Re: [syzbot] [mm?] KASAN: slab-use-after-free Read in mremap

[Hillf Danton <hdanton@sina.com>]

> Date: Fri, 15 Aug 2025 20:07:34 -0700	[thread overview]
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    931e46dcbc7e Add linux-next specific files for 20250814
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=16a66af0580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=bb7fbecfa2364d1c
> dashboard link: https://syzkaller.appspot.com/bug?extid=4e221abf50259362f4f4
> compiler:       Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11fb3c34580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=173c2dbc580000

#syz test

--- x/mm/mremap.c
+++ y/mm/mremap.c
@@ -1823,7 +1823,7 @@ static unsigned long remap_move(struct v
 	unsigned long target_addr = new_addr;
 	unsigned long res = -EFAULT;
 	unsigned long last_end;
-	bool seen_vma = false;
+	bool seen_vma = false, allowed;
 
 	VMA_ITERATOR(vmi, current->mm, start);
 
@@ -1865,7 +1865,8 @@ static unsigned long remap_move(struct v
 		vrm->new_addr = target_addr + offset;
 		vrm->old_len = vrm->new_len = len;
 
-		if (!vma_multi_allowed(vma)) {
+		allowed = vma_multi_allowed(vma);
+		if (!allowed) {
 			/* This is not the first VMA, abort immediately. */
 			if (seen_vma)
 				return -EFAULT;
@@ -1881,7 +1882,7 @@ static unsigned long remap_move(struct v
 			return res_vma;
 
 		if (!seen_vma) {
-			VM_WARN_ON_ONCE(vma_multi_allowed(vma) &&
+			VM_WARN_ON_ONCE(allowed &&
 					res_vma != new_addr);
 			res = res_vma;
 		}
--