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

Jonathan Cameron
2024-02-14T12:36:22Z

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);
> >