mm/hugetlb: fix PTE marker handling in hugetlb_change_protection()

Patch series "mm/hugetlb: uffd-wp fixes for hugetlb_change_protection()".

Playing with virtio-mem and background snapshots (using uffd-wp) on
hugetlb in QEMU, I managed to trigger a VM_BUG_ON().  Looking into the
details, hugetlb_change_protection() seems to not handle uffd-wp correctly
in all cases.

Patch #1 fixes my test case.  I don't have reproducers for patch #2, as it
requires running into migration entries.

I did not yet check in detail yet if !hugetlb code requires similar care.


This patch (of 2):

There are two problematic cases when stumbling over a PTE marker in
hugetlb_change_protection():

(1) We protect an uffd-wp PTE marker a second time using uffd-wp: we will
    end up in the "!huge_pte_none(pte)" case and mess up the PTE marker.

(2) We unprotect a uffd-wp PTE marker: we will similarly end up in the
    "!huge_pte_none(pte)" case even though we cleared the PTE, because
    the "pte" variable is stale. We'll mess up the PTE marker.

For example, if we later stumble over such a "wrongly modified" PTE marker,
we'll treat it like a present PTE that maps some garbage page.

This can, for example, be triggered by mapping a memfd backed by huge
pages, registering uffd-wp, uffd-wp'ing an unmapped page and (a)
uffd-wp'ing it a second time; or (b) uffd-unprotecting it; or (c)
unregistering uffd-wp. Then, ff we trigger fallocate(FALLOC_FL_PUNCH_HOLE)
on that file range, we will run into a VM_BUG_ON:

[  195.039560] page:00000000ba1f2987 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x0
[  195.039565] flags: 0x7ffffc0001000(reserved|node=0|zone=0|lastcpupid=0x1fffff)
[  195.039568] raw: 0007ffffc0001000 ffffe742c0000008 ffffe742c0000008 0000000000000000
[  195.039569] raw: 0000000000000000 0000000000000000 00000001ffffffff 0000000000000000
[  195.039569] page dumped because: VM_BUG_ON_PAGE(compound && !PageHead(page))
[  195.039573] ------------[ cut here ]------------
[  195.039574] kernel BUG at mm/rmap.c:1346!
[  195.039579] invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
[  195.039581] CPU: 7 PID: 4777 Comm: qemu-system-x86 Not tainted 6.0.12-200.fc36.x86_64 #1
[  195.039583] Hardware name: LENOVO 20WNS1F81N/20WNS1F81N, BIOS N35ET50W (1.50 ) 09/15/2022
[  195.039584] RIP: 0010:page_remove_rmap+0x45b/0x550
[  195.039588] Code: [...]
[  195.039589] RSP: 0018:ffffbc03c3633ba8 EFLAGS: 00010292
[  195.039591] RAX: 0000000000000040 RBX: ffffe742c0000000 RCX: 0000000000000000
[  195.039592] RDX: 0000000000000002 RSI: ffffffff8e7aac1a RDI: 00000000ffffffff
[  195.039592] RBP: 0000000000000001 R08: 0000000000000000 R09: ffffbc03c3633a08
[  195.039593] R10: 0000000000000003 R11: ffffffff8f146328 R12: ffff9b04c42754b0
[  195.039594] R13: ffffffff8fcc6328 R14: ffffbc03c3633c80 R15: ffff9b0484ab9100
[  195.039595] FS:  00007fc7aaf68640(0000) GS:ffff9b0bbf7c0000(0000) knlGS:0000000000000000
[  195.039596] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  195.039597] CR2: 000055d402c49110 CR3: 0000000159392003 CR4: 0000000000772ee0
[  195.039598] PKRU: 55555554
[  195.039599] Call Trace:
[  195.039600]  <TASK>
[  195.039602]  __unmap_hugepage_range+0x33b/0x7d0
[  195.039605]  unmap_hugepage_range+0x55/0x70
[  195.039608]  hugetlb_vmdelete_list+0x77/0xa0
[  195.039611]  hugetlbfs_fallocate+0x410/0x550
[  195.039612]  ? _raw_spin_unlock_irqrestore+0x23/0x40
[  195.039616]  vfs_fallocate+0x12e/0x360
[  195.039618]  __x64_sys_fallocate+0x40/0x70
[  195.039620]  do_syscall_64+0x58/0x80
[  195.039623]  ? syscall_exit_to_user_mode+0x17/0x40
[  195.039624]  ? do_syscall_64+0x67/0x80
[  195.039626]  entry_SYSCALL_64_after_hwframe+0x63/0xcd
[  195.039628] RIP: 0033:0x7fc7b590651f
[  195.039653] Code: [...]
[  195.039654] RSP: 002b:00007fc7aaf66e70 EFLAGS: 00000293 ORIG_RAX: 000000000000011d
[  195.039655] RAX: ffffffffffffffda RBX: 0000558ef4b7f370 RCX: 00007fc7b590651f
[  195.039656] RDX: 0000000018000000 RSI: 0000000000000003 RDI: 000000000000000c
[  195.039657] RBP: 0000000008000000 R08: 0000000000000000 R09: 0000000000000073
[  195.039658] R10: 0000000008000000 R11: 0000000000000293 R12: 0000000018000000
[  195.039658] R13: 00007fb8bbe00000 R14: 000000000000000c R15: 0000000000001000
[  195.039661]  </TASK>

Fix it by not going into the "!huge_pte_none(pte)" case if we stumble over
an exclusive marker.  spin_unlock() + continue would get the job done.

However, instead, make it clearer that there are no fall-through
statements: we process each case (hwpoison, migration, marker, !none,
none) and then unlock the page table to continue with the next PTE.  Let's
avoid "continue" statements and use a single spin_unlock() at the end.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 60dfaad ("mm/hugetlb: allow uffd wr-protect none ptes")
Signed-off-by: David Hildenbrand <[email protected]>
Reviewed-by: Peter Xu <[email protected]>
Reviewed-by: Mike Kravetz <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/hugetlb.c

@@ -6658,10 +6658,8 @@ unsigned long hugetlb_change_protection(struct vm_area_struct *vma,
 		}
 		pte = huge_ptep_get(ptep);
 		if (unlikely(is_hugetlb_entry_hwpoisoned(pte))) {
-			spin_unlock(ptl);
-			continue;
-		}
-		if (unlikely(is_hugetlb_entry_migration(pte))) {
+			/* Nothing to do. */
+		} else if (unlikely(is_hugetlb_entry_migration(pte))) {
 			swp_entry_t entry = pte_to_swp_entry(pte);
 			struct page *page = pfn_swap_entry_to_page(entry);
 
@@ -6682,18 +6680,13 @@ unsigned long hugetlb_change_protection(struct vm_area_struct *vma,
 				set_huge_pte_at(mm, address, ptep, newpte);
 				pages++;
 			}
-			spin_unlock(ptl);
-			continue;
-		}
-		if (unlikely(pte_marker_uffd_wp(pte))) {
-			/*
-			 * This is changing a non-present pte into a none pte,
-			 * no need for huge_ptep_modify_prot_start/commit().
-			 */
+		} else if (unlikely(is_pte_marker(pte))) {
+			/* No other markers apply for now. */
+			WARN_ON_ONCE(!pte_marker_uffd_wp(pte));
 			if (uffd_wp_resolve)
+				/* Safe to modify directly (non-present->none). */
 				huge_pte_clear(mm, address, ptep, psize);
-		}
-		if (!huge_pte_none(pte)) {
+		} else if (!huge_pte_none(pte)) {
 			pte_t old_pte;
 			unsigned int shift = huge_page_shift(hstate_vma(vma));