CVE-2024-47691

In the Linux kernel, the following vulnerability has been resolved:

f2fs: fix to avoid use-after-free in f2fsstopgc_thread()

syzbot reports a f2fs bug as below:

_dumpstack lib/dumpstack.c:88 [inline] dumpstacklvl+0x241/0x360 lib/dumpstack.c:114 printreport+0xe8/0x550 mm/kasan/report.c:491 kasanreport+0x143/0x180 mm/kasan/report.c:601 kasancheckrange+0x282/0x290 mm/kasan/generic.c:189 instrumentatomicreadwrite include/linux/instrumented.h:96 [inline] atomicfetchaddrelaxed include/linux/atomic/atomic-instrumented.h:252 [inline] _refcountadd include/linux/refcount.h:184 [inline] _refcountinc include/linux/refcount.h:241 [inline] refcountinc include/linux/refcount.h:258 [inline] gettaskstruct include/linux/sched/task.h:118 [inline] kthreadstop+0xca/0x630 kernel/kthread.c:704 f2fsstopgcthread+0x65/0xb0 fs/f2fs/gc.c:210 f2fsdoshutdown+0x192/0x540 fs/f2fs/file.c:2283 f2fsiocshutdown fs/f2fs/file.c:2325 [inline] _f2fsioctl+0x443a/0xbe60 fs/f2fs/file.c:4325 vfsioctl fs/ioctl.c:51 [inline] _dosysioctl fs/ioctl.c:907 [inline] _sesysioctl+0xfc/0x170 fs/ioctl.c:893 dosyscallx64 arch/x86/entry/common.c:52 [inline] dosyscall64+0xf3/0x230 arch/x86/entry/common.c:83 entrySYSCALL64afterhwframe+0x77/0x7f

The root cause is below race condition, it may cause use-after-free issue in sbi->gc_th pointer.

• remount
  • f2fs_remount
    • f2fsstopgcthread
      • kfree(gcth)
        • f2fsiocshutdown
          • f2fsdoshutdown
            • f2fsstopgcthread
              • kthreadstop(gcth->f2fsgctask) : sbi->gcthread = NULL;

We will call f2fsdoshutdown() in two paths: - for f2fsiocshutdown() path, we should grab sb->sumount semaphore for fixing. - for f2fsshutdown() path, it's safe since caller has already grabbed sb->s_umount semaphore.