cxl: Fix allowing bogus AFU descriptors with 0 maximum processes

If the AFU descriptor of an AFU directed AFU indicates that it supports
0 maximum processes, we will accept that value and attempt to use it.
The SPA will still be allocated (with 2 pages due to another minor bug
and room for 958 processes), and when a context is allocated we will
pass the value of 0 to idr_alloc as the maximum. However, idr_alloc will
treat that as meaning no maximum and will allocate a context number and
we return a valid context.

Conceivably, this could lead to a buffer overflow of the SPA if more
than 958 contexts were allocated, however this is mitigated by the fact
that there are no known AFUs in the wild with a bogus AFU descriptor
like this, and that only the root user is allowed to flash an AFU image
to a card.

Add a check when validating the AFU descriptor to reject any with 0
maximum processes.

We do still allow a dedicated process only AFU to indicate that it
supports 0 contexts even though that is forbidden in the architecture,
as in that case we ignore the value and use 1 instead. This is just on
the off-chance that such a dedicated process AFU may exist (not that I
am aware of any), since their developers are less likely to have cared
about this value at all.

Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Reviewed-by: Frederic Barrat <fbarrat@linux.vnet.ibm.com>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

diff --git a/drivers/misc/cxl/pci.c b/drivers/misc/cxl/pci.c
index 648817a2e219ebe4ed68e4f9c4530bcf30baa42b..58d7d8215964100da403ae907d9384d47463b833 100644 (file)
--- a/drivers/misc/cxl/pci.c
+++ b/drivers/misc/cxl/pci.c
@@ -775,6 +775,21 @@ static int cxl_afu_descriptor_looks_ok(struct cxl_afu *afu)
                }
        }
 
+       if ((afu->modes_supported & ~CXL_MODE_DEDICATED) && afu->max_procs_virtualised == 0) {
+               /*
+                * We could also check this for the dedicated process model
+                * since the architecture indicates it should be set to 1, but
+                * in that case we ignore the value and I'd rather not risk
+                * breaking any existing dedicated process AFUs that left it as
+                * 0 (not that I'm aware of any). It is clearly an error for an
+                * AFU directed AFU to set this to 0, and would have previously
+                * triggered a bug resulting in the maximum not being enforced
+                * at all since idr_alloc treats 0 as no maximum.
+                */
+               dev_err(&afu->dev, "AFU does not support any processes\n");
+               return -EINVAL;
+       }
+
        return 0;
 }