Re: [REGRESSION] NULL pointer dereference on ARM (AT91SAM9G25) during compaction

["Russell King (Oracle)" <linux@armlinux.org.uk>]

On Mon, Feb 10, 2025 at 05:49:38PM +0100, Ezra Buehler wrote:
> When running vanilla Linux 6.13 or newer (6.14-rc2) on the
> AT91SAM9G25-based GARDENA smart Gateway, we are seeing a NULL pointer
> dereference resulting in a kernel panic. The culprit seems to be commit
> fc9c45b71f43 ("arm: adjust_pte() usepte_offset_map_rw_nolock()").
> Reverting the commit apparently fixes the issue.

The blamed commit is buggy:

arch/arm/include/asm/tlbflush.h:
#define update_mmu_cache(vma, addr, ptep) \
        update_mmu_cache_range(NULL, vma, addr, ptep, 1)

So vmf can be NULL. This didn't used to matter before this commit,
because vmf was not used by ARM's update_mmu_cache_range(). However,
the commit introduced a dereference of it, which now causes a NULL
point dereference.

Not sure what the correct solution is, but at a guess, both:

	if (ptl != vmf->ptl)

need to become:

	if (!vmf || ptl != vmf->ptl)

but I haven't checked wha tthe locking context actually is here
(I've been out of MM stuff too long to know this off the top of my
head.)

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