mm/debug.c: do not dereference i_ino blindly

__dump_page() checks i_dentry is fetchable and i_ino is earlier in the
struct than i_ino, so it ought to work fine, but it's possible that struct
randomisation has reordered i_ino after i_dentry and the pointer is just
wild enough that i_dentry is fetchable and i_ino isn't.

Also print the inode number if the dentry is invalid.
Reported-by: NVlastimil Babka <vbabka@suse.cz>
Signed-off-by: NMatthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Reviewed-by: NJohn Hubbard <jhubbard@nvidia.com>
Reviewed-by: NMike Rapoport <rppt@linux.ibm.com>
Link: https://lkml.kernel.org/r/20200819185710.28180-1-willy@infradead.orgSigned-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/debug.c

@@ -120,6 +120,7 @@ void __dump_page(struct page *page, const char *reason)
 		struct hlist_node *dentry_first;
 		struct dentry *dentry_ptr;
 		struct dentry dentry;
+		unsigned long ino;
 
 		/*
 		 * mapping can be invalid pointer and we don't want to crash
@@ -136,21 +137,22 @@ void __dump_page(struct page *page, const char *reason)
 			goto out_mapping;
 		}
 
-		if (get_kernel_nofault(dentry_first, &host->i_dentry.first)) {
+		if (get_kernel_nofault(dentry_first, &host->i_dentry.first) ||
+		    get_kernel_nofault(ino, &host->i_ino)) {
 			pr_warn("aops:%ps with invalid host inode %px\n",
 					a_ops, host);
 			goto out_mapping;
 		}
 
 		if (!dentry_first) {
-			pr_warn("aops:%ps ino:%lx\n", a_ops, host->i_ino);
+			pr_warn("aops:%ps ino:%lx\n", a_ops, ino);
 			goto out_mapping;
 		}
 
 		dentry_ptr = container_of(dentry_first, struct dentry, d_u.d_alias);
 		if (get_kernel_nofault(dentry, dentry_ptr)) {
-			pr_warn("aops:%ps with invalid dentry %px\n", a_ops,
-					dentry_ptr);
+			pr_warn("aops:%ps ino:%lx with invalid dentry %px\n",
+					a_ops, ino, dentry_ptr);
 		} else {
 			/*
 			 * if dentry is corrupted, the %pd handler may still
@@ -158,7 +160,7 @@ void __dump_page(struct page *page, const char *reason)
 			 * corrupted struct page
 			 */
 			pr_warn("aops:%ps ino:%lx dentry name:\"%pd\"\n",
-					a_ops, host->i_ino, &dentry);
+					a_ops, ino, &dentry);
 		}
 	}
 out_mapping: