Re: [PATCH V2] riscv: kexec: Fixup synchronization problem between init_mm and active_mm

[Palmer Dabbelt]

On Tue, 11 Jul 2023 04:07:22 PDT (-0700), alex@xxxxxxxx wrote:
> Hi Guo,
>
>
> On 10/07/2023 07:40, guoren@xxxxxxxxxx wrote:
> > From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> >
> > The machine_kexec() uses set_memory_x to modify the direct mapping
> > attributes from RW to RWX. But set_memory_x only changes the init_mm's
> > attributes, not current->active_mm, so when kexec jumps into
> > control_buffer, the instruction page fault happens, and there is no
> > minor_pagefault for it, then panic.
>
>
> I think it needs more details like this:
>
> "The current implementation of set_memory_x does not split hugepages in
> the linear mapping and then when a PGD mapping is used, the whole PGD is
> marked as executable. But changing the permissions at the PGD level must
> be propagated to all the page tables."
>
>
> > The bug is found on an MMU_sv39 machine, and the direct mapping used a
> > 1GB PUD, the pgd entries. Here is the bug output:
> >
> >   kexec_core: Starting new kernel
> >   Will call new kernel at 00300000 from hart id 0
> >   FDT image at 747c7000
> >   Bye...
> >   Unable to handle kernel paging request at virtual address ffffffda23b0d000
> >   Oops [#1]
> >   Modules linked in:
> >   CPU: 0 PID: 53 Comm: uinit Not tainted 6.4.0-rc6 #15
> >   Hardware name: Sophgo Mango (DT)
> >   epc : 0xffffffda23b0d000
> >    ra : machine_kexec+0xa6/0xb0
> >   epc : ffffffda23b0d000 ra : ffffffff80008272 sp : ffffffc80c173d10
> >    gp : ffffffff8150e1e0 tp : ffffffd9073d2c40 t0 : 0000000000000000
> >    t1 : 0000000000000042 t2 : 6567616d69205444 s0 : ffffffc80c173d50
> >    s1 : ffffffd9076c4800 a0 : ffffffd9076c4800 a1 : 0000000000300000
> >    a2 : 00000000747c7000 a3 : 0000000000000000 a4 : ffffffd800000000
> >    a5 : 0000000000000000 a6 : ffffffd903619c40 a7 : ffffffffffffffff
> >    s2 : ffffffda23b0d000 s3 : 0000000000300000 s4 : 00000000747c7000
> >    s5 : 0000000000000000 s6 : 0000000000000000 s7 : 0000000000000000
> >    s8 : 0000000000000000 s9 : 0000000000000000 s10: 0000000000000000
> >    s11: 0000003f940001a0 t3 : ffffffff815351af t4 : ffffffff815351af
> >    t5 : ffffffff815351b0 t6 : ffffffc80c173b50
> >   status: 0000000200000100 badaddr: ffffffda23b0d000 cause: 000000000000000c
> >
> > The solution is to fix machine_kexec() to remap control code page outside
> > the linear mapping.
>
>
> "Given the current flaw in the set_memory_x implementation, the simplest
> solution is to ..."
>
>
> > Fixes: 3335068f8721 ("riscv: Use PUD/P4D/PGD pages for the linear mapping")
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
> > Cc: Alexandre Ghiti <alex@xxxxxxxx>
> > ---
> > Changelog:
> > V2:
> >   - Use vm_map_ram instead of modifying set_memory_x
> >   - Correct Fixes tag
> > ---
> >   arch/riscv/include/asm/kexec.h    |  1 +
> >   arch/riscv/kernel/machine_kexec.c | 14 ++++++++++----
> >   2 files changed, 11 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/riscv/include/asm/kexec.h b/arch/riscv/include/asm/kexec.h
> > index 2b56769cb530..17456e91476e 100644
> > --- a/arch/riscv/include/asm/kexec.h
> > +++ b/arch/riscv/include/asm/kexec.h
> > @@ -41,6 +41,7 @@ crash_setup_regs(struct pt_regs *newregs,
> >   struct kimage_arch {
> >   	void *fdt; /* For CONFIG_KEXEC_FILE */
> >   	unsigned long fdt_addr;
> > +	void *control_code_buffer;
> >   };
> >
> >   extern const unsigned char riscv_kexec_relocate[];
> > diff --git a/arch/riscv/kernel/machine_kexec.c b/arch/riscv/kernel/machine_kexec.c
> > index 2d139b724bc8..eeb209775107 100644
> > --- a/arch/riscv/kernel/machine_kexec.c
> > +++ b/arch/riscv/kernel/machine_kexec.c
> > @@ -86,7 +86,14 @@ machine_kexec_prepare(struct kimage *image)
> >
> >   	/* Copy the assembler code for relocation to the control page */
> >   	if (image->type != KEXEC_TYPE_CRASH) {
> > -		control_code_buffer = page_address(image->control_code_page);
> > +		control_code_buffer = vm_map_ram(&image->control_code_page,
> > +						 KEXEC_CONTROL_PAGE_SIZE/PAGE_SIZE,
> > +						 NUMA_NO_NODE);
> > +		if (control_code_buffer == NULL) {
> > +			pr_err("Failed to vm_map control page\n");
> > +			return -ENOMEM;
> > +		}
> > +
> >   		control_code_buffer_sz = page_size(image->control_code_page);
> >
> >   		if (unlikely(riscv_kexec_relocate_size > control_code_buffer_sz)) {
> > @@ -97,8 +104,7 @@ machine_kexec_prepare(struct kimage *image)
> >   		memcpy(control_code_buffer, riscv_kexec_relocate,
> >   			riscv_kexec_relocate_size);
> >
> > -		/* Mark the control page executable */
> > -		set_memory_x((unsigned long) control_code_buffer, 1);
> > +		internal->control_code_buffer = control_code_buffer;
>
>
> Where is this mapping marked as executable? I see that vm_map_ram() maps
> the pages as PAGE_KERNEL, which does not set PAGE_EXEC.
>
>
> >   	}
> >
> >   	return 0;
> > @@ -211,7 +217,7 @@ machine_kexec(struct kimage *image)
> >   	unsigned long this_cpu_id = __smp_processor_id();
> >   	unsigned long this_hart_id = cpuid_to_hartid_map(this_cpu_id);
> >   	unsigned long fdt_addr = internal->fdt_addr;
> > -	void *control_code_buffer = page_address(image->control_code_page);
> > +	void *control_code_buffer = internal->control_code_buffer;
> >   	riscv_kexec_method kexec_method = NULL;
> >
> >   #ifdef CONFIG_SMP
>
>
> Otherwise, you can add:
>
> Reviewed-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx>
>
> Thanks,
>
> Alex

Thanks for looking at this.  Guo: do you have a re-spit that fixes the 
issues Alex pointed out?  Sorry if I just missed it.