Re: [PATCH] powerpc/32: Clear volatile regs on syscall exit
From: Kees Cook <hidden>
Date: 2022-02-23 19:34:12
Also in:
lkml
On Wed, Feb 23, 2022 at 06:11:36PM +0100, Christophe Leroy wrote:
quoted hunk ↗ jump to hunk
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 <redacted> --- 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