Messages in this thread | | | Subject | Re: [PATCH 0/3] fs: reduce export usage of kerne_read*() calls | From | Scott Branden <> | Date | Fri, 5 Jun 2020 11:15:21 -0700 |
| |
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. The >>>>> 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. This interface reads the file into a buffer once, calls the >>>>> generic post security hook, before providing the buffer to the caller. >>>>> (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(). The 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. ima_post_read_file() > passes a buffer to process_measurement() instead. > > Scott, the change should be straight forward. The 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. Update 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? { enum ima_hooks func; u32 secid;
if (read_id != READING_FIRMWARE_PARTIAL_READ) return 0;
if (!file) { /* should never happen */ if (ima_appraise & IMA_APPRAISE_ENFORCE) return -EACCES; return 0; }
security_task_getsecid(current, &secid); return process_measurement(file, current_cred(), secid, NULL, 0, MAY_READ, FILE_CHECK); } > > The built-in IMA measurement policy contains a rule to measure > firmware. The policy can be specified on the boot command line by > specifying "ima_policy=tcb". After reading the firmware, the firmware > measurement should be in <securityfs>/ima/ascii_runtime_measurements. > > thanks, > > Mimi
| |