Date:   Fri, 15 Mar 2019 13:18:26 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     akpm@...ux-foundation.org
Cc:     mhocko@...e.com, anshuman.khandual@....com,
        william.kucharski@...cle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Oscar Salvador <osalvador@...e.de>
Subject: [PATCH] mm: Fix __dump_page when mapping->host is not set

While debugging something, I added a dump_page() into do_swap_page(),
and I got the splat from below.
The issue happens when dereferencing mapping->host in __dump_page():

...
else if (mapping) {
	pr_warn("%ps ", mapping->a_ops);
	if (mapping->host->i_dentry.first) {
		struct dentry *dentry;
		dentry = container_of(mapping->host->i_dentry.first, struct dentry, d_u.d_alias);
		pr_warn("name:\"%pd\" ", dentry);
	}
}
...

Swap address space does not contain an inode information, and so mapping->host
equals NULL.

Although the dump_page() call was added artificially into do_swap_page(),
I am not sure if we can hit this from any other path, so it looks worth
fixing it.
We can easily do that by cheking mapping->host first.

Splat:

kernel: page:ffffea0000630180 count:3 mapcount:0 mapping:0000000000000000 index:0x0
kernel: swap_aops
kernel: BUG: unable to handle kernel NULL pointer dereference at 0000000000000138
kernel: #PF error: [normal kernel read fault]
kernel: PGD 800000001eaea067 P4D 800000001eaea067 PUD 1eae9067 PMD 0
kernel: Oops: 0000 [#1] SMP PTI
kernel: CPU: 0 PID: 1522 Comm: __mremap Tainted: G            E     5.0.0-rc8-mm1-1-default+ #43
kernel: Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.0.0-prebuilt.qemu-project.org 04/01/2014
kernel: RIP: 0010:__dump_page+0x2d6/0x380
kernel: Code: ff 48 c7 c7 4d ac e3 81 31 c0 e8 e9 03 ef ff e9 27 fe ff ff 49 8b 75 70 31 c0 48 c7 c7 55 ac e3 81 e8 d2 03 ef ff 49 8b 45 00 <48> 8b 80 38 01 00 00 48 85 c0 0f 84 01 fe ff ff 48 8d b0 50 ff ff
kernel: RSP: 0000:ffffc900004c3ae0 EFLAGS: 00010296
kernel: RAX: 0000000000000000 RBX: ffffea0000630180 RCX: 0000000000000000
kernel: RDX: 000000000000000a RSI: ffffffff8276700c RDI: 0000000000000246
kernel: RBP: 0000000000000000 R08: ffffffff82767002 R09: 000000000000000a
kernel: R10: 00000000000005f2 R11: 0000000000012047 R12: ffffffff81e3b2d8
kernel: R13: ffff8880184e60a0 R14: ffff888013095100 R15: ffffea0000630180
kernel: FS:  00007f141813d4c0(0000) GS:ffff88801f200000(0000) knlGS:0000000000000000
kernel: CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
kernel: CR2: 0000000000000138 CR3: 000000001eaf2005 CR4: 00000000003606b0
kernel: DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
kernel: DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
kernel: Call Trace:
kernel:  dump_page+0xe/0x20
kernel:  do_swap_page+0x6a0/0xa10
kernel:  __handle_mm_fault+0xa7f/0xc00
kernel:  handle_mm_fault+0xfa/0x210
kernel:  __do_page_fault+0x1f4/0x490
kernel:  do_page_fault+0x32/0x140
kernel:  async_page_fault+0x1e/0x30
kernel: RIP: 0010:copy_user_generic_unrolled+0xa0/0xc0
kernel: Code: 7f 40 ff c9 75 b6 89 d1 83 e2 07 c1 e9 03 74 12 4c 8b 06 4c 89 07 48 8d 76 08 48 8d 7f 08 ff c9 75 ee 21 d2 74 10 89 d1 8a 06 <88> 07 48 ff c6 48 ff c7 ff c9 75 f2 31 c0 0f 01 ca c3 66 66 2e 0f
kernel: RSP: 0018:ffffc900004c3d90 EFLAGS: 00050202
kernel: RAX: 00000000013a620a RBX: 0000000000000001 RCX: 0000000000000001
kernel: RDX: 0000000000000001 RSI: ffffc9000025f07a RDI: 00000000013a6260
kernel: RBP: ffff8880195fa400 R08: ffffc9000025f000 R09: 000000000000001c
kernel: R10: 0000000000000001 R11: 0000000000000fe4 R12: 7fffffffffffffff
kernel: R13: 0000000000000000 R14: 00000000013a6260 R15: 0000000000000000
