linux-kernel - Re: [PATCH v9 1/7] ima: copy only complete measurement records across kexec

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Message-ID: <631f326006226e23f4f755fd32255792f6514a90.camel@linux.ibm.com>
Date: Tue, 11 Mar 2025 08:44:33 -0400
From: Mimi Zohar <zohar@...ux.ibm.com>
To: steven chen <chenste@...ux.microsoft.com>, Baoquan He <bhe@...hat.com>
Cc: stefanb@...ux.ibm.com, roberto.sassu@...weicloud.com,
        roberto.sassu@...wei.com, eric.snowberg@...cle.com,
        ebiederm@...ssion.com, paul@...l-moore.com, code@...icks.com,
        bauermann@...abnow.com, linux-integrity@...r.kernel.org,
        kexec@...ts.infradead.org, linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, madvenka@...ux.microsoft.com,
        nramas@...ux.microsoft.com, James.Bottomley@...senpartnership.com,
        vgoyal@...hat.com, dyoung@...hat.com
Subject: Re: [PATCH v9 1/7] ima: copy only complete measurement records
 across kexec

On Thu, 2025-03-06 at 21:51 -0500, Mimi Zohar wrote:
> On Thu, 2025-03-06 at 14:45 -0800, steven chen wrote:
> > On 3/5/2025 4:27 AM, Mimi Zohar wrote:
> > > On Wed, 2025-03-05 at 20:08 +0800, Baoquan He wrote:
> > > > On 03/04/25 at 11:03am, steven chen wrote:
> > > > > Carrying the IMA measurement list across kexec requires allocating a
> > > > > buffer and copying the measurement records.  Separate allocating the
> > > > > buffer and copying the measurement records into separate functions in
> > > > > order to allocate the buffer at kexec 'load' and copy the measurements
> > > > > at kexec 'execute'.
> > > > > 
> > > > > This patch includes the following changes:
> > > > I don't know why one patch need include so many changes. From below log,
> > > > it should be split into separate patches. It may not need to make one
> > > > patch to reflect one change, we should at least split and wrap several
> > > > kind of changes to ease patch understanding and reviewing. My personal
> > > > opinion.
> > > Agreed, well explained.
> > > 
> > > Mimi
> > > 
> > > > >   - Refactor ima_dump_measurement_list() to move the memory allocation
> > > > >     to a separate function ima_alloc_kexec_file_buf() which allocates
> > > > >     buffer of size 'kexec_segment_size' at kexec 'load'.
> > > > >   - Make the local variable ima_kexec_file in ima_dump_measurement_list()
> > > > >     a local static to the file, so that it can be accessed from
> > > > >     ima_alloc_kexec_file_buf(). Compare actual memory required to ensure
> > > > >     there is enough memory for the entire measurement record.
> > > > >   - Copy only complete measurement records.
> > > > >   - Make necessary changes to the function ima_add_kexec_buffer() to call
> > > > >     the above two functions.
> > > > >   - Compared the memory size allocated with memory size of the entire
> > > > >     measurement record. Copy only complete measurement records if there
> > > > >     is enough memory. If there is not enough memory, it will not copy
> > > > >     any IMA measurement records, and this situation will result in a
> > > > >     failure of remote attestation.
> > > > > 
> > > > > Suggested-by: Mimi Zohar <zohar@...ux.ibm.com>
> > > > > Signed-off-by: Tushar Sugandhi <tusharsu@...ux.microsoft.com>
> > > > > Signed-off-by: steven chen <chenste@...ux.microsoft.com>
> > 
> > I will split this patch into the following two patches:
> > 
> >      ima: define and call ima_alloc_kexec_file_buf
> >      ima: copy measurement records as much as possible across kexec
> 
> Steven, breaking up code into patches is in order to simplify patch review. 
> This is done by limiting each patch to a single "logical change" [1].  For
> example, the change below has nothing to do with "separate allocating the buffer
> and copying the measurement records into separate functions".
> 
>         /* This is an append-only list, no need to hold the RCU read lock */
>         list_for_each_entry_rcu(qe, &ima_measurements, later, true) {
> -               if (file.count < file.size) {
> +               entry_size += ima_get_binary_runtime_entry_size(qe->entry); 
> +               if (entry_size <= segment_size) {
>                         khdr.count++;
> -                       ima_measurements_show(&file, qe);
> +                       ima_measurements_show(&ima_kexec_file, qe);
>                 } else {
>                         ret = -EINVAL;
> +                       pr_err("IMA log file is too big for Kexec buf\n");
>                         break;
>                 }
>         }
> 
> The original code potentially copied a partial last measurement record, not a
> complete measurement record.  For ease of review, the above change is fine, but
> it needs to be a separate patch.
> 
> Patches:
> 1. ima: copy only complete measurement records across kexec
> 2. ima: define and call ima_alloc_kexec_file_buf()

Steven,

The alternative would be to revert using ima_get_binary_runtime_entry_size() and
simply use "ima_kexec_file.count < ima_kexec_file.size".  Only
ima_kexec_file.size would be initialized in ima_alloc_kexec_buf().  The rest
would remain in ima_dump_measurement_list().  get_binary_runtime_size() wouldn't
need to be made global.

To further simplify the patch review, first define a separate patch to just
rename the seq_file "file" to "ima_kexec_file".

Mimi