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

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Date:   Thu, 23 Apr 2020 18:26:57 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     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>,
        Joe Lawrence <joe.lawrence@...hat.com>
Subject: Re: [PATCH v2 6/9] s390/module: Use s390_kernel_write() for late
 relocations

On Thu, Apr 23, 2020 at 01:10:30PM -0500, Josh Poimboeuf wrote:
> On Thu, Apr 23, 2020 at 09:12:28AM -0500, Josh Poimboeuf wrote:
> > > > this is strange. While I would have expected an exception similar to
> > > > this, it really should have happened on the "sturg" instruction which
> > > > does the DAT-off store in s390_kernel_write(), and certainly not with
> > > > an ID of 0004 (protection). However, in your case, it happens on a
> > > > normal store instruction, with 0004 indicating a protection exception.
> > > > 
> > > > This is more like what I would expect e.g. in the case where you do
> > > > _not_ use the s390_kernel_write() function for RO module text patching,
> > > > but rather normal memory access. So I am pretty sure that this is not
> > > > related to the s390_kernel_write(), but some other issue, maybe some
> > > > place left where you still use normal memory access?
> > > 
> > > The call trace above also suggests that it is not a late relocation, no? 
> > > The path is from KLP module init function through klp_enable_patch. It should 
> > > mean that the to-be-patched object is loaded (it must be a module thanks 
> > > to a check klp_init_object_loaded(), vmlinux relocations were processed 
> > > earlier in apply_relocations()).
> > > 
> > > However, the KLP module state here must be COMING, so s390_kernel_write() 
> > > should be used. What are we missing?
> > 
> > I'm also scratching my head.  It _should_ be using s390_kernel_write()
> > based on the module state, but I don't see that on the stack trace.
> > 
> > This trace (and Gerald's comment) seem to imply it's using
> > __builtin_memcpy(), which might expected for UNFORMED state.
> > 
> > Weird...
> 
> 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;
-			ip[0] = 0x0d10e310;	/* basr 1,0  */
-			ip[1] = 0x100a0004;	/* lg	1,10(1) */
+
+			insn[0] = 0x0d10e310;	/* basr 1,0  */
+			insn[1] = 0x100a0004;	/* lg	1,10(1) */
 			if (IS_ENABLED(CONFIG_EXPOLINE) && !nospec_disable) {
 				unsigned int *ij;
 				ij = me->core_layout.base +
 					me->arch.plt_offset +
 					me->arch.plt_size - PLT_ENTRY_SIZE;
-				ip[2] = 0xa7f40000 +	/* j __jump_r1 */
+				insn[2] = 0xa7f40000 +	/* j __jump_r1 */
 					(unsigned int)(u16)
 					(((unsigned long) ij - 8 -
 					  (unsigned long) ip) / 2);
 			} else {
-				ip[2] = 0x07f10000;	/* br %r1 */
+				insn[2] = 0x07f10000;	/* br %r1 */
 			}
-			ip[3] = (unsigned int) (val >> 32);
-			ip[4] = (unsigned int) val;
+			insn[3] = (unsigned int) (val >> 32);
+			insn[4] = (unsigned int) val;
+
+			write(ip, insn, sizeof(insn));
 			info->plt_initialized = 1;
 		}
 		if (r_type == R_390_PLTOFF16 ||