kernel:  _copy_to_user+0x22/0x30
kernel:  n_tty_read+0x725/0x8d0
kernel:  ? do_wait_intr_irq+0xa0/0xa0
kernel:  tty_read+0x90/0xf0
kernel:  vfs_read+0x89/0x140
kernel:  ksys_read+0x42/0x90
kernel:  do_syscall_64+0x5b/0x180
kernel:  entry_SYSCALL_64_after_hwframe+0x44/0xa9
kernel: RIP: 0033:0x7f1417c53b41
kernel: Code: Bad RIP value.
kernel: RSP: 002b:00007fffa7377d88 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
kernel: RAX: ffffffffffffffda RBX: 00007f1417f1e9e0 RCX: 00007f1417c53b41
kernel: RDX: 0000000000000400 RSI: 00000000013a6260 RDI: 0000000000000000
kernel: RBP: 0000000000000d68 R08: 00007f1417f20880 R09: 00007f141813d4c0
kernel: R10: 000000000000019b R11: 0000000000000246 R12: 00007f1417f1a8e0
kernel: R13: 00007f1417f1b420 R14: 00007f1417f1b420 R15: 0000000000000000
kernel: Modules linked in: parport_pc(E) af_packet(E) xt_tcpudp(E) ipt_REJECT(E) xt_conntrack(E) nf_conntrack(E) nf_defrag_ipv4(E) ip_set(E) nfnetlink(E) ebtable_nat(E) ebtable_broute(E) bridge(E) stp(E) llc(E) iptable_mangle(E) iptable_raw(E) iptable_security(E) ebtable_filter(E) ebtables(E) iptable_filter(E) ip_tables(E) x_tables(E) kvm_intel(E) kvm(E) irqbypass(E) crct10dif_pclmul(E) crc32_pclmul(E) ghash_clmulni_intel(E) aesni_intel(E) bochs_drm(E) aes_x86_64(E) crypto_simd(E) ttm(E) cryptd(E) glue_helper(E) drm_kms_helper(E) virtio_net(E) drm(E) net_failover(E) pcspkr(E) failover(E) syscopyarea(E) sysfillrect(E) sysimgblt(E) fb_sys_fops(E) i2c_piix4(E) parport(E) button(E) btrfs(E) libcrc32c(E) xor(E) zstd_decompress(E) zstd_compress(E) xxhash(E) raid6_pq(E) sd_mod(E) ata_generic(E) ata_piix(E) crc32c_intel(E) serio_raw(E) ahci(E) libahci(E) virtio_pci(E) virtio_ring(E) virtio(E) libata(E) sg(E) scsi_mod(E) autofs4(E)
kernel: CR2: 0000000000000138
kernel: ---[ end trace b061d02f3cb1a1d1 ]---
kernel: RIP: 0010:__dump_page+0x2d6/0x380
kernel: Code: ff 48 c7 c7 4d ac e3 81 31 c0 e8 e9 03 ef ff e9 27 fe ff ff 49 8b 75 70 31 c0 48 c7 c7 55 ac e3 81 e8 d2 03 ef ff 49 8b 45 00 <48> 8b 80 38 01 00 00 48 85 c0 0f 84 01 fe ff ff 48 8d b0 50 ff ff
kernel: RSP: 0000:ffffc900004c3ae0 EFLAGS: 00010296
kernel: RAX: 0000000000000000 RBX: ffffea0000630180 RCX: 0000000000000000
kernel: RDX: 000000000000000a RSI: ffffffff8276700c RDI: 0000000000000246
kernel: RBP: 0000000000000000 R08: ffffffff82767002 R09: 000000000000000a
kernel: R10: 00000000000005f2 R11: 0000000000012047 R12: ffffffff81e3b2d8
kernel: R13: ffff8880184e60a0 R14: ffff888013095100 R15: ffffea0000630180
kernel: FS:  00007f141813d4c0(0000) GS:ffff88801f200000(0000) knlGS:0000000000000000
kernel: CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
kernel: CR2: 00007f1417c53b17 CR3: 000000001eaf2005 CR4: 00000000003606b0
kernel: DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
kernel: DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400

Fixes: 1c6fb1d89e73c ("mm: print more information about mapping in __dump_page")
Signed-off-by: Oscar Salvador <osalvador@...e.de>
---
 mm/debug.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/debug.c b/mm/debug.c
index c0b31b6c3877..7759f12a8fbb 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -79,7 +79,7 @@ void __dump_page(struct page *page, const char *reason)
 		pr_warn("ksm ");
 	else if (mapping) {
 		pr_warn("%ps ", mapping->a_ops);
-		if (mapping->host->i_dentry.first) {
+		if (mapping->host && mapping->host->i_dentry.first) {
 			struct dentry *dentry;
 			dentry = container_of(mapping->host->i_dentry.first, struct dentry, d_u.d_alias);
 			pr_warn("name:\"%pd\" ", dentry);
-- 
2.13.7

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