linux-kernel - Re: [PATCH v2 6/9] s390/module: Use s390_kernel

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Date:   Thu, 23 Apr 2020 22:35:21 -0400
From:   Joe Lawrence <joe.lawrence@...hat.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Miroslav Benes <mbenes@...e.cz>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Jessica Yu <jeyu@...nel.org>, linux-s390@...r.kernel.org,
        heiko.carstens@...ibm.com, Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [PATCH v2 6/9] s390/module: Use s390_kernel_write() for late
 relocations

On Thu, Apr 23, 2020 at 06:26:57PM -0500, Josh Poimboeuf wrote:
> On Thu, Apr 23, 2020 at 01:10:30PM -0500, Josh Poimboeuf wrote:
> > Mystery solved:
> > 
> >   $ CROSS_COMPILE=s390x-linux-gnu- scripts/faddr2line vmlinux apply_rela+0x16a/0x520
> >   apply_rela+0x16a/0x520:
> >   apply_rela at arch/s390/kernel/module.c:336
> > 
> > which corresponds to the following code in apply_rela():
> > 
> > 
> > 	case R_390_PLTOFF64:	/* 16 bit offset from GOT to PLT. */
> > 		if (info->plt_initialized == 0) {
> > 			unsigned int *ip;
> > 			ip = me->core_layout.base + me->arch.plt_offset +
> > 				info->plt_offset;
> > 			ip[0] = 0x0d10e310;	/* basr 1,0  */
> > 			ip[1] = 0x100a0004;	/* lg	1,10(1) */
> > 
> > 
> > Notice how it's writing directly to text... oops.
> 
> Here's a fix, using write() for the PLT and the GOT.
> 
> diff --git a/arch/s390/kernel/module.c b/arch/s390/kernel/module.c
> index 2798329ebb74..fe446f42818f 100644
> --- a/arch/s390/kernel/module.c
> +++ b/arch/s390/kernel/module.c
> @@ -297,7 +297,7 @@ static int apply_rela(Elf_Rela *rela, Elf_Addr base, Elf_Sym *symtab,
>  
>  			gotent = me->core_layout.base + me->arch.got_offset +
>  				info->got_offset;
> -			*gotent = val;
> +			write(gotent, &val, sizeof(*gotent));
>  			info->got_initialized = 1;
>  		}
>  		val = info->got_offset + rela->r_addend;
> @@ -330,25 +330,29 @@ static int apply_rela(Elf_Rela *rela, Elf_Addr base, Elf_Sym *symtab,
>  	case R_390_PLTOFF32:	/* 32 bit offset from GOT to PLT. */
>  	case R_390_PLTOFF64:	/* 16 bit offset from GOT to PLT. */
>  		if (info->plt_initialized == 0) {
> -			unsigned int *ip;
> +			unsigned int *ip, insn[5];
> +
>  			ip = me->core_layout.base + me->arch.plt_offset +
>  				info->plt_offset;

Would it be too paranoid to declare:

  			const unsigned int *ip = me->core_layout.base + 
						 me->arch.plt_offset +
  						 info->plt_offset;

That would trip an assignment to read-only error if someone were tempted
to write directly through the pointer in the future.  Ditto for Elf_Addr
*gotent pointer in the R_390_GOTPLTENT case.

-- Joe