From: Kees Cook <keescook@chromium.org>
To: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: linuxppc-dev@lists.ozlabs.org, Paul Mackerras <paulus@samba.org>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] powerpc/32: Clear volatile regs on syscall exit
Date: Wed, 23 Feb 2022 11:34:08 -0800 [thread overview]
Message-ID: <202202231131.08B7EC1@keescook> (raw)
In-Reply-To: <28b040bd2357a1879df0ca1b74094323f778a472.1645636285.git.christophe.leroy@csgroup.eu>
On Wed, Feb 23, 2022 at 06:11:36PM +0100, Christophe Leroy wrote:
> Commit a82adfd5c7cb ("hardening: Introduce CONFIG_ZERO_CALL_USED_REGS")
> added zeroing of used registers at function exit.
>
> At the time being, PPC64 clears volatile registers on syscall exit but
> PPC32 doesn't do it for performance reason.
>
> Add that clearing in PPC32 syscall exit as well, but only when
> CONFIG_ZERO_CALL_USED_REGS is selected.
>
> On an 8xx, the null_syscall selftest gives:
> - Without CONFIG_ZERO_CALL_USED_REGS : 288 cycles
> - With CONFIG_ZERO_CALL_USED_REGS : 305 cycles
> - With CONFIG_ZERO_CALL_USED_REGS + this patch : 319 cycles
>
> Note that (independent of this patch), with pmac32_defconfig,
> vmlinux size is as follows with/without CONFIG_ZERO_CALL_USED_REGS:
>
> text data bss dec hex filename
> 9578869 2525210 194400 12298479 bba8ef vmlinux.without
> 10318045 2525210 194400 13037655 c6f057 vmlinux.with
>
> That is a 7.7% increase on text size, 6.0% on overall size.
>
> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> ---
> arch/powerpc/kernel/entry_32.S | 15 +++++++++++++++
> 1 file changed, 15 insertions(+)
>
> diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
> index 7748c278d13c..199f23092c02 100644
> --- a/arch/powerpc/kernel/entry_32.S
> +++ b/arch/powerpc/kernel/entry_32.S
> @@ -151,6 +151,21 @@ syscall_exit_finish:
> bne 3f
> mtcr r5
>
> +#ifdef CONFIG_ZERO_CALL_USED_REGS
> + /* Zero volatile regs that may contain sensitive kernel data */
> + li r0,0
> + li r4,0
> + li r5,0
> + li r6,0
> + li r7,0
> + li r8,0
> + li r9,0
> + li r10,0
> + li r11,0
> + li r12,0
> + mtctr r0
> + mtxer r0
> +#endif
I think this should probably be unconditional -- if this is actually
leaking kernel pointers (or data) that's pretty bad. :|
If you really want to leave it build-time selectable, maybe add a new
config that gets "select"ed by CONFIG_ZERO_CALL_USED_REGS?
(And you may want to consider wiping all "unused" registers at syscall
entry as well.)
-Kees
> 1: lwz r2,GPR2(r1)
> lwz r1,GPR1(r1)
> rfi
> --
> 2.34.1
>
--
Kees Cook
next prev parent reply other threads:[~2022-02-23 19:34 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-23 17:11 [PATCH] powerpc/32: Clear volatile regs on syscall exit Christophe Leroy
2022-02-23 19:34 ` Kees Cook [this message]
2022-02-24 7:00 ` Christophe Leroy
2022-02-23 20:48 ` Gabriel Paubert
2022-02-23 23:27 ` Segher Boessenkool
2022-02-24 8:29 ` Gabriel Paubert
2022-02-24 12:49 ` Segher Boessenkool
2022-02-24 6:41 ` Christophe Leroy
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