Re: [PATCH] arm64: kprobes: check the return value of set_memory_rox()

[Yang Shi]

On 11/4/25 5:44 AM, Ryan Roberts wrote:
> On 04/11/2025 13:14, Ryan Roberts wrote:
> > On 03/11/2025 19:45, Yang Shi wrote:
> > > Since commit a166563e7ec3 ("arm64: mm: support large block mapping when
> > > rodata=full"), __change_memory_common has more chance to fail due to
> > > memory allocation fialure when splitting page table. So check the return
> > > value of set_memory_rox(), then bail out if it fails otherwise we may have
> > > RW memory mapping for kprobes insn page.
> > >
> > > Fixes: 195a1b7d8388 ("arm64: kprobes: call set_memory_rox() for kprobe page")
> > > Signed-off-by: Yang Shi <yang@xxxxxxxxxxxxxxxxxxxxxx>
> > This patch looks correct so:
> >
> > Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx>

Thank you.

> > but, I think I see an separate issue below...
> >
> > > ---
> > > I actually epxected 195a1b7d8388 ("arm64: kprobes: call set_memory_rox()
> > > for kprobe page") can be merged in 6.17-rcX, so I just restored it to
> > > before commit 10d5e97c1bf8 ("arm64: use PAGE_KERNEL_ROX directly in
> > > alloc_insn_page"), however it turned out to be merged in 6.18-rc1 and it
> > > is after commit a166563e7ec3 ("arm64: mm: support large block mapping when
> > > rodata=full"). So I made the fix tag point to it.
> > > And I don't think we need to backport this patch to pre-6.18.
> > >
> > >   arch/arm64/kernel/probes/kprobes.c | 5 ++++-
> > >   1 file changed, 4 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
> > > index 8ab6104a4883..43a0361a8bf0 100644
> > > --- a/arch/arm64/kernel/probes/kprobes.c
> > > +++ b/arch/arm64/kernel/probes/kprobes.c
> > > @@ -49,7 +49,10 @@ void *alloc_insn_page(void)
> > >   	addr = execmem_alloc(EXECMEM_KPROBES, PAGE_SIZE);
> > >   	if (!addr)
> > >   		return NULL;
> > > -	set_memory_rox((unsigned long)addr, 1);
> > > +	if (set_memory_rox((unsigned long)addr, 1)) {
> > How does x get cleared when freeing this memory? arm64's set_memory_x() sets
> > PTE_MAYBE_GP and clears PTE_PXN. The only function that will revert that is
> > set_memory_nx(). But that only gets called from module_enable_data_nx() (which I
> > don't think is applicable here) and execmem_force_rw() - but only if
> > CONFIG_ARCH_HAS_EXECMEM_ROX is enabled, which I don't think it is for arm64?
> >
> > So I think once we flip a page executable, it will be executable forever?
> >
> > Do we need to modify set_direct_map_default_noflush() to make the memory nx?
> > Then vm_reset_perms() will fix it up at vfree time?
>
> Dev just pointed this out to me. Panic over!

Aha, yes, it doesn't clear PXN at all.

Thanks,
Yang

> static int change_memory_common(unsigned long addr, int numpages,
> 				pgprot_t set_mask, pgprot_t clear_mask)
> {
> 	...
>
> 	/*
> 	 * If we are manipulating read-only permissions, apply the same
> 	 * change to the linear mapping of the pages that back this VM area.
> 	 */
> 	if (rodata_full && (pgprot_val(set_mask) == PTE_RDONLY ||
> 			    pgprot_val(clear_mask) == PTE_RDONLY)) {
> 		for (i = 0; i < area->nr_pages; i++) {
> 			__change_memory_common(...);
> 		}
> 	}
>
> 	...
> }
>
>
> > Thanks,
> > Ryan
> >
> > > +		execmem_free(addr);
> > > +		return NULL;
> > > +	}
> > >   	return addr;
> > >   }
> > >