[PATCH v2] mm: khugepaged: Fix kernel BUG in hpage_collapse_scan_file

[Ivan Orlov <ivan.orlov0322@gmail.com>]

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. It will
also add xarray state error checking via WARN_ON_ONCE macros,
to be sure that ENOMEM or other possible errors don't occur
at the places they shouldn't.

Tested via syzbot.

Reported-by: syzbot+9578faa5475acb35fa50@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?id=7d6bb3760e026ece7524500fe44fb024a0e959fc
Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
---
V1 -> V2: Add WARN_ON_ONCE error checking and comments

 mm/khugepaged.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 92e6f56a932d..8b6580b13339 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -55,6 +55,7 @@ enum scan_result {
 	SCAN_CGROUP_CHARGE_FAIL,
 	SCAN_TRUNCATED,
 	SCAN_PAGE_HAS_PRIVATE,
+	SCAN_STORE_FAILED,
 };
 
 #define CREATE_TRACE_POINTS
@@ -1840,6 +1841,15 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 					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;
 			}
@@ -1992,6 +2002,11 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 
 		/* 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);
@@ -2029,6 +2044,11 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 	/* 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:
-- 
2.34.1