 62 31 c0 4c 89 f9 e8 5d c9
>> [   79.981603] RSP: 0018:ffffb0d9c950fe70 EFLAGS: 00010202
>> [   79.986830] RAX: fffffffffffffffe RBX: ffff8beebe5383c0 RCX: ffffb0d9c950ff00
>> [   79.993963] RDX: 0000000000000001 RSI: 00007fd85b29e000 RDI: ffffe77a22000000
>> [   80.001095] RBP: 0000000000020000 R08: 0000000000000001 R09: 0000000000000000
>> [   80.008226] R10: 0000000000000000 R11: 0000000000000001 R12: 00007fd85b29e000
>> [   80.015358] R13: ffffffff893f0480 R14: 0000000000880000 R15: 00007fd85b29e000
>> [   80.022491] FS:  00007fd85b312800(0000) GS:ffff8c359fb00000(0000) knlGS:0000000000000000
>> [   80.030576] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> [   80.036321] CR2: fffffffffffffffe CR3: 0000004f54a38001 CR4: 00000000007606e0
>> [   80.043455] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>> [   80.050586] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>> [   80.057718] PKRU: 55555554
>> [   80.060428] Call Trace:
>> [   80.062877]  proc_reg_read+0x39/0x60
>> [   80.066459]  vfs_read+0x91/0x140
>> [   80.069686]  ksys_read+0x59/0xd0
>> [   80.072922]  do_syscall_64+0x59/0x1e0
>> [   80.076588]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>> [   80.081637] RIP: 0033:0x7fd85a7f5d75
>> ===================
>>
>> It turns out the panic was caused by the kpagecount_read() function
>> hitting an uninitialized page structure at PFN 0x880000 where all its
>> fields were set to -1. The compound_head value of -1 will mislead the
>> kernel to treat -2 as a pointer to the head page of the compound page
>> leading to the crash.
>>
>> The system have 12 GB of nvdimm ranging from PFN 0x880000-0xb7ffff.
>> However, only PFN 0x88c200-0xb7ffff are released by the nvdimm
>> driver to the kernel and initialized. IOW, PFN 0x880000-0x88c1ff
>> remain uninitialized. Perhaps these 196 MB of nvdimm are reserved for
>> internal use.
>>
>> To fix the panic, we need to find out if a page structure has been
>> initialized. This is done now by checking if the PFN is in the range
>> of a memory zone assuming that pages in a zone is either correctly
>> marked as not present in the mem_section structure or have their page
>> structures initialized.
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>  fs/proc/page.c | 68 +++++++++++++++++++++++++++++++++++++++++++++++---
>>  1 file changed, 65 insertions(+), 3 deletions(-)
> Would this not work equally well?
>
> +++ b/fs/proc/page.c
> @@ -46,7 +46,8 @@ static ssize_t kpagecount_read(struct file *file, char __user *buf,
>                         ppage = pfn_to_page(pfn);
>                 else
>                         ppage = NULL;
> -               if (!ppage || PageSlab(ppage) || page_has_type(ppage))
> +               if (!ppage || PageSlab(ppage) || page_has_type(ppage) ||
> +                               PagePoisoned(ppage))
>                         pcount = 0;
>                 else
>                         pcount = page_mapcount(ppage);
>
That is my initial thought too. However, I couldn't find out where the
memory of the uninitialized page structures may have been initialized
somehow. The only thing I found is when vm_debug is on that the page
structures are indeed poisoned. Without that it is probably just
whatever the content that the memory have when booting up the kernel.

It just happens on the test system that I used the memory of those page
structures turned out to be -1. It may be different in other systems
that can still crash the kernel, but not detected by the PagePoisoned()
check. That is why I settle on the current scheme which is more general
and don't rely on the memory get initialized in a certain way.

Cheers,
Longman