[PATCH V2] fs: perform the check when page without mapping but page->mapping contains junk or random bitscribble

[xiaosong.ma]

perform the check in dump_mapping() to print warning info and avoid crash with invalid non-NULL page->mapping.
For example, a panic with following backtraces show dump_page will show wrong info and panic when the bad page
is non-NULL mapping and page->mapping is 0x80000000000.

    crash_arm64> bt
    PID: 232    TASK: ffffff80e8c2c340  CPU: 0   COMMAND: "Binder:232_2"
     #0 [ffffffc013e5b080] sysdump_panic_event$b2bce43a479f4f7762201bfee02d7889 at ffffffc0108d7c2c
     #1 [ffffffc013e5b0c0] atomic_notifier_call_chain at ffffffc010300228
     #2 [ffffffc013e5b2c0] panic at ffffffc0102c926c
     #3 [ffffffc013e5b370] die at ffffffc010267670
     #4 [ffffffc013e5b3a0] die_kernel_fault at ffffffc0102808a4
     #5 [ffffffc013e5b3d0] __do_kernel_fault at ffffffc010280820
     #6 [ffffffc013e5b410] do_bad_area at ffffffc01028059c
     #7 [ffffffc013e5b440] do_translation_fault$4df5decbea5d08a63349aa36f07426b2 at ffffffc0111149c8
     #8 [ffffffc013e5b470] do_mem_abort at ffffffc0100a4488
     #9 [ffffffc013e5b5e0] el1_ia at ffffffc0100a6c00
     #10 [ffffffc013e5b5f0] __dump_page at ffffffc0104beecc
     #11 [ffffffc013e5b630] bad_page at ffffffc0104e6ffc
     #12 [ffffffc013e5b820] rmqueue_bulk at ffffffc0104e9128
     #13 [ffffffc013e5b950] rmqueue at ffffffc0104e7c3c
     #14 [ffffffc013e5b9c0] get_page_from_freelist at ffffffc0104e3e3c
     #15 [ffffffc013e5ba50] __alloc_pages_nodemask at ffffffc0104e3a7c
     #16 [ffffffc013e5bac0] pagecache_get_page at ffffffc01047d0e4
     #17 [ffffffc013e5bb20] grab_cache_page_write_begin at ffffffc010480e3c
     #18 [ffffffc013e5bb50] block_write_begin at ffffffc010586204
     #19 [ffffffc013e5bb90] blkdev_write_begin$75b353f60767e771433fc3b19ba260ab at ffffffc01058cc48
     #20 [ffffffc013e5bc00] generic_perform_write at ffffffc010480f1c
     #21 [ffffffc013e5bc60] __generic_file_write_iter at ffffffc01048115c
     #22 [ffffffc013e5bcf0] blkdev_write_iter at ffffffc01058c0a8
     #23 [ffffffc013e5bda0] __vfs_write at ffffffc01052d808
     #24 [ffffffc013e5bdd0] vfs_write at ffffffc01052da5c
     #25 [ffffffc013e5be30] __arm64_sys_pwrite64 at ffffffc01052e09c
     #26 [ffffffc013e5be60] el0_svc_common at ffffffc010272224
     #27 [ffffffc013e5bea0] el0_svc_handler at ffffffc010272148
     #28 [ffffffc013e5bff0] el0_svc at ffffffc0100a7ec4

Signed-off-by: xiaosong.ma <Xiaosong.Ma@xxxxxxxxxx>
---
 fs/inode.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/inode.c b/fs/inode.c
index f453eb5..c9021e5 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -564,7 +564,8 @@ void dump_mapping(const struct address_space *mapping)
 	 * If mapping is an invalid pointer, we don't want to crash
 	 * accessing it, so probe everything depending on it carefully.
 	 */
-	if (get_kernel_nofault(host, &mapping->host) ||
+	if (get_kernel_nofault(mapping, &mapping) ||
+	    get_kernel_nofault(host, &mapping->host) ||
 	    get_kernel_nofault(a_ops, &mapping->a_ops)) {
 		pr_warn("invalid mapping:%px\n", mapping);
 		return;
-- 
1.9.1