Messages in this thread | | | Subject | Re: [PATCH 4/4] x86,module: Detect CRn and DRn manipulation | From | Jan Kiszka <> | Date | Wed, 8 Apr 2020 07:58:53 +0200 |
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On 07.04.20 23:48, Steven Rostedt wrote: > On Tue, 07 Apr 2020 13:02:40 +0200 > Peter Zijlstra <peterz@infradead.org> wrote: > >> Since we now have infrastructure to analyze module text, disallow >> modules that write to CRn and DRn registers. >> >> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> >> --- >> arch/x86/kernel/module.c | 21 +++++++++++++++++++++ >> 1 file changed, 21 insertions(+) >> >> --- a/arch/x86/kernel/module.c >> +++ b/arch/x86/kernel/module.c >> @@ -266,6 +266,22 @@ static bool insn_is_vmx(struct insn *ins >> return false; >> } >> >> +static bool insn_is_mov_CRn(struct insn *insn) >> +{ >> + if (insn->opcode.bytes[0] == 0x0f && insn->opcode.bytes[1] == 0x22) >> + return true; >> + >> + return false; >> +} >> + >> +static bool insn_is_mov_DRn(struct insn *insn) >> +{ >> + if (insn->opcode.bytes[0] == 0x0f && insn->opcode.bytes[1] == 0x23) >> + return true; >> + >> + return false; >> +} >> + >> static int decode_module(struct module *mod, void *text, void *text_end, bool sld_safe) >> { >> bool allow_vmx = sld_safe || !split_lock_enabled(); >> @@ -285,6 +301,11 @@ static int decode_module(struct module * >> return -ENOEXEC; >> } >> >> + if (insn_is_mov_CRn(&insn) || insn_is_mov_DRn(&insn)) { >> + pr_err("Module writes to CRn or DRn, please use the proper accessors: %s\n", mod->name); >> + return -ENOEXEC; >> + } > > Hmm, wont this break jailhouse?
Yes, possibly. We load the hypervisor binary via request_firmware into executable memory and then jump into it. So most of the "suspicious" code is there - except two cr4_init_shadow() calls to propagate the non-transparent update of VMXE into that shadow. We could hide that CR4 flag, but that could mislead root Linux to try to use VMX while in jail.
Jan
-- Siemens AG, Corporate Technology, CT RDA IOT SES-DE Corporate Competence Center Embedded Linux
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