mm: khugepaged: fix kernel BUG in hpage_collapse_scan_file()

Syzkaller reported the following issue:

kernel BUG at mm/khugepaged.c:1823!
invalid opcode: 0000 [#1] PREEMPT SMP KASAN
CPU: 1 PID: 5097 Comm: syz-executor220 Not tainted 6.2.0-syzkaller-13154-g857f1268a591 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/16/2023
RIP: 0010:collapse_file mm/khugepaged.c:1823 [inline]
RIP: 0010:hpage_collapse_scan_file+0x67c8/0x7580 mm/khugepaged.c:2233
Code: 00 00 89 de e8 c9 66 a3 ff 31 ff 89 de e8 c0 66 a3 ff 45 84 f6 0f 85 28 0d 00 00 e8 22 64 a3 ff e9 dc f7 ff ff e8 18 64 a3 ff <0f> 0b f3 0f 1e fa e8 0d 64 a3 ff e9 93 f6 ff ff f3 0f 1e fa 4c 89
RSP: 0018:ffffc90003dff4e0 EFLAGS: 00010093
RAX: ffffffff81e95988 RBX: 00000000000001c1 RCX: ffff8880205b3a80
RDX: 0000000000000000 RSI: 00000000000001c0 RDI: 00000000000001c1
RBP: ffffc90003dff830 R08: ffffffff81e90e67 R09: fffffbfff1a433c3
R10: 0000000000000000 R11: dffffc0000000001 R12: 0000000000000000
R13: ffffc90003dff6c0 R14: 00000000000001c0 R15: 0000000000000000
FS: 00007fdbae5ee700(0000) GS:ffff8880b9900000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fdbae6901e0 CR3: 000000007b2dd000 CR4: 00000000003506e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
madvise_collapse+0x721/0xf50 mm/khugepaged.c:2693
madvise_vma_behavior mm/madvise.c:1086 [inline]
madvise_walk_vmas mm/madvise.c:1260 [inline]
do_madvise+0x9e5/0x4680 mm/madvise.c:1439
__do_sys_madvise mm/madvise.c:1452 [inline]
__se_sys_madvise mm/madvise.c:1450 [inline]
__x64_sys_madvise+0xa5/0xb0 mm/madvise.c:1450
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

The xas_store() call during page cache scanning can potentially translate
'xas' into the error state (with the reproducer provided by the syzkaller
the error code is -ENOMEM). However, there are no further checks after
the 'xas_store', and the next call of 'xas_next' at the start of the
scanning cycle doesn't increase the xa_index, and the issue occurs.

This patch will add the xarray state error checking after the xas_store()
and the corresponding result error code.

Tested via syzbot.

[[e-mail]: update include/trace/events/huge_memory.h's SCAN_STATUS]
Link: https://lkml.kernel.org/r/[e-mail]
Link: https://syzkaller.appspot.com/bug?id=7d6bb3760e026ece7524500fe44fb024a0e959fc
Signed-off-by: Ivan Orlov <[e-mail]>
Reported-by: [e-mail]
Tested-by: Zach O'Keefe <[e-mail]>
Cc: Yang Shi <[e-mail]>
Cc: Himadri Pandya <[e-mail]>
Cc: Ivan Orlov <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Cc: Song Liu <[e-mail]>
Cc: Rik van Riel <[e-mail]>
Cc: Kirill A. Shutemov <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/trace/events/huge_memory.h

···
 	EM( SCAN_ALLOC_HUGE_PAGE_FAIL,	"alloc_huge_page_failed")	\
 	EM( SCAN_CGROUP_CHARGE_FAIL,	"ccgroup_charge_failed")	\
 	EM( SCAN_TRUNCATED,		"truncated")			\
-	EMe(SCAN_PAGE_HAS_PRIVATE,	"page_has_private")		\
+	EM( SCAN_PAGE_HAS_PRIVATE,	"page_has_private")		\
+	EMe(SCAN_STORE_FAILED,		"store_failed")
 
 #undef EM
 #undef EMe

mm/khugepaged.c

···
 	SCAN_CGROUP_CHARGE_FAIL,
 	SCAN_TRUNCATED,
 	SCAN_PAGE_HAS_PRIVATE,
+	SCAN_STORE_FAILED,
 };
 
 #define CREATE_TRACE_POINTS
···
 					goto xa_locked;
 				}
 				xas_store(&xas, hpage);
+				if (xas_error(&xas)) {
+					/* revert shmem_charge performed
+					 * in the previous condition
+					 */
+					mapping->nrpages--;
+					shmem_uncharge(mapping->host, 1);
+					result = SCAN_STORE_FAILED;
+					goto xa_locked;
+				}
 				nr_none++;
 				continue;
 			}
···
 
 		/* Finally, replace with the new page. */
 		xas_store(&xas, hpage);
+		/* We can't get an ENOMEM here (because the allocation happened before)
+		 * but let's check for errors (XArray implementation can be
+		 * changed in the future)
+		 */
+		WARN_ON_ONCE(xas_error(&xas));
 		continue;
 out_unlock:
 		unlock_page(page);
···
 	/* Join all the small entries into a single multi-index entry */
 	xas_set_order(&xas, start, HPAGE_PMD_ORDER);
 	xas_store(&xas, hpage);
+	/* Here we can't get an ENOMEM (because entries were
+	 * previously allocated) But let's check for errors
+	 * (XArray implementation can be changed in the future)
+	 */
+	WARN_ON_ONCE(xas_error(&xas));
 xa_locked:
 	xas_unlock_irq(&xas);
 xa_unlocked: