f2fs: fix to avoid use-after-free issue in f2fs_filemap_fault

syzbot reports a f2fs bug as below:

BUG: KASAN: slab-use-after-free in f2fs_filemap_fault+0xd1/0x2c0 fs/f2fs/file.c:49
Read of size 8 at addr ffff88807bb22680 by task syz-executor184/5058

CPU: 0 PID: 5058 Comm: syz-executor184 Not tainted 6.7.0-syzkaller-09928-g052d5343 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/17/2023
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0x1e7/0x2d0 lib/dump_stack.c:106
 print_address_description mm/kasan/report.c:377 [inline]
 print_report+0x163/0x540 mm/kasan/report.c:488
 kasan_report+0x142/0x170 mm/kasan/report.c:601
 f2fs_filemap_fault+0xd1/0x2c0 fs/f2fs/file.c:49
 __do_fault+0x131/0x450 mm/memory.c:4376
 do_shared_fault mm/memory.c:4798 [inline]
 do_fault mm/memory.c:4872 [inline]
 do_pte_missing mm/memory.c:3745 [inline]
 handle_pte_fault mm/memory.c:5144 [inline]
 __handle_mm_fault+0x23b7/0x72b0 mm/memory.c:5285
 handle_mm_fault+0x27e/0x770 mm/memory.c:5450
 do_user_addr_fault arch/x86/mm/fault.c:1364 [inline]
 handle_page_fault arch/x86/mm/fault.c:1507 [inline]
 exc_page_fault+0x456/0x870 arch/x86/mm/fault.c:1563
 asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:570

The root cause is: in f2fs_filemap_fault(), vmf->vma may be not alive after
filemap_fault(), so it may cause use-after-free issue when accessing
vmf->vma->vm_flags in trace_f2fs_filemap_fault(). So it needs to keep vm_flags
in separated temporary variable for tracepoint use.

Fixes: 87f3afd3 ("f2fs: add tracepoint for f2fs_vm_page_mkwrite()")
Reported-and-tested-by: syzbot+763afad57075d3f862f2@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/lkml/000000000000e8222b060f00db3b@google.com
Cc: Ed Tsai <Ed.Tsai@mediatek.com>
Suggested-by: Hillf Danton <hdanton@sina.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/file.c

@@ -39,6 +39,7 @@
 static vm_fault_t f2fs_filemap_fault(struct vm_fault *vmf)
 {
 	struct inode *inode = file_inode(vmf->vma->vm_file);
+	vm_flags_t flags = vmf->vma->vm_flags;
 	vm_fault_t ret;
 
 	ret = filemap_fault(vmf);
@@ -46,7 +47,7 @@ static vm_fault_t f2fs_filemap_fault(struct vm_fault *vmf)
 		f2fs_update_iostat(F2FS_I_SB(inode), inode,
 					APP_MAPPED_READ_IO, F2FS_BLKSIZE);
 
-	trace_f2fs_filemap_fault(inode, vmf->pgoff, vmf->vma->vm_flags, ret);
+	trace_f2fs_filemap_fault(inode, vmf->pgoff, flags, ret);
 
 	return ret;
 }