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

[Matthew Wilcox <willy@infradead.org>]

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);