[PATCH] mm: Fix mmap_assert_locked() in follow_pte()

[Pei Li]

This patch fixes this warning by acquiring read lock before entering
untrack_pfn() while write lock is not held.

syzbot has tested the proposed patch and the reproducer did not
trigger any issue.

Reported-by: syzbot+35a4414f6e247f515443@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=35a4414f6e247f515443
Tested-by: syzbot+35a4414f6e247f515443@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Pei Li <peili.dev@xxxxxxxxx>
---
Syzbot reported the following warning in follow_pte():

WARNING: CPU: 3 PID: 5192 at include/linux/rwsem.h:195 rwsem_assert_held include/linux/rwsem.h:195 [inline]
WARNING: CPU: 3 PID: 5192 at include/linux/rwsem.h:195 mmap_assert_locked include/linux/mmap_lock.h:65 [inline]
WARNING: CPU: 3 PID: 5192 at include/linux/rwsem.h:195 follow_pte+0x414/0x4c0 mm/memory.c:5980

This is because we are assuming that mm->mmap_lock should be held when
entering follow_pte(). This is added in commit c5541ba378e3 (mm:
follow_pte() improvements).

However, in the following call stack, we are not acquring the lock:
 follow_phys arch/x86/mm/pat/memtype.c:957 [inline]
 get_pat_info+0xf2/0x510 arch/x86/mm/pat/memtype.c:991
 untrack_pfn+0xf7/0x4d0 arch/x86/mm/pat/memtype.c:1104
 unmap_single_vma+0x1bd/0x2b0 mm/memory.c:1819
 zap_page_range_single+0x326/0x560 mm/memory.c:1920

In zap_page_range_single(), we passed mm_wr_locked as false, as we do
not expect write lock to be held.
In the special case where vma->vm_flags is set as VM_PFNMAP, we are
hitting untrack_pfn() which eventually calls into follow_phys.

This patch fixes this warning by acquiring read lock before entering
untrack_pfn() while write lock is not held. 

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Tested on:

commit:         9d9a2f29 Merge tag 'mm-hotfixes-stable-2024-07-10-13-1..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=13be8021980000
kernel config:  https://syzkaller.appspot.com/x/.config?x=3456bae478301dc8
dashboard link: https://syzkaller.appspot.com/bug?extid=35a4414f6e247f515443
compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
patch:          https://syzkaller.appspot.com/x/patch.diff?x=145e3441980000

Note: testing is done by a robot and is best-effort only.
---
 mm/memory.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index d10e616d7389..75d7959b835b 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1815,9 +1815,16 @@ static void unmap_single_vma(struct mmu_gather *tlb,
 	if (vma->vm_file)
 		uprobe_munmap(vma, start, end);
 
-	if (unlikely(vma->vm_flags & VM_PFNMAP))
+	if (unlikely(vma->vm_flags & VM_PFNMAP)) {
+		if (!mm_wr_locked)
+			mmap_read_lock(vma->vm_mm);
+
 		untrack_pfn(vma, 0, 0, mm_wr_locked);
 
+		if (!mm_wr_locked)
+			mmap_read_unlock(vma->vm_mm);
+	}
+
 	if (start != end) {
 		if (unlikely(is_vm_hugetlb_page(vma))) {
 			/*

---
base-commit: 734610514cb0234763cc97ddbd235b7981889445
change-id: 20240710-bug12-fecfe4bb0dcb

Best regards,
-- 
Pei Li <peili.dev@xxxxxxxxx>