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From: Eric Biggers <ebiggers@kernel.org>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Matthew Wilcox <willy@infradead.org>,
	David Laight <David.Laight@aculab.com>,
	Christoph Hellwig <hch@lst.de>,
	David Hildenbrand <david@redhat.com>,
	Greg KH <gregkh@linuxfoundation.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	"kernel-team@android.com" <kernel-team@android.com>,
	Andrew Morton <akpm@linux-foundation.org>,
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	David Howells <dhowells@redhat.com>,
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	<linux-arm-kernel@lists.infradead.org>,
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Subject: Re: Buggy commit tracked to: "Re: [PATCH 2/9] iov_iter: move rw_copy_check_uvector() into lib/iov_iter.c"
Date: Thu, 22 Oct 2020 13:59:32 -0700
Message-ID: <20201022205932.GB3613750@gmail.com> (raw)
In-Reply-To: <CAKwvOdnq-yYLcF_coo=jMV-RH-SkuNp_kMB+KCBF5cz3PwiB8g@mail.gmail.com>

On Thu, Oct 22, 2020 at 10:00:44AM -0700, Nick Desaulniers wrote:
> On Thu, Oct 22, 2020 at 9:40 AM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > On Thu, Oct 22, 2020 at 04:35:17PM +0000, David Laight wrote:
> > > Wait...
> > > readv(2) defines:
> > >       ssize_t readv(int fd, const struct iovec *iov, int iovcnt);
> >
> > It doesn't really matter what the manpage says.  What does the AOSP
> > libc header say?
> 
> Same: https://android.googlesource.com/platform/bionic/+/refs/heads/master/libc/include/sys/uio.h#38
> 
> Theoretically someone could bypass libc to make a system call, right?
> 
> >
> > > But the syscall is defined as:
> > >
> > > SYSCALL_DEFINE3(readv, unsigned long, fd, const struct iovec __user *, vec,
> > >                 unsigned long, vlen)
> > > {
> > >         return do_readv(fd, vec, vlen, 0);
> > > }
> >
> 

FWIW, glibc makes the readv() syscall assuming that fd and vlen are 'int' as
well.  So this problem isn't specific to Android's libc.

From objdump -d /lib/x86_64-linux-gnu/libc.so.6:

	00000000000f4db0 <readv@@GLIBC_2.2.5>:
	   f4db0:       64 8b 04 25 18 00 00    mov    %fs:0x18,%eax
	   f4db7:       00
	   f4db8:       85 c0                   test   %eax,%eax
	   f4dba:       75 14                   jne    f4dd0 <readv@@GLIBC_2.2.5+0x20>
	   f4dbc:       b8 13 00 00 00          mov    $0x13,%eax
	   f4dc1:       0f 05                   syscall
	   ...

There's some code for pthread cancellation, but no zeroing of the upper half of
the fd and vlen arguments, which are in %edi and %edx respectively.  But the
glibc function prototype uses 'int' for them, not 'unsigned long'
'ssize_t readv(int fd, const struct iovec *iov, int iovcnt);'.

So the high halves of the fd and iovcnt registers can contain garbage.  Or at
least that's what gcc (9.3.0) and clang (9.0.1) assume; they both compile the
following

void g(unsigned int x);

void f(unsigned long x)
{
        g(x);
}

into f() making a tail call to g(), without zeroing the top half of %rdi.

Also note the following program succeeds on Linux 5.9 on x86_64.  On kernels
that have this bug, it should fail.  (I couldn't get it to actually fail, so it
must depend on the compiler and/or the kernel config...)

	#include <fcntl.h>
	#include <stdio.h>
	#include <sys/syscall.h>
	#include <sys/uio.h>
	#include <unistd.h>

	int main()
	{
		int fd = open("/dev/zero", O_RDONLY);
		char buf[1000];
		struct iovec iov = { .iov_base = buf, .iov_len = sizeof(buf) };
		long ret;

		ret = syscall(__NR_readv, fd, &iov, 0x100000001);
		if (ret < 0)
			perror("readv failed");
		else
			printf("read %ld bytes\n", ret);
	}

I think the right fix is to change the readv() (and writev(), etc.) syscalls to
take 'unsigned int' rather than 'unsigned long', as that is what the users are
assuming...

