Date: Wed, 2 Dec 2015 22:15:33 +0100 From: Hans Petter Selasky <hps@selasky.org> To: Mateusz Guzik <mjguzik@gmail.com> Cc: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r291481 - head/sys/compat/linuxkpi/common/include/linux Message-ID: <565F5F75.20806@selasky.org> In-Reply-To: <20151202205417.GC30250@dft-labs.eu> References: <201511300924.tAU9OC7o049788@repo.freebsd.org> <20151202202958.GA30250@dft-labs.eu> <565F58EF.1030707@selasky.org> <20151202205417.GC30250@dft-labs.eu>
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On 12/02/15 21:54, Mateusz Guzik wrote: > On Wed, Dec 02, 2015 at 09:47:43PM +0100, Hans Petter Selasky wrote: >> On 12/02/15 21:29, Mateusz Guzik wrote: >>> On Mon, Nov 30, 2015 at 09:24:12AM +0000, Hans Petter Selasky wrote: >>>> Author: hselasky >>>> Date: Mon Nov 30 09:24:12 2015 >>>> New Revision: 291481 >>>> URL: https://svnweb.freebsd.org/changeset/base/291481 >>>> >>>> Log: >>>> Add more functions and types to the LinuxKPI. >>>> >>>> MFC after: 1 week >>>> Sponsored by: Mellanox Technologies >>>> >>>> Modified: >>>> head/sys/compat/linuxkpi/common/include/linux/file.h >>>> head/sys/compat/linuxkpi/common/include/linux/workqueue.h >>>> >>>> Modified: head/sys/compat/linuxkpi/common/include/linux/file.h >>>> ============================================================================== >>>> --- head/sys/compat/linuxkpi/common/include/linux/file.h Mon Nov 30 09:13:04 2015 (r291480) >>>> +++ head/sys/compat/linuxkpi/common/include/linux/file.h Mon Nov 30 09:24:12 2015 (r291481) >>>> @@ -2,7 +2,7 @@ >>>> * Copyright (c) 2010 Isilon Systems, Inc. >>>> * Copyright (c) 2010 iX Systems, Inc. >>>> * Copyright (c) 2010 Panasas, Inc. >>>> - * Copyright (c) 2013 Mellanox Technologies, Ltd. >>>> + * Copyright (c) 2013-2015 Mellanox Technologies, Ltd. >>>> * All rights reserved. >>>> * >>>> * Redistribution and use in source and binary forms, with or without >>>> @@ -125,6 +125,21 @@ get_unused_fd(void) >>>> return fd; >>>> } >>>> >>>> +static inline int >>>> +get_unused_fd_flags(int flags) >>>> +{ >>>> + struct file *file; >>>> + int error; >>>> + int fd; >>>> + >>>> + error = falloc(curthread, &file, &fd, flags); >>>> + if (error) >>>> + return -error; >>>> + /* drop the extra reference */ >>>> + fdrop(file, curthread); >>>> + return fd; >>>> +} >>>> + >>> >>> This does not look right. >>> >>> AFAIR Linux drivers are not going to install fds into kernel threads. So >>> this would be used for a userspace thread, but then it would completely >>> insecure. >>> >>> Linux model is to reserve a slot in the fd table, obtain a 'file' object >>> and install it as the last step. >>> >>> FreeBSD installs the file right away, but this means an extra reference >>> has to be held in case something else using the table closes the fd. >>> >>> As such, this fdrop can lead to a use-after-free as the file can be >>> freed from this poin. >>> >>> I'm afraid there is no way around patching improted consumers. >>> >> >> Hi Mateusz, >> >> Thanks for your input. Yes, there is a potential race there, but no >> use-after-free from what I can see, because the LinuxKPI always >> retrieve the file pointer by the file number using "fget_unlocked()". >> >> I'll look into if we can delay the fdrop() until after the fd_install(). >> > > I grepped an example consumer: > >> ssize_t ib_uverbs_create_comp_channel(struct ib_uverbs_file *file, >> struct ib_device *ib_dev, >> const char __user *buf, int in_len, >> int out_len) >> { >> struct ib_uverbs_create_comp_channel cmd; >> struct ib_uverbs_create_comp_channel_resp resp; >> struct file *filp; >> int ret; >> >> if (out_len < sizeof resp) >> return -ENOSPC; >> >> if (copy_from_user(&cmd, buf, sizeof cmd)) >> return -EFAULT; >> >> ret = get_unused_fd_flags(O_CLOEXEC); >> if (ret < 0) >> return ret; >> resp.fd = ret; >> > > So this is supposed to get the slot. > >> filp = ib_uverbs_alloc_event_file(file, ib_dev, 0); > > file object is allocated separately. > >> if (IS_ERR(filp)) { >> put_unused_fd(resp.fd); >> return PTR_ERR(filp); >> } >> >> if (copy_to_user((void __user *) (unsigned long) cmd.response, >> &resp, sizeof resp)) { >> put_unused_fd(resp.fd); >> fput(filp); >> return -EFAULT; >> } >> >> fd_install(resp.fd, filp); > > And here is the install. > >> return in_len; >> } > > If you drop, and have some magic to grab the file obtained from > get_unused_fd_flags, the file object is unsafe to use. > > In general, I don't see how you can get around having to edit such > functions. If they get edited, you can just use the FreeBSD scheme. > > Fortunately as far as error handling goes, it behaves the same way. > Hi, I've made a patch here: https://reviews.freebsd.org/D4351 Does it make sense? --HPS
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