linux-kernel - Re: [PATCH v4 2/3] powerpc/modules: Don't try to restore r2 after a sibling call

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Tue, 14 Nov 2017 15:59:21 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     Balbir Singh <bsingharora@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        live-patching@...r.kernel.org
Subject: Re: [PATCH v4 2/3] powerpc/modules: Don't try to restore r2 after a
 sibling call

Kamalesh Babulal wrote:
> From: Josh Poimboeuf <jpoimboe@...hat.com>
> 
> When attempting to load a livepatch module, I got the following error:
> 
>   module_64: patch_module: Expect noop after relocate, got 3c820000
> 
> The error was triggered by the following code in
> unregister_netdevice_queue():
> 
>   14c:   00 00 00 48     b       14c <unregister_netdevice_queue+0x14c>
>                          14c: R_PPC64_REL24      net_set_todo
>   150:   00 00 82 3c     addis   r4,r2,0
> 
> GCC didn't insert a nop after the branch to net_set_todo() because it's
> a sibling call, so it never returns.  The nop isn't needed after the
> branch in that case.
> 
> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> Signed-off-by: Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>
> ---
>  arch/powerpc/kernel/module_64.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
> index 39b01fd..9e5391f 100644
> --- a/arch/powerpc/kernel/module_64.c
> +++ b/arch/powerpc/kernel/module_64.c
> @@ -489,6 +489,10 @@ static int restore_r2(u32 *instruction, struct module *me)
>  	if (is_early_mcount_callsite(instruction - 1))
>  		return 1;
> 
> +	/* Sibling calls don't return, so they don't need to restore r2 */
> +	if (instruction[-1] == PPC_INST_BRANCH)
> +		return 1;
> +

This looks quite fragile, unless we know for sure that gcc will _always_
emit this instruction form for sibling calls with relocations.

As an alternative, does it make sense to do the following check instead?
	if ((instr_is_branch_iform(insn) || instr_is_branch_bform(insn))
		&& !(insn & 0x1))


- Naveen