Re: [PATCH v2 3/3] lib/firmware_table: Provide buffer length argument to cdat_table_parse()

[Jonathan Cameron <Jonathan.Cameron@Huawei.com>]

On Fri, 9 Feb 2024 12:23:39 +0100
Robert Richter <rrichter@amd.com> wrote:

> On 26.01.24 16:46:03, Jonathan Cameron wrote:
> > On Mon, 8 Jan 2024 12:48:33 +0100
> > Robert Richter <rrichter@amd.com> wrote:
> >   
> > > The last entry in the CDAT table may not mark the end of the CDAT
> > > table buffer specified by the length field in the CDAT header. It can
> > > be shorter with trailing unused (zero'ed) data. The actual table
> > > length is determined when reading all CDAT entries of the table with
> > > DOE.  
> > 
> > Can you give some reasons why this would occur?  
> 
> I have seen card implementations where the CDAT table is some sort of
> fix buffer, but with entries filled in that do not fill the whole
> table length size. Which means that the last DOE ends earlier than the
> table end that then contains padding bytes. Spec is not entierly clear
> here. It could be interpreted as a spec violation, but DOE is the card
> vendor's firmware there is not much that can be done to fix that
> there...

We can fall back on the usual kernel developer nudge solution.
Print a rude message and hope their customers tell them to fix it ;)

> 
> > 
> > Need to be clear if this is:
> > 1) Hardening against device returning borked table.  
> 
> So this was the main motivation. It will be likely there are more
> cards with that issue.
> 
> > 2) Hardening against in flight update of CDAT racing with the readout
> >    (not sure table can change size, but maybe.. I haven't checked).  
> 
> That is a side effect I realized while implementing this patch. To
> prevent an out of bound buffer access, either the buffer needs a
> length argument or the length field in the header needs to be checked.
> The earlier is more reasonable and natural to me as no buffer insight
> is needed for that.
> 
> > 3) DW read back vs packed structures?  
> 
> Good point. But that was not the reason. IIR the PCI DOE code
> correctly, there is a DW access but only the missing bytes are written
> to the buffer. The CDAT structures are all DW aligned, but the DOE
> header isn't.

Yes. I hoped that was still working :)

> 
> > 
> > Patch seems reasonable to me, I'd just like a clear statement of why
> > it happens!  
> 
> So regardless of padding bytes or not, a length check is required in
> any case, one or the other way.
> 
> Will update the patch description.

