PCI: endpoint: pci-epf-test: Fix double free that causes kernel to oops

Fix a kernel oops found while testing the stm32_pcie Endpoint driver
with handling of PERST# deassertion:

During EP initialization, pci_epf_test_alloc_space() allocates all BARs,
which are further freed if epc_set_bar() fails (for instance, due to no
free inbound window).

However, when pci_epc_set_bar() fails, the error path:

  pci_epc_set_bar() ->
    pci_epf_free_space()

does not clear the previous assignment to epf_test->reg[bar].

Then, if the host reboots, the PERST# deassertion restarts the BAR
allocation sequence with the same allocation failure (no free inbound
window), creating a double free situation since epf_test->reg[bar] was
deallocated and is still non-NULL.

Thus, make sure that pci_epf_alloc_space() and pci_epf_free_space()
invocations are symmetric, and as such, set epf_test->reg[bar] to NULL
when memory is freed.

Reviewed-by: Niklas Cassel <cassel@kernel.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Christian Bruel <christian.bruel@foss.st.com>
Link: https://lore.kernel.org/r/20250124123043.96112-1-christian.bruel@foss.st.com
[kwilczynski: commit log]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>

Author: bruelc

drivers/pci/endpoint/functions/pci-epf-test.c

@@ -738,6 +738,7 @@ static int pci_epf_test_set_bar(struct pci_epf *epf)
 		if (ret) {
 			pci_epf_free_space(epf, epf_test->reg[bar], bar,
 					   PRIMARY_INTERFACE);
+			epf_test->reg[bar] = NULL;
 			dev_err(dev, "Failed to set BAR%d\n", bar);
 			if (bar == test_reg_bar)
 				return ret;
@@ -929,6 +930,7 @@ static void pci_epf_test_free_space(struct pci_epf *epf)
 
 		pci_epf_free_space(epf, epf_test->reg[bar], bar,
 				   PRIMARY_INTERFACE);
+		epf_test->reg[bar] = NULL;
 	}
 }