Re: [PATCH 0/3] fs: reduce export usage of kerne_read*() calls
From: Scott Branden
Date: Fri Jun 05 2020 - 14:15:33 EST
Hi Mimi,
On 2020-05-23 7:52 p.m., Mimi Zohar wrote:
On Fri, 2020-05-22 at 16:25 -0700, Scott Branden wrote:
Hi Kees,
On 2020-05-22 4:04 p.m., Kees Cook wrote:
On Fri, May 22, 2020 at 03:24:32PM -0700, Scott Branden wrote:
On 2020-05-18 5:37 a.m., Mimi Zohar wrote:
On Sun, 2020-05-17 at 23:22 -0700, Christoph Hellwig wrote:
On Fri, May 15, 2020 at 09:29:33PM +0000, Luis Chamberlain wrote:
On Wed, May 13, 2020 at 11:17:36AM -0700, Christoph Hellwig wrote:
Can you also move kernel_read_* out of fs.h? That header gets pulled
in just about everywhere and doesn't really need function not related
to the general fs interface.
Sure, where should I dump these?
Maybe a new linux/kernel_read_file.h? Bonus points for a small top
of the file comment explaining the point of the interface, which I
still don't get :)
Instead of rolling your own method of having the kernel read a file,
which requires call specific security hooks, this interface provides a
single generic set of pre and post security hooks.ÂÂThe
kernel_read_file_id enumeration permits the security hook to
differentiate between callers.
To comply with secure and trusted boot concepts, a file cannot be
accessible to the caller until after it has been measured and/or the
integrity (hash/signature) appraised.
In some cases, the file was previously read twice, first to measure
and/or appraise the file and then read again into a buffer for
use.ÂÂThis interface reads the file into a buffer once, calls the
generic post security hook, before providing the buffer to the caller.
Â(Note using firmware pre-allocated memory might be an issue.)
Partial reading firmware will result in needing to pre-read the entire
file, most likely on the security pre hook.
The entire file may be very large and not fit into a buffer.
Hence one of the reasons for a partial read of the file.
For security purposes, you need to change your code to limit the amount
of data it reads into a buffer at one time to not consume or run out of much
memory.
Hm? That's not how whole-file hashing works. :)
These hooks need to finish their hashing and policy checking before they
can allow the rest of the code to move forward. (That's why it's a
security hook.) If kernel memory utilization is the primary concern,
then sure, things could be rearranged to do partial read and update the
hash incrementally, but the entire file still needs to be locked,
entirely hashed by hook, then read by the caller, then unlocked and
released.
Exactly.
So, if you want to have partial file reads work, you'll need to
rearchitect the way this works to avoid regressing the security coverage
of these operations.
I am not familiar with how the security handling code works at all.
Is the same security check run on files opened from user space?
A file could be huge.
If it assumes there is there is enough memory available to read the
entire file into kernel space then the improvement below can be left as
a memory optimization to be done in an independent (or future) patch series.
There are two security hooks - security_kernel_read_file(),
security_kernel_post_read_file - in kernel_read_file(). ÂThe first
hook is called before the file is read into a buffer, while the second
hook is called afterwards.
For partial reads, measuring the firmware and verifying the firmware's
signature will need to be done on the security_kernel_read_file()
hook.
So, probably, the code will look something like:
file = kernel_open_file_for_reading(...)
file = open...
disallow_writes(file);
while (processed < size-of-file) {
buf = read(file, size...)
security_file_read_partial(buf)
}
ret = security_file_read_finished(file);
if (ret < 0) {
allow_writes(file);
return PTR_ERR(ret);
}
return file;
while (processed < size-of-file) {
buf = read(file, size...)
firmware_send_partial(buf);
}
kernel_close_file_for_reading(file)
allow_writes(file);
Right, the ima_file_mmap(), ima_bprm_check(), and ima_file_check()
hooks call process_measurement() to do this. Âima_post_read_file()
passes a buffer to process_measurement() instead.
Scott, the change should be straight forward. ÂThe additional patch
needs to:
- define a new kernel_read_file_id enumeration, like
FIRMWARE_PARTIAL_READ.
- Currently ima_read_file() has a comment about pre-allocated firmware
buffers. ÂUpdate ima_read_file() to call process_measurement() for the
new enumeration FIRMWARE_PARTIAL_READ and update ima_post_read_file()
to return immediately.
Should this be what is in ima_read_file?
{
ÂÂÂ enum ima_hooks func;
ÂÂÂ u32 secid;
ÂÂÂ if (read_id != READING_FIRMWARE_PARTIAL_READ)
ÂÂÂ ÂÂÂ return 0;
ÂÂÂ if (!file) { /* should never happen */
ÂÂÂ ÂÂÂ if (ima_appraise & IMA_APPRAISE_ENFORCE)
ÂÂÂ ÂÂÂ ÂÂÂ return -EACCES;
ÂÂÂ ÂÂÂ return 0;
ÂÂÂ }
ÂÂÂ security_task_getsecid(current, &secid);
ÂÂÂ return process_measurement(file, current_cred(), secid, NULL,
ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂ 0, MAY_READ, FILE_CHECK);
}
The built-in IMA measurement policy contains a rule to measure
firmware. ÂThe policy can be specified on the boot command line by
specifying "ima_policy=tcb". ÂAfter reading the firmware, the firmware
measurement should be in <securityfs>/ima/ascii_runtime_measurements.
thanks,
Mimi