Great.
> 
> Thanks,
> 
> -Robert
> 
> > 
> > Jonathan
> >   
> > > 
> > > If the table is greater than expected (containing zero'ed trailing
> > > data), the CDAT parser fails with:
> > > 
> > >  [   48.691717] Malformed DSMAS table length: (24:0)
> > >  [   48.702084] [CDAT:0x00] Invalid zero length
> > >  [   48.711460] cxl_port endpoint1: Failed to parse CDAT: -22
> > > 
> > > In addition, the table buffer size can be different from the size
> > > specified in the length field. This may cause out-of-bound access then
> > > parsing the CDAT table.
> > > 
> > > Fix that by providing an optonal buffer length argument to
> > > acpi_parse_entries_array() that can be used by cdat_table_parse() to
> > > propagate the buffer size down to its users.
> > > 
> > > Cc: "Rafael J. Wysocki" <rafael@kernel.org>
> > > Cc: Len Brown <lenb@kernel.org>
> > > Signed-off-by: Robert Richter <rrichter@amd.com>
> > > ---
> > >  drivers/acpi/tables.c    |  2 +-
> > >  drivers/cxl/core/cdat.c  |  6 +++---
> > >  include/linux/fw_table.h |  4 +++-
> > >  lib/fw_table.c           | 15 ++++++++++-----
> > >  4 files changed, 17 insertions(+), 10 deletions(-)
> > > 
> > > diff --git a/drivers/acpi/tables.c b/drivers/acpi/tables.c
> > > index b07f7d091d13..b976e5fc3fbc 100644
> > > --- a/drivers/acpi/tables.c
> > > +++ b/drivers/acpi/tables.c
> > > @@ -253,7 +253,7 @@ int __init_or_acpilib acpi_table_parse_entries_array(
> > >  
> > >  	count = acpi_parse_entries_array(id, table_size,
> > >  					 (union fw_table_header *)table_header,
> > > -					 proc, proc_num, max_entries);
> > > +					 0, proc, proc_num, max_entries);
> > >  
> > >  	acpi_put_table(table_header);
> > >  	return count;
> > > diff --git a/drivers/cxl/core/cdat.c b/drivers/cxl/core/cdat.c
> > > index 6fe11546889f..012d8f2a7945 100644
> > > --- a/drivers/cxl/core/cdat.c
> > > +++ b/drivers/cxl/core/cdat.c
> > > @@ -149,13 +149,13 @@ static int cxl_cdat_endpoint_process(struct cxl_port *port,
> > >  	int rc;
> > >  
> > >  	rc = cdat_table_parse(ACPI_CDAT_TYPE_DSMAS, cdat_dsmas_handler,
> > > -			      dsmas_xa, port->cdat.table);
> > > +			      dsmas_xa, port->cdat.table, port->cdat.length);
> > >  	rc = cdat_table_parse_output(rc);
> > >  	if (rc)
> > >  		return rc;
> > >  
> > >  	rc = cdat_table_parse(ACPI_CDAT_TYPE_DSLBIS, cdat_dslbis_handler,
> > > -			      dsmas_xa, port->cdat.table);
> > > +			      dsmas_xa, port->cdat.table, port->cdat.length);
> > >  	return cdat_table_parse_output(rc);
> > >  }
> > >  
> > > @@ -511,7 +511,7 @@ void cxl_switch_parse_cdat(struct cxl_port *port)
> > >  		return;
> > >  
> > >  	rc = cdat_table_parse(ACPI_CDAT_TYPE_SSLBIS, cdat_sslbis_handler,
> > > -			      port, port->cdat.table);
> > > +			      port, port->cdat.table, port->cdat.length);
> > >  	rc = cdat_table_parse_output(rc);
> > >  	if (rc)
> > >  		dev_dbg(&port->dev, "Failed to parse SSLBIS: %d\n", rc);
> > > diff --git a/include/linux/fw_table.h b/include/linux/fw_table.h
> > > index 95421860397a..3ff4c277296f 100644
> > > --- a/include/linux/fw_table.h
> > > +++ b/include/linux/fw_table.h
> > > @@ -40,12 +40,14 @@ union acpi_subtable_headers {
> > >  
> > >  int acpi_parse_entries_array(char *id, unsigned long table_size,
> > >  			     union fw_table_header *table_header,
> > > +			     unsigned long max_length,
> > >  			     struct acpi_subtable_proc *proc,
> > >  			     int proc_num, unsigned int max_entries);
> > >  
> > >  int cdat_table_parse(enum acpi_cdat_type type,
> > >  		     acpi_tbl_entry_handler_arg handler_arg, void *arg,
> > > -		     struct acpi_table_cdat *table_header);
> > > +		     struct acpi_table_cdat *table_header,
> > > +		     unsigned long length);
> > >  
> > >  /* CXL is the only non-ACPI consumer of the FIRMWARE_TABLE library */
> > >  #if IS_ENABLED(CONFIG_ACPI) && !IS_ENABLED(CONFIG_CXL_BUS)
> > > diff --git a/lib/fw_table.c b/lib/fw_table.c
> > > index 1e5e0b2f7012..ddb67853b7ac 100644
> > > --- a/lib/fw_table.c
> > > +++ b/lib/fw_table.c
> > > @@ -132,6 +132,7 @@ static __init_or_fwtbl_lib int call_handler(struct acpi_subtable_proc *proc,
> > >   *
> > >   * @id: table id (for debugging purposes)
> > >   * @table_size: size of the root table
> > > + * @max_length: maximum size of the table (ignore if 0)
> > >   * @table_header: where does the table start?
> > >   * @proc: array of acpi_subtable_proc struct containing entry id
> > >   *        and associated handler with it
> > > @@ -153,10 +154,11 @@ static __init_or_fwtbl_lib int call_handler(struct acpi_subtable_proc *proc,
> > >  int __init_or_fwtbl_lib
> > >  acpi_parse_entries_array(char *id, unsigned long table_size,
> > >  			 union fw_table_header *table_header,
> > > +			 unsigned long max_length,
> > >  			 struct acpi_subtable_proc *proc,
> > >  			 int proc_num, unsigned int max_entries)
> > >  {
> > > -	unsigned long table_end, subtable_len, entry_len;
> > > +	unsigned long table_len, table_end, subtable_len, entry_len;
> > >  	struct acpi_subtable_entry entry;
> > >  	enum acpi_subtable_type type;
> > >  	int count = 0;
> > > @@ -164,8 +166,10 @@ acpi_parse_entries_array(char *id, unsigned long table_size,
> > >  	int i;
> > >  
> > >  	type = acpi_get_subtable_type(id);
> > > -	table_end = (unsigned long)table_header +
> > > -		    acpi_table_get_length(type, table_header);
> > > +	table_len = acpi_table_get_length(type, table_header);
> > > +	if (max_length && max_length < table_len)
> > > +		table_len = max_length;
> > > +	table_end = (unsigned long)table_header + table_len;
> > >  
> > >  	/* Parse all entries looking for a match. */
> > >  
> > > @@ -220,7 +224,8 @@ int __init_or_fwtbl_lib
> > >  cdat_table_parse(enum acpi_cdat_type type,
> > >  		 acpi_tbl_entry_handler_arg handler_arg,
> > >  		 void *arg,
> > > -		 struct acpi_table_cdat *table_header)
> > > +		 struct acpi_table_cdat *table_header,
> > > +		 unsigned long length)
> > >  {
> > >  	struct acpi_subtable_proc proc = {
> > >  		.id		= type,
> > > @@ -234,6 +239,6 @@ cdat_table_parse(enum acpi_cdat_type type,
> > >  	return acpi_parse_entries_array(ACPI_SIG_CDAT,
> > >  					sizeof(struct acpi_table_cdat),
> > >  					(union fw_table_header *)table_header,
> > > -					&proc, 1, 0);
> > > +					length, &proc, 1, 0);
> > >  }
> > >  EXPORT_SYMBOL_FWTBL_LIB(cdat_table_parse);  
> >