Re: [PATCH v10 2/8] ima: define and call ima_alloc_kexec_file_buf()

[Mimi Zohar]

On Wed, 2025-03-19 at 16:09 +0800, Baoquan He wrote:
> On 03/17/25 at 06:04pm, 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:
> >  - 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().
> >  - Make necessary changes to the function ima_add_kexec_buffer() to call
> >    the above two functions.
> 
> We may not need above details about code change because it's not so
> difficult to get them from patch.

Agreed.  The changes don't even reflect the current patch.  Please remove the
entire section.

> 
> > 
> > Signed-off-by: Tushar Sugandhi <tusharsu@xxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: steven chen <chenste@xxxxxxxxxxxxxxxxxxx>
> > ---
> >  security/integrity/ima/ima_kexec.c | 67 +++++++++++++++++++++++++-----
> >  1 file changed, 56 insertions(+), 11 deletions(-)
> > 
> > diff --git a/security/integrity/ima/ima_kexec.c b/security/integrity/ima/ima_kexec.c
> > index 8567619889d1..45170e283272 100644
> > --- a/security/integrity/ima/ima_kexec.c
> > +++ b/security/integrity/ima/ima_kexec.c
> > @@ -15,6 +15,48 @@
> >  #include "ima.h"
> >  
> >  #ifdef CONFIG_IMA_KEXEC
> > +static struct seq_file ima_kexec_file;
> > +
> > +static void ima_reset_kexec_file(struct seq_file *sf)
> > +{
> > +	sf->buf = NULL;
> > +	sf->size = 0;
> > +	sf->read_pos = 0;
> > +	sf->count = 0;
> > +}
> > +
> > +static void ima_free_kexec_file_buf(struct seq_file *sf)
> > +{
> > +	vfree(sf->buf);
> > +	ima_reset_kexec_file(sf);
> > +}
> > +
> > +static int ima_alloc_kexec_file_buf(size_t segment_size)
> > +{
> > +	/*
> > +	 * kexec 'load' may be called multiple times.
> > +	 * Free and realloc the buffer only if the segment_size is
> > +	 * changed from the previous kexec 'load' call.
> > +	 */
> > +	if (ima_kexec_file.buf && ima_kexec_file.size == segment_size)
> > +		goto out;

The call to ima_reset_kexec_file() in ima_add_kexec_buffer() resets
ima_kexec_file.buf() hiding the fact that the above test always fails and falls
through.  As a result, 'buf' is always being re-allocated.

> > +
> > +	ima_free_kexec_file_buf(&ima_kexec_file);
> > +
> > +	/* segment size can't change between kexec load and execute */
> > +	ima_kexec_file.buf = vmalloc(segment_size);
> > +	if (!ima_kexec_file.buf)
> > +		return -ENOMEM;
> > +
> > +	ima_kexec_file.size = segment_size;
> > +
> > +out:
> > +	ima_kexec_file.read_pos = 0;
> > +	ima_kexec_file.count = sizeof(struct ima_kexec_hdr);	/* reserved space */
> > +
> > +	return 0;
> > +}
> > +
> >  /*
> >   * Copy the measurement list to the allocated memory
> >   * compare the size of IMA measurement list with the size of the allocated memory
> > @@ -26,23 +68,16 @@
> >  static int ima_dump_measurement_list(unsigned long *buffer_size, void **buffer,
> >  				     unsigned long segment_size)
> >  {
> > -	struct seq_file ima_kexec_file;
> >  	struct ima_queue_entry *qe;
> >  	struct ima_kexec_hdr khdr;
> >  	int ret = 0;
> >  
> >  	/* segment size can't change between kexec load and execute */
> > -	ima_kexec_file.buf = vmalloc(segment_size);
> >  	if (!ima_kexec_file.buf) {
> > -		ret = -ENOMEM;
> > -		goto out;
> > +		pr_err("Kexec file buf not allocated\n");
> > +		return -EINVAL;
> >  	}
> >  
> > -	ima_kexec_file.file = NULL;
> > -	ima_kexec_file.size = segment_size;
> > -	ima_kexec_file.read_pos = 0;
> > -	ima_kexec_file.count = sizeof(khdr);	/* reserved space */
> > -
> >  	memset(&khdr, 0, sizeof(khdr));
> >  	khdr.version = 1;
> >  	/* This is an append-only list, no need to hold the RCU read lock */
> > @@ -79,8 +114,6 @@ static int ima_dump_measurement_list(unsigned long *buffer_size, void **buffer,
> >  	*buffer_size = ima_kexec_file.count;
> >  	*buffer = ima_kexec_file.buf;
> >  out:
> > -	if (ret == -EINVAL)
> > -		vfree(ima_kexec_file.buf);
> >  	return ret;
> >  }
> >  
> > @@ -119,6 +152,12 @@ void ima_add_kexec_buffer(struct kimage *image)
> >  		return;
> >  	}
> >  
> > +	ret = ima_alloc_kexec_file_buf(kexec_segment_size);
> > +	if (ret < 0) {
> > +		pr_err("Not enough memory for the kexec measurement buffer.\n");
> > +		return;
> > +	}
> > +
> >  	ima_dump_measurement_list(&kexec_buffer_size, &kexec_buffer,
> >  				  kexec_segment_size);
> >  	if (!kexec_buffer) {
> > @@ -140,6 +179,12 @@ void ima_add_kexec_buffer(struct kimage *image)
> >  	image->ima_buffer_size = kexec_segment_size;
> >  	image->ima_buffer = kexec_buffer;
> >  
> > +	/*
> > +	 * kexec owns kexec_buffer after kexec_add_buffer() is called
> > +	 * and it will vfree() that buffer.
> > +	 */
> > +	ima_reset_kexec_file(&ima_kexec_file);
> 
> I can't see why we need call ima_reset_kexec_file() here. If we need
> reuse the buffer, we will reset the needed fields at the end of
> ima_alloc_kexec_file_buf(). Not sure if I miss anything.

Without ima_reset_kexec_file(), calling 'kexec load' consecutively without
"kexec -s -e" in between fails.

# kexec -s -l /boot/vmlinuz-`uname -r` --initrd=/boot/initramfs-`uname -r`.img --reuse-cmdline
# kexec -s -l /boot/vmlinuz-`uname -r` --initrd=/boot/initramfs-`uname -r`.img --reuse-cmdline
Killed

As mentioned above, the call to ima_reset_kexec_file() resets
ima_kexec_file.buf, so the segment size test always fails and the memory is
being allocated.

Mimi

> 
> static int ima_alloc_kexec_file_buf(size_t segment_size)
> {
> ......
> out:
>         ima_kexec_file.read_pos = 0;
>         ima_kexec_file.count = sizeof(struct ima_kexec_hdr);    /* reserved space */
> 
>         return 0;
> }
> 
>