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.le...@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