linux-hardening - Re: [PATCH] PCI: Test for bit underflow in pcie_set

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Message-ID: <202509081437.1F92F56@keescook>
Date: Mon, 8 Sep 2025 14:39:50 -0700
From: Kees Cook <kees@...nel.org>
To: Bjorn Helgaas <helgaas@...nel.org>
Cc: Bjorn Helgaas <bhelgaas@...gle.com>,
	Linux Kernel Functional Testing <lkft@...aro.org>,
	Anders Roxell <anders.roxell@...aro.org>,
	Naresh Kamboju <naresh.kamboju@...aro.org>,
	lkft-triage@...ts.linaro.org,
	Linux Regressions <regressions@...ts.linux.dev>,
	Arnd Bergmann <arnd@...db.de>,
	Dan Carpenter <dan.carpenter@...aro.org>,
	Ben Copeland <benjamin.copeland@...aro.org>,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH] PCI: Test for bit underflow in pcie_set_readrq()

On Mon, Sep 08, 2025 at 03:53:49PM -0500, Bjorn Helgaas wrote:
> On Thu, Sep 04, 2025 at 10:28:41PM -0700, Kees Cook wrote:
> > After commit cbc654d18d37 ("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. :)
> > 
> > Fixes: cbc654d18d37 ("bitops: Add __attribute_const__ to generic ffs()-family implementations")
> 
> What's your plan for merging cbc654d18d37?  I suppose it's intended
> for v6.18?  If it will appear in v6.17, let me know so I can merge
> this for it as well.

I had it planned for v6.18.

> Maybe this should go in v6.17 regardless, to avoid a warning
> regression between this patch and cbc654d18d37?

Sure, or I could take it as part of the ffs series?

-Kees

-- 
Kees Cook