Commit 00e58ff9 authored Sep 04, 2025 by Kees Cook

PCI: Test for bit underflow in pcie_set_readrq()



In preparation for the future commit ("bitops: Add __attribute_const__ to generic
ffs()-family implementations"), which allows GCC's value range tracker
to see past ffs(), GCC 8 on ARM thinks that it might be possible that
"ffs(rq) - 8" used here:

	v = FIELD_PREP(PCI_EXP_DEVCTL_READRQ, ffs(rq) - 8);

could wrap below 0, leading to a very large value, which would be out of
range for the FIELD_PREP() usage:

drivers/pci/pci.c: In function 'pcie_set_readrq':
include/linux/compiler_types.h:572:38: error: call to '__compiletime_assert_471' declared with attribute error: FIELD_PREP: value too large for the field
...
drivers/pci/pci.c:5896:6: note: in expansion of macro 'FIELD_PREP'
  v = FIELD_PREP(PCI_EXP_DEVCTL_READRQ, ffs(rq) - 8);
      ^~~~~~~~~~

If the result of the ffs() is bounds checked before being used in
FIELD_PREP(), the value tracker seems happy again. :)

Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Closes: https://lore.kernel.org/linux-pci/CA+G9fYuysVr6qT8bjF6f08WLyCJRG7aXAeSd2F7=zTaHHd7L+Q@mail.gmail.com/


Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20250905052836.work.425-kees@kernel.org


Signed-off-by: Kees Cook <kees@kernel.org>

parent c17b750b

drivers/pci/pci.c

+5 −1

Original line number	Diff line number	Diff line
		@@ -5932,6 +5932,7 @@ int pcie_set_readrq(struct pci_dev *dev, int rq)
		{
		u16 v;
		int ret;
		unsigned int firstbit;
		struct pci_host_bridge *bridge = pci_find_host_bridge(dev->bus);

		if (rq < 128 \|\| rq > 4096 \|\| !is_power_of_2(rq))
		@@ -5949,7 +5950,10 @@ int pcie_set_readrq(struct pci_dev *dev, int rq)
		rq = mps;
		}

		v = FIELD_PREP(PCI_EXP_DEVCTL_READRQ, ffs(rq) - 8);
		firstbit = ffs(rq);
		if (firstbit < 8)
		return -EINVAL;
		v = FIELD_PREP(PCI_EXP_DEVCTL_READRQ, firstbit - 8);

		if (bridge->no_inc_mrrs) {
		int max_mrrs = pcie_get_readrq(dev);