Re: [PATCHv2 2/3] powerpc/execmem: Allocate kprobes insn pages from the modules area on book3s/32

[Mike Rapoport <rppt@kernel.org>]

On Fri, Nov 13, 2026 at 06:01:37PM +0530, Ritesh Harjani (IBM) wrote:
> On book3s/32, a kprobe on an instruction that can't be emulated and has
> to be single stepped out of line, crashes the kernel. For e.g. with the
> following cmds...
>     ~> echo 9 > /proc/sys/kernel/printk
>     ~> mount -t debugfs debugfs /sys/kernel/debug
>     ~> echo 'p:wx filldir64+0xb8' > /sys/kernel/debug/tracing/kprobe_events
>     ~> echo 1 > /sys/kernel/debug/tracing/events/kprobes/wx/enable
>     ~> echo 1 > /sys/kernel/debug/tracing/events/kprobes/enable
> 
> ...we can see the following crash:
>     kernel tried to execute exec-protected page (f101a008) - exploit attempt? (uid: 0)
>     BUG: Unable to handle kernel instruction fetch
>     Faulting instruction address: 0xc02b919c
>     Vector: 400 (Instruction Access) at [f19a9d60]
>         pc: c02b919c: filldir64+0xb8/0x2dc
>         lr: c02b9148: filldir64+0x64/0x2dc
>         sp: f19a9e20
>        msr: 10029032
>       current = 0xc112e900
>         pid   = 102, comm = ls
>     enter ? for help
>     [f19a9e20] c1a8d420 (unreliable)
>     [f19a9e50] c02e4074 dcache_readdir+0x208/0x2a8
>     [f19a9e90] c02b8c5c iterate_dir+0x88/0x1fc
>     [f19a9ec0] c02b96e0 sys_getdents64+0x88/0x198
>     [f19a9f10] c0014400 system_call_exception+0xf8/0x20c
>     [f19a9f30] c00191ac ret_from_syscall+0x0/0x2c
>     ---- Exception: c00 (System Call) at a7596164
>     SP (af802310) is in userspace
> 
> This happens because on book3s/32 the exec permission is per segment
> (256MB), not per page. The page tables have no hardware exec bit, only
> the segment registers have a no-execute (N) bit. mmu_mark_initmem_nx()
> sets N on all the kernel segments except the ones of the modules area.
> So nothing in the vmalloc area can be executed, even when the page is
> mapped with PAGE_KERNEL_ROX or PAGE_KERNEL_EXEC, because _PAGE_EXEC is
> a software only bit there and the segment's N bit is what the MMU
> enforces.
> 
> Before commit 1b750c2fbf82 ("powerpc: extend execmem_params for kprobes
> allocations"), the kprobes insn pages came from module_alloc(). That
> commit moved them to the whole of the vmalloc space, on the grounds
> that kprobes use a breakpoint and don't need to be near the kernel
> text. That is fine on platforms where the exec permission is per page,
> as PAGE_KERNEL_ROX is then enough to make a vmalloc page executable.
> It is not on book3s/32, where the modules area is not only close to
> the kernel text but is also the only page-mapped area whose segment is
> left executable. So the insn pages must keep coming from there.
> 
> Hence, use the modules area for EXECMEM_KPROBES on book3s/32.
 
Right, I missed that on book3s/32 does vmalloc is not executable and its
protection can't be changed.

Could it be that 8xx also need a special treatment for kprobes?

> Fixes: 1b750c2fbf82 ("powerpc: extend execmem_params for kprobes allocations")
> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>

Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
>  arch/powerpc/mm/mem.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
> index 77b63b52d8f2..ae1bc7359353 100644
> --- a/arch/powerpc/mm/mem.c
> +++ b/arch/powerpc/mm/mem.c
> @@ -391,6 +391,7 @@ struct execmem_info __init *execmem_arch_setup(void)
>  	pgprot_t kprobes_prot = strict_kernel_rwx_enabled() ? PAGE_KERNEL_ROX : PAGE_KERNEL_EXEC;
>  	pgprot_t prot = strict_module_rwx_enabled() ? PAGE_KERNEL : PAGE_KERNEL_EXEC;
>  	unsigned long fallback_start = 0, fallback_end = 0;
> +	unsigned long kprobes_start = VMALLOC_START, kprobes_end = VMALLOC_END;
>  	unsigned long start, end;
> 
>  	/*
> @@ -410,6 +411,11 @@ struct execmem_info __init *execmem_arch_setup(void)
>  	}
> 
>  	end = MODULES_END;
> +
> +	if (IS_ENABLED(CONFIG_PPC_BOOK3S_32)) {
> +		kprobes_start = MODULES_VADDR;
> +		kprobes_end = MODULES_END;
> +	}
>  #else
>  	start = VMALLOC_START;
>  	end = VMALLOC_END;
> @@ -428,8 +434,8 @@ struct execmem_info __init *execmem_arch_setup(void)
>  				.fallback_end	= fallback_end,
>  			},
>  			[EXECMEM_KPROBES] = {
> -				.start	= VMALLOC_START,
> -				.end	= VMALLOC_END,
> +				.start	= kprobes_start,
> +				.end	= kprobes_end,
>  				.pgprot	= kprobes_prot,
>  				.alignment = 1,
>  			},
> --
> 2.39.5
> 
> 

-- 
Sincerely yours,
Mike.