- Eric

  reply index

Thread overview: 66+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-25  4:51 let import_iovec deal with compat_iovecs as well v4 Christoph Hellwig
2020-09-25  4:51 ` [PATCH 1/9] compat.h: fix a spelling error in <linux/compat.h> Christoph Hellwig
2020-09-25  4:51 ` [PATCH 2/9] iov_iter: move rw_copy_check_uvector() into lib/iov_iter.c Christoph Hellwig
2020-10-21 16:13   ` Buggy commit tracked to: "Re: [PATCH 2/9] iov_iter: move rw_copy_check_uvector() into lib/iov_iter.c" Greg KH
2020-10-21 20:59     ` David Laight
2020-10-21 23:39     ` Al Viro
2020-10-22  8:26       ` Greg KH
2020-10-22  8:35         ` David Hildenbrand
2020-10-22  8:40           ` David Laight
2020-10-22  8:48             ` David Hildenbrand
2020-10-22  9:01               ` Greg KH
2020-10-22  9:06                 ` David Laight
2020-10-22  9:19                 ` David Hildenbrand
2020-10-22  9:25                   ` David Hildenbrand
2020-10-22  9:32                     ` David Laight
2020-10-22  9:36                       ` David Hildenbrand
2020-10-22 10:48                         ` Greg KH
2020-10-22 12:18                           ` Greg KH
2020-10-22 12:42                             ` David Hildenbrand
2020-10-22 12:57                               ` Greg KH
2020-10-22 13:50                                 ` Greg KH
     [not found]                                   ` <CAK8P3a1B7OVdyzW0-97JwzZiwp0D0fnSfyete16QTvPp_1m07A@mail.gmail.com>
2020-10-22 14:40                                     ` Greg KH
2020-10-22 16:15                                       ` David Laight
2020-10-23 12:46                                   ` David Laight
2020-10-23 13:09                                     ` David Hildenbrand
2020-10-23 14:33                                       ` David Hildenbrand
2020-10-23 14:39                                         ` David Laight
2020-10-23 14:47                                           ` 'Greg KH'
2020-10-23 16:33                                             ` David Hildenbrand
2020-11-02  9:06                                             ` David Laight
2020-11-02 13:52                                               ` 'Greg KH'
2020-11-02 18:23                                                 ` David Laight
2020-10-23 17:58                                       ` Al Viro
2020-10-23 18:27                                         ` Segher Boessenkool
2020-10-23 21:28                                           ` David Laight
2020-10-24 17:29                                             ` Segher Boessenkool
2020-10-24 21:12                                               ` David Laight
     [not found]                                     ` <CAK8P3a1n+b8hOMhNQSDzgic03dyXbmpccfTJ3C1bGKvzsgMXbg@mail.gmail.com>
2020-10-23 13:28                                       ` David Laight
2020-10-22 13:23                         ` Christoph Hellwig
2020-10-22 16:35                           ` David Laight
2020-10-22 16:40                             ` Matthew Wilcox
2020-10-22 16:50                               ` David Laight
2020-10-22 17:00                               ` Nick Desaulniers
2020-10-22 20:59                                 ` Eric Biggers [this message]
2020-10-22 21:28                                   ` Al Viro
2020-10-22 18:19                               ` Al Viro
2020-10-22 17:54                             ` Nick Desaulniers
     [not found]                               ` <CAK8P3a3LjG+ZvmQrkb9zpgov8xBkQQWrkHBPgjfYSqBKGrwT4w@mail.gmail.com>
2020-10-22 19:04                                 ` Nick Desaulniers
2020-10-22 19:24                                   ` Al Viro
2020-10-22 19:27                                     ` Al Viro
2020-10-22 20:06                                     ` Al Viro
2020-10-22 20:09                                       ` Al Viro
2020-10-22 20:11                                     ` Nick Desaulniers
2020-10-22 22:07                                     ` David Laight
2020-10-23 13:12                                     ` David Hildenbrand
2020-10-22 22:04                                   ` David Laight
2020-10-22  9:28                   ` David Laight
2020-10-22  9:02               ` David Laight
2020-09-25  4:51 ` [PATCH 3/9] iov_iter: refactor rw_copy_check_uvector and import_iovec Christoph Hellwig
2020-09-25  4:51 ` [PATCH 4/9] iov_iter: transparently handle compat iovecs in import_iovec Christoph Hellwig
2020-09-25  4:51 ` [PATCH 5/9] fs: remove various compat readv/writev helpers Christoph Hellwig
2020-09-25  4:51 ` [PATCH 6/9] fs: remove the compat readv/writev syscalls Christoph Hellwig
2020-09-25  4:51 ` [PATCH 7/9] fs: remove compat_sys_vmsplice Christoph Hellwig
2020-09-25  4:51 ` [PATCH 8/9] mm: remove compat_process_vm_{readv,writev} Christoph Hellwig
2020-09-25  4:51 ` [PATCH 9/9] security/keys: remove compat_keyctl_instantiate_key_iov Christoph Hellwig
2020-09-25 15:23 ` let import_iovec deal with compat_iovecs as well v4 Al Viro

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