> Date: Fri, 2 Oct 2020 10:32:27 +0200
> From: Martin Pieuchot <[email protected]>
>
> On 01/10/20(Thu) 21:44, Mark Kettenis wrote:
> > > Date: Thu, 1 Oct 2020 14:10:56 +0200
> > > From: Martin Pieuchot <[email protected]>
> > >
> > > While studying a bug report from naddy@ in 2017 when testing guenther@'s
> > > amap/anon locking diff I figured out that we have been too optimistic in
> > > the !MAP_ANON case.
> > >
> > > The reported panic involves, I'd guess, a race between fd_getfile() and
> > > vref():
> > >
> > > panic: vref used where vget required
> > > db_enter() at db_enter+0x5
> > > panic() at panic+0x129
> > > vref(ffffff03b20d29e8) at vref+0x5d
> > > uvn_attach(1010000,ffffff03a5879dc0) at uvn_attach+0x11d
> > > uvm_mmapfile(7,ffffff03a5879dc0,2,1,13,100000012) at uvm_mmapfile+0x12c
> > > sys_mmap(c50,ffff8000225f82a0,1) at sys_mmap+0x604
> > > syscall() at syscall+0x279
> > > --- syscall (number 198) ---
> > > end of kernel
> > >
> > > Removing the KERNEL_LOCK() from file mapping was out of the scope of this
> > > previous work, so I'd like to go back to a single KERNEL_LOCK/UNLOCK dance
> > > in this code path to remove any false positive.
> > >
> > > Note that this code is currently always run under KERNEL_LOCK() so this
> > > will only have effect once the syscall will be unlocked.
> > >
> > > ok?
> >
> > Hmm, I thought fd_getfile() was fully mpsafe.
>
> It is to get a reference on `fp'. However if the current thread
> releases the KERNEL_LOCK() before calling vref(9) it might lose a
> race.
I don't see the race. The function returns a 'fp' with a reference,
so 'fp' will be valid regardless of whether we hold the kernel lock or
not. So we should be able to take the kernel lock after the
fd_getfile() call isn't it?
> > But I suppose the kernel lock needs to be grabbed before we start
> > looking at the vnode?
>
> Yes, or to say it otherwise not released.
So the problem is that while we have an 'fp', its f_data member points
to a vnode that has already been put on the freelist and therefore has
v_usecount set to zero? How does that happen?
> > Your diff makes the locking a bit convoluted, but I suppose adding a
> > KERNEL_UNLOCK() before every "goto out" is worse?
>
> I tried to keep the diff as small as possible to not obfuscate the change.
> If we want cleaner code we can move the !ANON case in a different function.
Splitting would be hard because of the "goto is_anon".
> > > Index: uvm/uvm_mmap.c
> > > ===================================================================
> > > RCS file: /cvs/src/sys/uvm/uvm_mmap.c,v
> > > retrieving revision 1.161
> > > diff -u -p -r1.161 uvm_mmap.c
> > > --- uvm/uvm_mmap.c 4 Mar 2020 21:15:39 -0000 1.161
> > > +++ uvm/uvm_mmap.c 28 Sep 2020 09:48:26 -0000
> > > @@ -288,8 +288,11 @@ sys_mmap(struct proc *p, void *v, regist
> > >
> > > /* check for file mappings (i.e. not anonymous) and verify file. */
> > > if ((flags & MAP_ANON) == 0) {
> > > - if ((fp = fd_getfile(fdp, fd)) == NULL)
> > > - return (EBADF);
> > > + KERNEL_LOCK();
> > > + if ((fp = fd_getfile(fdp, fd)) == NULL) {
> > > + error = EBADF;
> > > + goto out;
> > > + }
> > >
> > > if (fp->f_type != DTYPE_VNODE) {
> > > error = ENODEV; /* only mmap vnodes! */
> > > @@ -313,6 +316,7 @@ sys_mmap(struct proc *p, void *v, regist
> > > flags |= MAP_ANON;
> > > FRELE(fp, p);
> > > fp = NULL;
> > > + KERNEL_UNLOCK();
> > > goto is_anon;
> > > }
> > >
> > > @@ -362,9 +366,7 @@ sys_mmap(struct proc *p, void *v, regist
> > > * EPERM.
> > > */
> > > if (fp->f_flag & FWRITE) {
> > > - KERNEL_LOCK();
> > > error = VOP_GETATTR(vp, &va, p->p_ucred, p);
> > > - KERNEL_UNLOCK();
> > > if (error)
> > > goto out;
> > > if ((va.va_flags & (IMMUTABLE|APPEND)) == 0)
> > > @@ -390,9 +392,9 @@ sys_mmap(struct proc *p, void *v, regist
> > > goto out;
> > > }
> > > }
> > > - KERNEL_LOCK();
> > > error = uvm_mmapfile(&p->p_vmspace->vm_map, &addr, size, prot,
> > > maxprot, flags, vp, pos, lim_cur(RLIMIT_MEMLOCK), p);
> > > + FRELE(fp, p);
> > > KERNEL_UNLOCK();
> > > } else { /* MAP_ANON case */
> > > if (fd != -1)
> > > @@ -428,7 +430,10 @@ is_anon: /* label for SunOS style /dev/z
> > > /* remember to add offset */
> > > *retval = (register_t)(addr + pageoff);
> > >
> > > + return (error);
> > > +
> > > out:
> > > + KERNEL_UNLOCK();
> > > if (fp)
> > > FRELE(fp, p);
> > > return (error);
> > >
> > >
>
