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From: Jeff Layton <jlayton@kernel.org>
To: Eric Biggers <ebiggers@kernel.org>, Luo Meng <luomeng12@huawei.com>
Cc: bfields@fieldses.org, viro@zeniv.linux.org.uk,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH] locks: Fix UBSAN undefined behaviour in flock64_to_posix_lock
Date: Thu, 22 Oct 2020 13:48:58 -0400	[thread overview]
Message-ID: <b2dc8ec275e9fc379398f95aba237e1224c86330.camel@kernel.org> (raw)
In-Reply-To: <20201022172500.GA3613750@gmail.com>

On Thu, 2020-10-22 at 10:25 -0700, Eric Biggers wrote:
> On Thu, Oct 22, 2020 at 10:03:41AM +0800, Luo Meng wrote:
> > When the sum of fl->fl_start and l->l_len overflows,
> > UBSAN shows the following warning:
> > 
> > UBSAN: Undefined behaviour in fs/locks.c:482:29
> > signed integer overflow: 2 + 9223372036854775806
> > cannot be represented in type 'long long int'
> > Call Trace:
> >  __dump_stack lib/dump_stack.c:77 [inline]
> >  dump_stack+0xe4/0x14e lib/dump_stack.c:118
> >  ubsan_epilogue+0xe/0x81 lib/ubsan.c:161
> >  handle_overflow+0x193/0x1e2 lib/ubsan.c:192
> >  flock64_to_posix_lock fs/locks.c:482 [inline]
> >  flock_to_posix_lock+0x595/0x690 fs/locks.c:515
> >  fcntl_setlk+0xf3/0xa90 fs/locks.c:2262
> >  do_fcntl+0x456/0xf60 fs/fcntl.c:387
> >  __do_sys_fcntl fs/fcntl.c:483 [inline]
> >  __se_sys_fcntl fs/fcntl.c:468 [inline]
> >  __x64_sys_fcntl+0x12d/0x180 fs/fcntl.c:468
> >  do_syscall_64+0xc8/0x5a0 arch/x86/entry/common.c:293
> >  entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > 
> > Fix it by moving -1 forward.
> > 
> > Signed-off-by: Luo Meng <luomeng12@huawei.com>
> > ---
> >  fs/locks.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/fs/locks.c b/fs/locks.c
> > index 1f84a03601fe..8489787ca97e 100644
> > --- a/fs/locks.c
> > +++ b/fs/locks.c
> > @@ -542,7 +542,7 @@ static int flock64_to_posix_lock(struct file *filp, struct file_lock *fl,
> >  	if (l->l_len > 0) {
> >  		if (l->l_len - 1 > OFFSET_MAX - fl->fl_start)
> >  			return -EOVERFLOW;
> > -		fl->fl_end = fl->fl_start + l->l_len - 1;
> > +		fl->fl_end = fl->fl_start - 1 + l->l_len;
> >  
> 
> Given what the bounds check just above does, wouldn't it make more sense to
> parenthesize 'l->l_len - 1' instead?  So:
> 
> 		fl->fl_end = fl->fl_start + (l->l_len - 1);
> 
> Also FWIW, the Linux kernel uses the -fwrapv compiler flag, so signed integer
> overflow is defined.  IMO it's still best avoided though...
> 

That does seem less ambiguous.

Luo, if you're OK with that approach, I can just fix it up in-tree.
-- 
Jeff Layton <jlayton@kernel.org>


      reply	other threads:[~2020-10-22 17:49 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-22  2:03 [PATCH] locks: Fix UBSAN undefined behaviour in flock64_to_posix_lock Luo Meng
2020-10-22 13:21 ` Jeff Layton
2020-10-22 14:51   ` Matthew Wilcox
2020-10-22 17:25 ` Eric Biggers
2020-10-22 17:48   ` Jeff Layton [this message]

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