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Date: Mon, 08 Jun 2020 09:27:42 -0400
From: Mimi Zohar <zohar@...ux.ibm.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Scott Branden <scott.branden@...adcom.com>,
Luis Chamberlain <mcgrof@...nel.org>,
Wolfram Sang <wsa@...nel.org>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
David Brown <david.brown@...aro.org>,
Alexander Viro <viro@...iv.linux.org.uk>,
Shuah Khan <shuah@...nel.org>, bjorn.andersson@...aro.org,
Shuah Khan <skhan@...uxfoundation.org>,
Arnd Bergmann <arnd@...db.de>,
"Rafael J . Wysocki" <rafael@...nel.org>,
linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
linux-fsdevel@...r.kernel.org,
BCM Kernel Feedback <bcm-kernel-feedback-list@...adcom.com>,
Olof Johansson <olof@...om.net>,
Andrew Morton <akpm@...ux-foundation.org>,
Dan Carpenter <dan.carpenter@...cle.com>,
Colin Ian King <colin.king@...onical.com>,
Kees Cook <keescook@...omium.org>,
Takashi Iwai <tiwai@...e.de>, linux-kselftest@...r.kernel.org,
Andy Gross <agross@...nel.org>,
linux-integrity@...r.kernel.org,
linux-security-module@...r.kernel.org,
Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH v7 1/8] fs: introduce kernel_pread_file* support
On Mon, 2020-06-08 at 09:22 -0400, Mimi Zohar wrote:
> On Mon, 2020-06-08 at 06:16 -0700, Matthew Wilcox wrote:
> > On Mon, Jun 08, 2020 at 09:03:21AM -0400, Mimi Zohar wrote:
> > > On Sat, 2020-06-06 at 08:52 -0700, Matthew Wilcox wrote:
> > > > On Fri, Jun 05, 2020 at 10:04:51PM -0700, Scott Branden wrote:
> > > > > -int kernel_read_file(struct file *file, void **buf, loff_t *size,
> > > > > - loff_t max_size, enum kernel_read_file_id id)
> > > > > -{
> > > > > - loff_t i_size, pos;
> > > > > +int kernel_pread_file(struct file *file, void **buf, loff_t *size,
> > > > > + loff_t pos, loff_t max_size,
> > > > > + enum kernel_pread_opt opt,
> > > > > + enum kernel_read_file_id id)
> > > > > +{
> > > > > + loff_t alloc_size;
> > > > > + loff_t buf_pos;
> > > > > + loff_t read_end;
> > > > > + loff_t i_size;
> > > > > ssize_t bytes = 0;
> > > > > int ret;
> > > > >
> > > >
> > > > Look, it's not your fault, but this is a great example of how we end
> > > > up with atrocious interfaces. Someone comes along and implements a
> > > > simple DWIM interface that solves their problem. Then somebody else
> > > > adds a slight variant that solves their problem, and so on and so on,
> > > > and we end up with this bonkers API where the arguments literally change
> > > > meaning depending on other arguments.
> > > >
> > > > > @@ -950,21 +955,31 @@ int kernel_read_file(struct file *file, void **buf, loff_t *size,
> > > > > ret = -EINVAL;
> > > > > goto out;
> > > > > }
> > > > > - if (i_size > SIZE_MAX || (max_size > 0 && i_size > max_size)) {
> > > > > +
> > > > > + /* Default read to end of file */
> > > > > + read_end = i_size;
> > > > > +
> > > > > + /* Allow reading partial portion of file */
> > > > > + if ((opt == KERNEL_PREAD_PART) &&
> > > > > + (i_size > (pos + max_size)))
> > > > > + read_end = pos + max_size;
> > > > > +
> > > > > + alloc_size = read_end - pos;
> > > > > + if (i_size > SIZE_MAX || (max_size > 0 && alloc_size > max_size)) {
> > > > > ret = -EFBIG;
> > > > > goto out;
> > > >
> > > > ... like that.
> > > >
> > > > I think what we actually want is:
> > > >
> > > > ssize_t vmap_file_range(struct file *, loff_t start, loff_t end, void **bufp);
> > > > void vunmap_file_range(struct file *, void *buf);
> > > >
> > > > If end > i_size, limit the allocation to i_size. Returns the number
> > > > of bytes allocated, or a negative errno. Writes the pointer allocated
> > > > to *bufp. Internally, it should use the page cache to read in the pages
> > > > (taking appropriate reference counts). Then it maps them using vmap()
> > > > instead of copying them to a private vmalloc() array.
> > > >
> > > > kernel_read_file() can be converted to use this API. The users will
> > > > need to be changed to call kernel_read_end(struct file *file, void *buf)
> > > > instead of vfree() so it can call allow_write_access() for them.
> > > >
> > > > vmap_file_range() has a lot of potential uses. I'm surprised we don't
> > > > have it already, to be honest.
> > >
> > > Prior to kernel_read_file() the same or verify similar code existed in
> > > multiple places in the kernel. The kernel_read_file() API
> > > consolidated the existing code adding the pre and post security hooks.
> > >
> > > With this new design of not using a private vmalloc, will the file
> > > data be accessible prior to the post security hooks? From an IMA
> > > perspective, the hooks are used for measuring and/or verifying the
> > > integrity of the file.
> >
> > File data is already accessible prior to the post security hooks.
> > Look how kernel_read_file works:
> >
> > ret = deny_write_access(file);
> > ret = security_kernel_read_file(file, id);
> > *buf = vmalloc(i_size);
> > bytes = kernel_read(file, *buf + pos, i_size - pos, &pos);
> > ret = security_kernel_post_read_file(file, *buf, i_size, id);
> >
> > kernel_read() will read the data into the page cache and then copy it
> > into the vmalloc'd buffer. There's nothing here to prevent read accesses
> > to the file.
>
> The post security hook needs to access to the file data in order to
> calculate the file hash. The question is whether prior to returning
> from kernel_read_file() the caller can access the file data.
In the case of firmware, I'm asking if the device will be able to
access the file data before kernel_read_file() returns.
Mimi
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