Re: [PATCH] eventfd: avoid overflow to ULLONG_MAX when ctx->count is 0

[Christian Brauner]

On Mon, Jul 10, 2023 at 11:02:33PM +0800, Wen Yang wrote:
> 
> On 2023/7/10 22:12, Christian Brauner wrote:
> > On Sun, Jul 09, 2023 at 02:54:51PM +0800, wenyang.linux@xxxxxxxxxxx wrote:
> > > From: Wen Yang <wenyang.linux@xxxxxxxxxxx>
> > > 
> > > For eventfd with flag EFD_SEMAPHORE, when its ctx->count is 0, calling
> > > eventfd_ctx_do_read will cause ctx->count to overflow to ULLONG_MAX.
> > > 
> > > Fixes: cb289d6244a3 ("eventfd - allow atomic read and waitqueue remove")
> > > Signed-off-by: Wen Yang <wenyang.linux@xxxxxxxxxxx>
> > > Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> > > Cc: Jens Axboe <axboe@xxxxxxxxx>
> > > Cc: Christian Brauner <brauner@xxxxxxxxxx>
> > > Cc: Christoph Hellwig <hch@xxxxxx>
> > > Cc: Dylan Yudaken <dylany@xxxxxx>
> > > Cc: David Woodhouse <dwmw@xxxxxxxxxxxx>
> > > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > > Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> > > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > > ---
> > So this looks ok but I would like to see an analysis how the overflow
> > can happen. I'm looking at the callers and it seems that once ctx->count
> > hits 0 eventfd_read() won't call eventfd_ctx_do_read() anymore. So is
> > there a caller that can call directly or indirectly
> > eventfd_ctx_do_read() on a ctx->count == 0?
> eventfd_read() ensures that ctx->count is not 0 before calling
> eventfd_ctx_do_read() and it is correct.
> 
> But it is not appropriate for eventfd_ctx_remove_wait_queue() to call
> eventfd_ctx_do_read() unconditionally,
> 
> as it may not only causes ctx->count to overflow, but also unnecessarily
> calls wake_up_locked_poll().
> 
> 
> I am sorry for just adding the following string in the patch:
> Fixes: cb289d6244a3 ("eventfd - allow atomic read and waitqueue remove")
> 
> 
> Looking forward to your suggestions.
> 
> --
> 
> Best wishes,
> 
> Wen
> 
> 
> > I'm just slightly skeptical about patches that fix issues without an
> > analysis how this can happen.
> > 
> > >   fs/eventfd.c | 4 +++-
> > >   1 file changed, 3 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/fs/eventfd.c b/fs/eventfd.c
> > > index 8aa36cd37351..10a101df19cd 100644
> > > --- a/fs/eventfd.c
> > > +++ b/fs/eventfd.c
> > > @@ -189,7 +189,7 @@ void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt)
> > >   {
> > >   	lockdep_assert_held(&ctx->wqh.lock);
> > > -	*cnt = (ctx->flags & EFD_SEMAPHORE) ? 1 : ctx->count;
> > > +	*cnt = ((ctx->flags & EFD_SEMAPHORE) && ctx->count) ? 1 : ctx->count;
> > >   	ctx->count -= *cnt;
> > >   }
> > >   EXPORT_SYMBOL_GPL(eventfd_ctx_do_read);
> > > @@ -269,6 +269,8 @@ static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t c
> > >   		return -EFAULT;
> > >   	if (ucnt == ULLONG_MAX)
> > >   		return -EINVAL;
> > > +	if ((ctx->flags & EFD_SEMAPHORE) && !ucnt)
> > > +		return -EINVAL;

Hm, why is bit necessary though? What's wrong with specifying ucnt == 0
with EFD_SEMAPHORE? This also looks like a (very low potential) uapi
break.